ICAL POSI

21 MAY 1987

TECHNICAL SYMPOSIUM

WATKINS-JOHNSON COMPANY SYSTEMS GROUP

Miniaturization of VHF and UHF Receivers by Hybridization

Presented by Harmon W. Banning CEI Division

id Broad Band Signal Aquisition a New Tool

Presented by Allen L. Shapiro CEI Division

Calibration Techniques Enhance ESM System Performance

Presented by Dave Hawthorne ESM Division

Theory and Application of a O. 5 to 18. 0 GHz Tunable IFM Receiver

Presented by Ralph Williams ESM Division

Automatic Test Equipment for EW /ECM Equipment

Presented by Steve Adams SSE Division

Direction-Finding Displays Using Digital Techniques

Presented by David Z. Janich SSE Division ..

Computer Graphics for Three-Channel ~ction Finding Data

Presented by Philip O. Morse Special Projects Division

Spurious Signals: Specifications and Testing Procedures

Presented by Robert E. Watson Special Projects Division

CALIBRATION TECHNIQUES ENHANCE ESM SYSTEM PERFORMANCE

Presented by

Dave Hawthorne

Prepared by

Dave Hawthorne

WATKINS-JOHNSON COMPANY

HSM DIVISION

CALIBRATION TECHNIQUES ENHANCE

ESM SYSTEM PERFORMANCE

Electronic warfare support measures (ESM), electronic counter measures

(ECM) and electronic counter-countermeasures (ECCM) are the basic subdivi­

sions of electronic warfare (EW). ESM involves actions to "search for, inter­

cept, locate, and immediately identify radiant electromagnetic energy for the

purpose of immediate threat recognition. 111 Information prq4~ced by ESM

equipment can be used to initiate ECM , ECCM, avoidance , t;fgeting or other

tactical responses.

A fundamental requirement for an ESM system is "to .Jilrt the timely assessment of an enemy's intentions and position by interi~~ting and pro­

cessing RF ~ffiissions as$p9:;a.ted with Iµ@ ope,#ijtions. 112 To: ;;tisfy this re-

~~r:;:n:~i :::t:Jll:::g:":roilt:Jl tt::::-11 b;e•c:i::~~·:: large ' mu!ti~\\\¥;,;,eceiver ,:!!~:•. A i%? .,,.•,,.·•.·,,• •• ,•.,.•',.,.,,•,,.•,,•, .• ,·.i·',, ',.',,,'',,.',,._,, .·', ':, ·,, ..•• .••. .. ,.·,,.·,,. ·,,. · •• . ·,,·,.,: ~',.·,,·,,.·:.·, •.• ,,· .. ·,, :,,,.·',.,.::':.·.·,,,.'',,.''•,.'',.,•',.,'',.,•,.a•.·,,•,,•.,,•.,•,, .• c'··,,.•., .• ,,.,,.,,.,.q'.·,:.·····.•.\.!u" ;1·····•s• ition. Regardless of !i~~ji!iw I~~ ~~lf~ffi I?:~ perform O o~+w Super-

heterodyne, crystal video, instantaneous frequency measurement (IFM) ,

channelized, microscan and acoustooptic receivers have been used for signal

acquisition. Each receiver type has its own inherent strengths and weak­

nesses, as shown in Table 1. Selection of a receiver type is critical to the

development of an architecture for:. a specific application.

As the state of the art of receiving equipment has advanced, considerable

information has been compiled on the advantages and disadvantages of differ­

ent receiver technologies. The debate continues as to which receiver type is

most desirable for ESM applications, for no single receiver is ideal for all

applications. Arriving at an optimal solution to a specific problem involves

-1-________________________ UJ •

Table 1. Features Comparison of the Principal Receiver Architectures

~ Wide-Band TRF Narrow-Band Wide-Band Acou.sto-Crystal Video Crystal IFM Superhet Superhet Channelized Microscan Optic

pe

Video e

Instantaneou.s 500- 10.000· 25 500-10,000 0.5-20 500 1280 250 soo Analysis Bandwidth (MHz) ·10 2 10 2 4 5 4 4

Frequency 500-10,000 2S 0.52-3 0.5-20 0.5-20 500 10 2 Resolution "

(MHz) .· I 3 9 8 6 8 9

Sensitivity (dBm) -46 to -84 -80 to -83 -37 to -80 -101 -87 -72 -90 -67 T-290°K 5 8 4 10 9 4 7 9 9

Dynamic 70-73 50-80 70 60 70 70 60 35 Range (DB) 7 5 7 6 7 7 6 4

Relative Speed 1 700 1 875 35 13.67 70 35 of Signal Acquisition 9 1 9 1 7 8 6 7

Short Pulse 40 75 40 50 40 100 500 8000 Width Capability (ns) 7 7 7 7 7 7 5 1

Measurable TOA, PW, PA RF, TOA, PW RF, TOA RF, TOA, PA RF, PW RF. TOA RF, TOA Signal SRI PA, SRI PA, SRI. PW PA, PW, SRI TDA, SRI, PA PW, SRI PA, SRI PW Characteristics 8 10 10 10 10 10 8 6

Exotic Signals ii> ii> "'· !J,.f ii> "'· !J.f ii>. !J,.f tJ.f.CW !J,.f

Applicable 3 3 6 3 6 6 6 3

High Signal Performance I 3 2 3 I 5 7 6

Simultaneous Signal Capability I 5 3 7 6 5 7 7

Processing Complexity 4 5 5 4 4 4 I 2

Immunity from Jamming I 5 I 9 3 2 7 7

Size (in3) s 53 1656 2500 1515 4320 1390 1900

9 2 3 2 3 I 3 3

Power (W) I.S 7. 1 320 ISO 12 1200 80 71 Requirements 9 8 4 5 8 I 6 6

Cost (SK) 20 50 75 125 100 2500 1300 133 8 7 7 6 6 I 3 6

*Upper number or letters are nominal receiver capabilities and lower number are relative EW /ESM ratings based on scale of 10 to I.

• ________ - 2_-_____ ~u

San Jose, California

consideration of performance requirements and parameters such as relative

system cost, size, weight, and reliability.

Tradeoffs in probability of intercept, response time, frequency coverage,

sensitivity, dynamic range, resolution, selectivity, and accuracy must be

considered in the selection of a receiver type. Frequently, the best approach

to an ESM requirement is the use of a hybrid architecture which incorporates

multiple receivers of different types, thus combining the strengths and

possibly overcoming the weaknesses of the individual receivers.

In order to manage the multiple resources of a hybrid ESM s_ystem and to

attain the maximum performance from a system at a reasonabl~ .cost , computer

control is required. Computers are also required to procJgJ the data ac­

quired by a large system to produce useful information abq:µJ the environ­

ment. Using embedded microprocessors in the RF distribu!&J and tuners,

and single board computers for system control, signal proce$.$Wg and display

generation, results in a powerful, cost effective architecturei f

;:;;;::I~~l=:1:;::~l:;:~::1

:;jlJ;7jf l~:~:~~::Jl:;:~~:·:::~ ;~: • st:::::•·~--~·iiirl1!::::i!iliiif t~J;08:o!::o:~:~ system . complements the hybrid receiver architecture, resulting in system

capabilities superior to those achieved without calibration.

This paper shall consider a hybrid ESM system architecture, and discuss the . calibration techniques that can be appliecf to enhance system performance.

Although the techniques are discussed i~ the context of a particular archi­

tecture, the techniques can be readily applied to other systems with similar

requirements. The hybrid architecture utilizes a scanning IFM and a set of

narrowband superheterodyne receivers under computer control. Such equip­

ment is currently produced by the ESM Division of Watkins-Johnson Company

in San Jose, California.

-3-_______________________ UJ •

BLOCK DIAGRAM

Figure 1 presents the block diagram of a hybrid ESM system. The system

combines a tunable IFM with a bank of narrowband superheterodyne receiv­

ers. The system shown in the figure is based on standard receivers and

utilizes typical signal distribution, processing and control hardware. The

components that comprise the hybrid ESM system shall be described in the

following paragraphs.

The tunable IFM is a high probability of intercept receiver which performs

rapid search and simultaneous signal processing to interqept, detect, mea­

sure, analyze and report signals found in the environment. The architecture

of the tunable IFM is different than conventional IFM's, fgi is discussed

below.

OMNI ANTENNAS

y y . . . y I INPUT SWITCH I CONTROL

I I ..... ..... ... GPIB

RF DISTRIBUTION ·1 AUX

RF IF RF RF RF

TN-122 TN-218 I+-TN-218 CONTROL TN-218

TUNER ~

TUNER TUNER TUNER . . . lIF lIF lIF lrF

r---+ IFM MD-100· I+-

MD-100 CONTROL MD-100 DEMOD DEMOD DEMOD . . . DEMOD

IF 1 lVIDED IF lvrDEO IFl lVIDEO

I SWITCH ASSEMBLY I I

IFM VIDEOS THRESHOLD SELECTEDl SELECTED ! VIDEOS IF VIDEO

TTL CONTROL TTL CONTROL TUNER CONTROL VIDEO PROCESSOR AND

CONTROLLER TUNER CONTROL GPIB

87. R.1142

Figure 1. Hybrid ES Sys Block Diagram

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The bank of superheterodyne tuners are used for both signal acquisition and

automatic or manual analysis. The tuning accuracy, selectivity, and sensitiv­

ity of these tuners provide capabilities that augment the basic IFM system.

The number of tuners incorporated into such a system architecture is dictat­

ed by the requirements of the application. The concept of the hybrid ESM

architecture has been implemented with a single narrowband tuner, as well

as with multiple sets of superhet tuners.

CONVENTIONAL IFM RECEIVER

An IFM, or Instantaneous Frequency Measurement, receiver is defined as a

"receiver with a wide acceptance bandwidth which determines frequency from

phase information in power split signal components traversµi.g unequal path

delays. 113 A block diagram of a conventional IFM receiving ~;stem is shown

in Figure 2, The receiver consists of an antenna connected to a hybrid

power splitter, a delay line in one of the signal legs, a p~lt~\e discriminator

and display. The principle of the IFM operation is to introdll¢e a time delay

in one of the splitter outpllts which translates to a phase diit~rence between

the compone#tS which is <fj#oportional t§ freqij~l'lCY. 4 The llase difference

can be mea$;tired by daegc······r·.al······.·.· .. ·e.·.•.··.·./···1····.·0··b····· .. sr.ateTdhephda

1.

8s .... •.·.·.rt·e·;··.·.· .·.·.•

1·.·.•.•.•a.•.•.· .. dyiesdck .. : .. : ..•.. 1 .. L.•.•.m.•.•.•n:•.• .. • .. •.1.·.gnlaetor and !i§played as an angle from Q\to 360 i:,, a indicat,; dhe frequency

offset of thJ~iinal with~~~T band 0{1IJ IFM,i:t,

87.R.1131

111 CROWA VE PHASE OISCRl~INATOR

UA E -r-,.-----------1 LAW I OU A ORA !URE COUPLERS !DETECTORS I & PHASE SHIFTERS I

I PASSIVE STRIPLINE II I NE TlfORK L ____________ ..J _ _,

RF SI GNH PROCESS ING

VIDEO DETECTION

DIFFERENTIAL AMPLIFIERS

Figure 2. Conventional IFM Receiver

-5-

---------------------- UJ •

Figure 3 shows a basic phase discriminator. The phase discriminator accepts

two inputs which are derived from a single RF signal, using the power

divider and delay line. The two outputs of the phase discriminator are

proportional to sin 0 and cos 0 , where 0 = 2 7T f ~ L / c. 0 is the phase angle

between the two inputs, ~ L is the difference in path lengths to the inputs of

the phase discriminator and c is the propagation velocity of the wave m the

delay lines. As the two input signals are of equal amplitude, the ratio of the

output voltages equals the tangent of the phase angle between the two

components.

If the sine and cosine outputs of the phase discriminato·r are applied to the

X and Y inputs of an X-Y monitor, the displayed vector will have an angular

displacement equivalent to the phase angle 0. This angle can be interpreted as

the signal frequency since a fixed time delay in one of the phase discrimina­

tor inputs translates to a linear phase change over frequency within the

bandwidth of the discriminator. The unambiguous frequency range, or

mkA 2

c =-2 cose

= SIGNAL Acosr/1 INPUT

PASSIVE STRIPLINE NETWORK

87.R.1132

Figure 3. Phase Discriminator

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bandwidth, of the discriminator is determined by the delay line length. The

phase angle will rotate through 360 degrees for each octave frequency

change. The nominal frequency of the discriminator is given by the equation

fo = c/ AL = 1/T

where T is the difference in the path delay times.

TUNABLE IFM

As indicated by Table 1, IFM receivers exhibit many favorable characteristics

for ESM applications. IFM's have a wide instantaneous analysis bandwidth

resulting in a high probability of intercept, very good dypamic range to

allow operation in environments with signals of vastly different strengths,

high signal acquisition speed and the ability to handle exqt}P signal types.

These characteristics make the IFM attractive for many appH!l~ions.

On the other J1and, the I;F'.l\1 receiver typically has poor sed~i.iivity, average

frequency Jli1ution, de!t:ided perfor&IJce ifi [environmen1it tr high signal

:;:;~ty~g:n11t:::::g1:l::t: ~::11~;u:r,r:::: ;: 1~=~ t:: j;:~ ::: ::::::511;;;.tll;~;,1,t:~::~1~\,,:a;:::; :::: but limited, ra~gJT F~r broadbti:rid applications, it bth~mes necessary to

operate multiple IFM's in parallel to achieve the required coverage.

The tunable IFM concept was developed at Watkins-fohnson Company to

partially overcome the shortcomings of t~e conventional IFM. By placing a

wideband tuner in front of an I:FM demodulator, the frequency coverage of

the conventional IFM can be expanded without incurring the cost increase

associated with distributing input signals to multiple IFM's and correlating

the various outputs. The TN-122 tuner shown in Figure 1 is swept across

the spectrum at a rate of up to 40 GHz per second, allowing coverage of a

very wide range in a short time. The TN-122 also supports 500 MHz of

instantaneous bandwidth, to maintain a high probability of intercept under

high scan-rate conditions.

_____________ -_1-_________ UJ I

•

As the TN-122 scans the environment , the IFM demodulator produces log and

FM video signals from the 1 GHz IF of the tuner. The IFM demodulator

actually contains two phase discriminators, with different delay line lengths,

as shown in Figure 7. The addition of the second phase discriminator im­

proves the effective resolution of the tunable IFM.

The outputs of the IFM demodulator are digitized and processed by circuitry

in the Video Processor and Controll~r unit. The computers contained in the

unit automatically an[:!lyze the data, deinterleave pulse trains, determine

signal characteristics, identify signals and output results. In addition, .. processing techniques have been developed to handle high environmental

densities, exotic signals, CW and high duty cycle signals. :J;'pe tu~able IFM

also has the ability to automatically switch to a narrowband mode in high

density areas, or at operator request.

The performance characteristics of the tunable IFM are:

Wide Instantgn~ous Analys[i{Bandwidth: }

High FrequiJl Resolutiol! )((((

Good SensittJi~~k .

Wide Dynamic Range: 60 dB

High Speed of Acquisition - less than 0.5 seconds in 0.5 to 18 GHz range

for continuously detectable signals

SUPERHETERODYNE RECEIVERS

Although the tunable IFM represents a cost effective solution to the problem

of providing high probability of intercept and rapid acquisition time, it does

not perform as well as a narrowband superheterodyne tuner in the areas of

frequency resolution, sensitivity, high density en vironment performance,

simultaneous signal processing or im uni "amming. The narrowband

• _____________ ·wu San Jose, California

superhet exhibits high selectivity and can be tuned to a signal frequency

with high accuracy when local oscillators are synchronized to a system

reference. This allows collection of signal parameter data necessary for

detailed analysis of emitter characteristics.

The TN-218 tuner is a digitally controlled, single conversion, super­

heterodyne tuner. According to the IEEE definition, super heterodyne recep­

tion is a "method of receiving radio waves in which the process of hetero­

dyne reception is used to convert the voltage of the received wave into a

voltage of an intermediate, but usually superaudible, frequency that is then

detected." Heterodyne reception (beat reception) is defined as "t:tie process

of reception in which a received high frequency wave is combined in a

nonlinear device with a locally generated wave, with the result that in the

output there are frequencies equal to the sum and difference of the combin­

ing frequencies." The TN-218 converts frequencies in the 2 to 18 GHz

range to an intermediate frequency of 160 MHz. The IF ol1.tput is detected

and processed downstream.

. :(<>.:> \: ::· · ,: Detailed analysis through a narrowband superhet channel takes time, and in

dense environments multiple tuner channels b~come a necessity. Combining

the tunable IFM with multiple superhet .iUners ;ekults in a system which can

continuously monitor the environment with an ~~tremely htgh probability of

intercept, while <pe'I'forming. autom1tic; detailed ;rii1'#:~¥~ <()f previously inter­

cepted signals. This is accomplished by initially scanning all receivers (IFM

and superhets) against the environment and automatically assigning super-. hets to the task of analyzing encountered signals as required. Tuner assign-

ments are determined by the controller unit base~ on operator inputs, signal

intercepts, relative signal priorities and available resources.

For the system to operate effectively, it is essential that the controller have

the ability to transfer, or handoff, signals from an intercepting receiver (be

it IFM or scanning superhet) to an analysis receiver quickly and accurately.

-9-

.-

•

In order to optimize this process, calibration techniques have been developed

to correct for tuning inaccuracies caused by the properties of the individual

receivers. These calibration techniques will be discussed below, in the

context of the architecture shown in Figure 1.

RF DISTRIBUTION

Inputs to the system are received through omnidirectional antennas, which

are selected to cover the frequency range of interest. The antenna outputs

are connected to an - input switch unit, which allows the antennas to be

disconnected from the system and provides for proper termination of input

lines when in the disconnect state. The input switch unit is required for

calibration , as the antennas must be disconnected for prop¢r results. The

input switch unit is controlled by the RF Distribution unit in response to

commands from the Video Processor and Controller unit.

••••••••••••••••••••

The RF Distribution unit is designed to accept signals in J~8h of the input

bands determined by the antenna array. A block diagram of}tlie RF Distribu-

:::w: i:h:J.t:a;:r;oj!ith:.::•:;r Ji;t;~ll::s ~:1:Jlllh:~:\:::d~: preamplified~ t and convert,il. if necesi~i:ir. t~ ihe frequen~~ >range of the

tuners . Aft~t £Onversio1}; fij~ signals >~f~ split i9 each of JP-~ channel selec­

tor switches. ''fp.$ijg ,i:;wjh:fh4$ iU9W ,~ijc,itf tuner td AP~r~t~ grj §ignals in any of

the input band;, 1HaJS~~dent ar iH~ .,~,~her tuner~. ¥Bi 'Bui~uts of the chan-

nel selector switches are connected to the tunable IFM and the bank of

narrowband, super heterodyne tuners.

Also pictured in Figure 4 is a comb generator circuit. As will be discussed

later, the comb generator is used during the calibration procedures to

generate signals at known frequencies (every 100 MHz ). The comb signal is

generated by amplifying the output of a 100 MHz voltage cont rolled oscillator

(VCO) and applying the signal to a step recovery diode (S RD ) . The nonlin-

ear transfer fu nction of the SRD resll.!. s in

the or iginal 100 MHz signal. The spectra: as comb- like appearance -hen vie ed

co b generator .

e generation of harmonics of

of t he harmonics has a

analyzer, hence the name

•

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-

ANTENNA C>--INPlJT )--- --- --- ! --·

(TYPICAL}

AUX ff

GP1B

87.R.1133

COMB

S WIT C tt CONT AOL

OOHNCONVEATEA (AS AEGILJ11-1 E OJ

COMB

DIST

- OIVIDEA

-_ . -•

"TO OHIE R INPUT BANOS

FROM -. + ~!~!5~ _. SWITCH

CHANNE L 1 ·• OUTPUT

FROH -. EJ-WITCH -~ Cli0AUN1NPF.U\ 2 OTHER • S ANDS-·

SWITCM -_, CHANNEL N

oun:iur

Figure 4. RF Distribution Block Diagram

A transfer ilil\iitch and $\fl{r located <!fti\r t1\l!II SRD are Jllded to allow selection of i ! $ingle comb/!i J;quency. T;; Juseftj~less of the r,4J~r will become

::a:::e:nil~::;:r:e:Jlct::e t:r::lt :lf in:~t:::~tlt:::t;:::~ ator is not ·iii use, th&. 'VCQ • .. Js dislib)~d, prJ¥!~tiHK J.~~iii~ of the comb

frequencies in~6>twi ~Ii:1 ;:tH1I >

An auxiliary IF input is provided for calibration of the tunable IFM. The aux "

IF signal is used as an additional input to the first channel input selector

switch. This allows the aux IF signal _to be connected to the input of the

first super heterodyne tuner.

The RF Distribution is under microprocessor control . The microprocessor

communicates over an IEEE-488 instrumentation bus ( GPIB) with the system

controller. Control of the comb generator and switches in the unit, as well

as the Input Switch unit, is exercised by the microprocessor.

-11- •

TN-218 TUNER

The selected RF signal from the RF Distribution unit is input to the TN- 218

tuner. The TN-218 is illustrated in Figure 5. The input signal passes

through a YIG-tuned filter to a mixer. YIG (Yttrium-Iron-Garnet) crystals

are used as the tuning elements in many devices, because of the material's

high Q resonance characteristics. 5 The YIG oscillator and filter shown in

the figure are examples of such devices. The filter, or preselector, is a six

stage device with an instantaneous banclwidth of between 20 and 70 MHz,

depending on the tuned frequency.

The preselector is under the control of the microprocessor, which generates

the appropriate digital tuning words for each frequency. 'rp~ tuning words

are converted to an analog voltage by a digital to analog converter (DAC).

The preselector driver converts the tuning voltage to a \ qµrrent, which

actually controls the magnetic field within the device. YIG ffit~rs · are highly

linear , magnetically tuned devices. The resonant frequency jti}/ a YIG sphere

is directly p:r.9portional to the magnetic field which surround~ it. 6

F<F ltJPU T >-- --t~~E;_~- f - •~ ~~[>----------~IF OU TPUT

VEFI P AD ~L L --- AMP LIFIER AMP LIFIER

- - --------,

----- •-----· -- -~ LO SAMP LE

OIVIOER

RE~;:,~cE )- - - ----· -------·-- SYNCHR ONI ZER., _ ___ ~

,,,,,,,,,,,,,, I

DAC I C~,::~L ;-.!-------------------------~

87.R.1136 Figure 5. TN-218 Tuner Block Diagram

• _ ___ _______ -_12_-_______ UJu

San Jose, Cali forni a

A mixer is used to combine the RF signal with a local oscillator signal (LO)

to produce the desired intermediate frequency, or IF, signal. The IF fre­

quency is fixed at 160 MHz, which greatly simplifies the process of amplifica­

tion, filtering and detection. Changing the tuned frequency of the receiver

is accomplished by controlling the frequency of the LO signal. As the LO

frequency is changed, the preselector is made to track the tuned frequency

of the receiver.

The conversion from RF to IF in the mixer is a non-linear process in which

new frequencies are generated and added to the two input frequencies. The

first sidebands, or sum and difference frequencies, are generally used as

the IF signal, such that

fIF = fLO ± fRF'

In the TN-218, the IF is 160 MHz, and the input frequenc}'\,ange is 2 to 18

GHz. The LP/ tunes from Zi 16 GHz to ')..$.16 Qfflz to cover fffl s range. The

:~:~tu::~ch1:::~·:::he~Y111d ::I::~:~Jif ~:tlf :o;~:::~f lt:':~u~~:: ~~

Ps1~gonvailde1· s thacec)/:oli tm•_-._,_•_•_o_._•,_•p•._',,•li· __ ·.· ..• fs .. _.r,h_._.e_.e.q_.d. u._., ..... •e,_·,-,u_·_·.n_,_·•.•s•,-.c_._i_.1·.•.1_:;ne./gs_•_._••.·. . .,,;:;,,,,., , ··.· .,,,•,•,•,•;:;•,:;; .-;:,,,,,,;:;,_,,,.,.· lh~ §PPT iJwitch ti~:i:J,~,r rnJ9rpfizj6~essor control.

The microproc;;~d,ii ~1§6/ co~f±-61; fh~ >~uned fre~iilrid;, 8¥\ the YTO via the

DAC and YIG driver circuit.

A second input to the YIG os.cillator is generated by a synchronizer. The

synchronizer is enabled by the . microprocessor in the ,.fixed frequency or

manually-tuned mode to improve tuning · acc;:uracy. The synchronizer samples

the LO signal and phase locks the frequency to an external, crystal refer­

ence frequency. With the synchronizer active, the tuner may be tuned in 100

kHz steps with accuracies determined by the external reference. Typical

synchronized accuracies are 1 part in 108 . Under scanning conditions, the

"open-loop" tuning accuracy is 30 MHz, maximum.

-13-

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•

The open-loop tuning errors encountered with YIG-tuned receivers can

significantly affect the performance of ESM receivers. The accuracy and

repeatability of frequency measurements as well as the speed with which a

receiver can "set-on" an intercepted signal are directly determined by the

tuning accuracy of the receiver. The factors which influence tuning accura­

cy, and the techniques which may be used to improve tuning accuracy will

be discussed in a subsequent section of this paper.

Following the mixer, the signal encounters amplifiers and a digitally con­

trolled attenuator. The amplifiers are required to supply the desired RF to

IF gain, and the attenuator is used for automatic noise leveling. The auto­

matic noise leveling function (ANL) is an automatic, calibr~ted response of

the microprocessor to changes in tuned frequency. The ANL feature allows

the tuner to maintain a constant noise level at its output, independent of

gain or noise figure variations of the system across the bati(i. The ANL is

important in small and large ESM systems to optimize detecti~h thresholds for : · .-: ·.

high probability of intercept for weak signals while minimj iing false alarm

;;~s ~:c::lt•::nt::;•IL~~:~ro:jfl::ru11i:t:::.:: •••••••••••••••••••• st &tus information

c:Sri.f ains lines for

tuned frequJrie_ .• •.·••.J .... ... ".··.• ...... tu11~r.·•••.•• ~dd;pe§sir1_.g_._.;._? tiining irlride~ hysN~i'~iis, phase lock ••••c<>a· l1'brat1'on ( d\a·••.ft •. ••. ••.•. ·ar _ .••.•..•. _ ••.••... •.•••.•._•c/o)nt tfr?o· l. The parallel indication, R~ bl~h)k.fri~\ r;JJtU ~rid

control interface allows multiple tuners to be controlled over a single bus.

Each tuner microprocessor is capable of receiving and executing commands

every 100 microseconds.

The performance characteristics of the TN-218 are summarized in Table 2.

MD-100 DEMODULATOR

The MD-100 demodulator provides the functions of IF filter selection and

video detection. The unit contains selectable IF filters which set the receiver

bandwidth from 1 to 20 MHz . The output of the selected filter is power

divided , and sent to both a buffer amplifier and a log detector amplifier.

• ________ -_--_____ ·U.Ju San Jose, California

The buffered signal is routed to a connector and used as an IF output to

the switch assembly.

The log detector produces a video output that is proportional to the log of

the amplitude of the IF signal envelope. The log amp is commonly used in

ESM systems to produce pulsed video signals, as the log amp can detect

signals covering a dynamic range of up to 60 dB.

The demodulator is controlled using dedicated TTL control lines. These lines

allow selection of IF filters by the controller unit.

SWITCH ASSEMBLY

The switch assembly contains a set of IF and video s witches t o select t he

output of one of the superhet receivers for processing or}:giµibration . The

switch assembly is under TTL control of the controller unit.if :

Table 2. TN-218 Tuner Functional PerformaJI /

RF Input I~/idance:

.. .. ::;rtr:m;

5~ ~hms l!fuinal

RF Frequen¢j1j~

IF Frequenci&I\ 1!6 MHz Hdfuinal

RF BandwidttlS:> ,. : :.... ••/ :Ii MHz mihtMum Frequency Acc~:i8Y: t n···· ···?•••••••<•>•••<t>(io MHz typ~~!1\ Iitt ··

(Non-Synchornized): 30 MHz maximum

Incidental FM:

Step Size:

Image Rejection:

Lo Radiation :

1 dB Input Compression Point:

Third-Order Input Intercept Point:

25 kHz typical

40 kHz maximum

100 kHz nominal

80 dB typical

60 dB minimum

-90 dBm typical

-80 dBm maximum

+5 dBm typical

0 dBm minimum

+ 10 dBm typical

+5 dBm minimum

-15-_ ____________________ lU San Jasie.

•

TN-122 TUNER

The tunable IFM incorporates a TN-122 wideband tuner, which covers the

0.5 to 18 GHz range and supports a 500 MHz instantaneous bandwitdh at an

IF of 1 GHz. Performance specifications for the TN-122 are shown in Table

3.

The block diagram of the TN-122 is shown in Figure 6. The unit is a digital­

ly controlled, dual conversion , super heterodyne tuner. RF inputs pass

through an input -· filter/switch assembly to route the RF energy through

either a low-band channel (0.5 to 8.0 GHz) or a high-hand channel (8.0 to

18.0 GHz).

In each channel the signal encounters a mixer, used to perform the first of

two conversions. The first local oscillator is a YIG-tuned GaAs FET oscilla­

tor. For signals in the low-band channel the first LO swelpij• from 10. 5 to

18 .5 GHz, producing a constant IF output of 10 GHz. T~ij )!ll rirst LO must

make two sweeps to convert all frequencies in the high-baril channel. The

first sweep Jl~vers signaii between 8 iii 14 :\gllz, where ttil\ LO is always

!~q~::ci:!1F!:h:4 t::e~~if lt~~e:;:~ JTite:11{:::;pt:::lt+t~e :::~::: the tuned f;i;uency Thti<lroduces a ij~hstan~iit)f output atm\J\~ 5 GHz for all inputs betwe~w:1•••:•• ,n~•··••·1~ /:a~•i.•~:: ... , •••••......... ·.·.·

The first LO is controlled by the unit microprocessor. The microprocessor

produces tuning words that are converted to a tuning voltage and finally to

a coil current, as described for the T N-218 tuner. This tuner utilizes a dual

conversion scheme, however, to cover the range from O • 5 to 18 GHz, as

opposed to the single conversion architecture of the TN-218.

The first local oscillator is not synchronized, as is the TN -218 tuner. The

tunable IFM is normally used as a scanning receiver, because of its high

probability of intercept and because the narrowband superheterodyne tuners

provide set-on and manual tuning capabilities. The synchronizer is not

• ___________ -_is_-_______ UJu

San Jose, California

87.R.1134

B. 4 GHZ LP FILTER

LOW-BAND CHANNEL 0. :5 ro 8 GHZ

HIGH-BAND CHANNEL 8TD1BGHZ

Figure 6. TN-122 Tuner Architecture

_I'..........__ -t i GHZ If ~/- -- OUTPUt

useable in J!j~!.t scan moli['ibecause of l.i t!Jlilqulred fJl!I. loop to lock up at each $tip. The TN+ti2 , thereforif does• bbt contain J ~}nchronizer in

this ap P]IC·~~ . . •.• •. ·.•·.•o••·.·.··.·.•.•s•.··.··.•.;.o,.·.•.·.··.·.···"•j··1·••.·1·•· .. ·atttolir! 'l,tihro•••.uw·······.·······.·,,. .. • .. •.·.•.· ... ·v.·.·.··.· .. •.· .. •.·.·e·.··.· .. •.t.· •. •••.·.·.: .. • .. • .. •,•-!i 111 ; IIlh. <1,lll!)i The second l=~~l ~~ ~ ·~·· f~i a diele!tjil iji,Bt~f\~~~, phase-locked

oscillator with a fixed frequency of 4 .5 GHz. This frequency is used to

convert the 3.5 GHz signal from the high-band mixer to a final IF of 1 GHz . . The 4.5 GHz LO is also doubled to 9.0 GHz for use in the second conversion

of the low-band signals from 10 GHz to 1 GHz.

The output of either the low-band or _high-band channels is selected through

the SPDT switch, and amplified before being output to the IFM Demodulator.

The TN-122 is controlled by the internal microprocessor. The microprocessor

communicates with the Video Processor and Controller unit over a parallel

control interface similar to the one used for the TN-218 tuners.

-17- •

Table 3. TN-122 Tuner Functional Performance

Input Frequency Range

IF Output Center Frequency

RF To IF Bandwidth (3 dB)

Step Size (minimum)

Noise Figure

RF To IF Gain

Input 3rd Order Intercept Point

Input ldB Compression Point

Single-Sign~} ~pur Free J:?ynamic

Range

0.5 To 18 GHz

1.0 GHz

500 MHz Nominal

500 kHz

23 dB maximum O. 5 To 18 GHz 16 dB typical O .5 To 8 GHz 18 dB typical 8 to 18 GHz

15 dB typical, Higher Gain Optional

0 dBm minimum +5 dBm typical

-7 dBm minimum -5 to O dBm typical

57 dBm>minimum

~·~ :aBmt~;pical

Image Rejectiqp. .:::.::::::::·:.

LO Radiatio~ > .11~~dl;::u~~ dBmjli\ical

'''1Ii!@rn mtii!Tif ~~~/~h::4tra:ii\t };;cal Size

Weight

IFM DEMODULATOR

7.44 inches x 19.65 inches x 4. 94 inches (AlD)

25 pounds

The IFM Demodulator processes the 1 GHz IF to produce log video and

discriminator outputs from which the amplitude, time of arrival and frequen­

cy of the signal can be measured. The block diagram of the unit is shown in

Figure 7.

•

-------------~u San Jose, Ca lifornia

~

i E

I .... ~ I

•

1 GHz IF INPUT DIGITALLY ---••~I CONTROLLED

ATTENUATOR

SPOT SWITCH

1 GHz CENTER FREQ. 500 MHz BW

IF LIMITING AMP

87.A.8821

SPF

oo

INI "-.

goo IN

goo

oo

SPOT SWITCH

LONG DELAY

1 GHz CENTER FREQ. 500 MHz BW

BPF

oo-oo

LINE........_. STRETCHER

IF

LINE

IJ1111u~"'" ~o

~ goo

goo

14.56 nsec

oo

I 0°

LINE .___. STRETCHER

Figure 7. IFM Demodulator Block Diagram

LOG AMP

LIMITED IF OUTPUT

LOG VIDEO

OUTPUT

50 MHz BW

VIDEO AMPLIFIER

,..., COURSE

Sin (21rftd)

COURSE Cos (21rftd)

~

,._

FINE Sin (21rftd)

The 1 GHz IF input from the TN-122 tuner is applied to a digitally controlled

attenuator, which is used for the automatic noise leveling function as de­

scribed for the TN-218. The attenuator is controlled using TTL lines from

the controller unit to vary the IF gain of the unit, maintaining a constant

noise floor at the detectors .

The output of the attenuator is connected to a pair of switches that allow a

30 MHz bandpass filter to be _switched into the signal path. This filter allows

the tunable IFM to switch to a narrowband mode for response to CW signals

and dense environments, overcoming the problem of . simultaneous signal

processing in the IFM.

The signal is then filtered to set the 500 MHz bandwidth the IFM. The

output of the filter is connected to a log detector amplifier , .. >which produces

the log video voltage. A limited IF signal is a natural bypf§~uct of the log

detection process' and this output is used by the discriminatb:is.

The limited lit' signal is fj@(}l.er limited M/@id IQtW'ed again. II~ limited IF is

then power (U'tided, and 4~itributed to f#o d&lii line disc~tffl~nators. These

discriminatoi~ measure ttlj phase diffetl#.ce oltleen delayed Jnd undelayed

::~n:e1:::~1;: ~o T:~.,1;::·~;;:t,~~:;!la::,:h:a#l!1:d t:0

:::: ::

the circuit de;dHBi! fri ;ectldh JitJ i;cept thai::-ifJ \ i.J: used as phase

detectors. 7

Two delay line discriminators are used, with delay lines whose lengths are

related by a factor of 8. 0. The sine and cosine outputs from each discrimin­

ator are processed by the Video Processor and Controller unit to determine

the frequency offset of the input signal within the 500 MHz IF bandwidth of

the IFM.

• ___________ -_20_-_______ U.Ju

San Jose, California

VIDEO PROCESSOR AND CONTROLLER UNIT

The Video Processor and Controller unit contains video circuitry and analog

to digital converters to process the signals generated by the narrowband

super heterodyne tuners and the tunable IFM. The unit also contains multiple

single board computers for data analysis and control.

The video outputs from each of the narrowband superhet tuner channels are

input to the unit for purposes of signal detection, as shown in Figure 8.

The log video amplitude is compared to a programmable threshold voltage by

the intercept circuitry to indicate the presence of signal energy. The thres­

hold voltage is set relative to the noise floor to correspond to a specific

signal to noise ratio.

The two basic sensitivity requirements for an ESM system gf¢ determination

of the presence of a signal ( detection) and determination d( the parameters

of a signal (processing). The detection sensitivity of the \l&-ltem is deter­

mined by the capability of the threshold comparator to detect. the presence of

a weak sig~~[? at the inpllJ tf the systJ~j Thil level of sen~~"t.ivity is deter­

mined by tfi/J threshold &-iitage appliel lo th.I comparator~ !k1ative to the ::;:::::;::::;:•:

noise level at/ the signal il!it.

t PN •tGEOS ;:-.1---

INTERCEPT

CHlCU1TAY

A/0 C0f'IVERTEA5

TUNED FAEOUENCJES

INTERCEPT FAEO UE.NCIES

PflEPAOCESSOA "NU CUNlAOLLfA

FH'O MEMORY

PARALLEL SYSTEM

BU•

1/0

------1--+ TN J .!2 cor~TMl>l.

>-------1--+ T J,_ Cl.)t,ll ftDL

. R. 1135 Figure 8. Video Processor and Controller Block Diagram

-21- •

Adjustment of the threshold voltage involves trade-offs between probability

of detection and probability of false alarm. 8 False alarms are generated by

noise voltages that exceed the threshold voltage at the comparator, so the

threshold voltage cannot be set too low. Variations in the noise level relative

to the fixed threshold voltage affect the false alarm rate, further degrading

system performance. For this reason, the ANL function is incorporated in

the system, maintaining a relatively constant noise level and allowing the

lowest possible threshold voltage _ ( corresponding to the acceptable false alarm

rate) to be used.

. When a signal exceeds the intercept threshold, the tuned frequency of the

tuner is recorded. The tuned frequency is used by the controller to assign

a tuner to the task of collecting signal parameter data at lit! frequency of

intercept for purposes of analysis and identification. Inte:rg~pt frequencies

are loaded into a first in, first out (FIFO) memory, and latef, $ead by one of

the single board computers .

The single J?g~rd computi~~ used in tij~ unit ijfe Intel iS~d 386/100 32-bit

::::::·::: ·llt:·;s:0::.;Jlt .. ar;u~:::l tJl::~:uss;:tllr~:~tu:::ti:: II parallel i&~tem bus J(if~B) allows,!/ @ultip\¢1I single bo~~¢1} computers to

operate in ;1;~u~1. µ§ifil i ii d s / > t : perf: rm uHtt>:f n tions Th

result is a •.·.· .......... ·.·.·.·.·.·.·.·.·.·.·.·.· .. ... · .. · ·.·.·...... "'e/ xrapr·····r.· ... ·af .. L.• ... 1.• .•. ·~.··· .. ·.·.•• .... a····.• ... · ... ·.r ... L ......... ~ ... ·1···· .. ···:e\· 'Ua)rrcche1s' tec0tu' r>er.•.• .. ,•.• .... • ......... .............. ......

0 ... •.w•.•·.r.· .. ··h·• ...... •.•1: ... •c•·:.•.1:•fi: .. ••.• .. • .. • .. • .. ·······cua::n:::.. ubec adapt

0

ed teo Ve;y pawifffil, nM IJ

a variety of specific applications .

In the block diagram for the unit three single board computers are shown.

The first computer interfaces to the intercept circuit, acts as the system

GPIB controller and interfaces to a hardware tuner control circuit. The

tuner control circuit is state-machine driven, and allows multiple super­

heterodyne tuners to be controlled on one or more tuner control interfaces.

The single board computer loads scan limit s or tuned frequencies into the

tuner control circuit to perform the tuner allocation function. It also controls

t he demodulators and switch assembly over parallel TTL lines accessed

t hrough the I/0 board on the iPSB .

• ___ ________ -_22_-_______ UJu

San Jose, Ca liforn ia

The second computer is dedicated to data analysis. Analysis is performed on

both IFM data and collected superhet data to determine signal characteris­

tics. Analysis data is transmitted to the computer over the iPSB interface

from the interface microprocessor. The interface microprocessor is a board

developed at Watkins-Johnson Company for application-specific interfaces to

the Intel iPSB.

Selected IF and video signals are received by the unit from the switch

assembly unit. The IF and log video from a set-on superhet tuner are pro­

cessed by the unit to collect detailed pulse data for fine grain analysis. The

IF input is converted to FM video by a limiter and discriminator. The FM

video and log video are processed by the video processor Hircuitry to pro­

duce amplitude, time of arrival, pulsewidth, fine frequency, and frequency

deviation data for each received pulse. Pulse data is loag~g into a FIFO

memory for transmission to the analysis computer. :i::::/}li\iii

Inputs from the IFM Demodulator are digitized by the analoi ~o digital con­

verter (A/D): circuitry. Ai Jp.reshold cortjp!!trat<>:!MQn this boata.tdetects signal

~::::o::J~t:;;~ 8

::e jit::::: stat,1/fuacJi to initiate i/~je conversion. where some Il~ta manlpulan Is p~r:Jlr:e:l:::;a:\li:~le:::::: Output data f#>:rn Jhe p;rjt,:f-it§~mr t~Jtji£<1.ed intd i §~conq ffiJmb memory.

The status of the FIFO memories is monitored by the interface microproces­

sor. Transfer of data to the analysis CPU is performed when the FIFO

reaches a predetermined level. ·The interface microprocessor also controls the

superhet and IFM processing circuitry in response to commands from the

iPSB. The unit may be controlled remotely by incorporating a communications

processor or locally by integrating the operator interface functions into the

front panel controls.

The third single board computer on the iPSB is used for overall unit control

as well as control of the TN-122 tuner. This tuner is normally scanning at a

high rate of speed and requires more complex control than the superhets. A

dedicated computer is used for the TN-122 control for this reason.

-23-

SYSTEM PERFORMANCE

Table 4 lists the preferred order of receiver features for various applica­

tions, including ESM. The first three features on the list of preferred ESM

features -- sensitivity, instantaneous analysis bandwidth and high signal

density performance -- are provided by the hybrid ESM architecture dis­

cussed herein. The fourth item on the list, speed of signal acquisition, is

affected as much by the operation of the system and strategic utilization of

resources as it is by the actual -sy~tem design.

Speed of signal acquisition is related not only to the ..detection of the pres­

ence of signal energy, but also to the response of the system to the initial

detection. With the tunable IFM, the detection of a signal dijj~es an immedi­

ate digitization of the IFM videos. Preprocessing and analysis immediately

follow, occurring simultaneous to the scanning of the rece',iy.er. Hence, no

time is required for the receiver to return to the signal fr~§# ency prior to

performing signal measurements.

However, dil~ to the hfif scan rate t!r thtP receiver,

m6.i:; than ten : .;; fifteen conilcuti-V,~ pulses fro~t i~ single pulse receiving :=:=::::;.::;.;:;::

train is not y~ry high. jij )addition, tfj.ij} prep[tgfessor limit) }the number of

~:~::• t:1 ::!Jl;:;m;t11,;i;:~f lt:::·1!t~;:i-1ll'::::::a:~1:: train analysis diffibiiTt, and ibWJr the confide~JJ >ii~;l associated with

analysis results.

For this reason, it is advantageous to assign a set-on receiver to newly

intercepted signal frequencies to collect a sufficient number of pulses for

detailed pulse train characterization . In the system presented here, the

narrowband superhets are used as set-on receivers when new signals are

intercepted by the IFM or a scanning superhet. Outputs from the IFM data.

analysis are used by the controller to maintain a file of emitters in the

environment. Each time a new emitter is encountered, the controller allocates

a superhet tuner to the task of collecting signal data on that emitter. The

results of the superhet data analysis are used to improve the information in

the file of intercepted emitters.

• ___________ -_24_-_______ UJu

San Jose, Cal if ornia

Table 4. Preferred Order of RF Receiver Features for Indicated Applications (1-10, higher is better)

General ESM ECM RWR Airborne Ground-Bued

Speed or Speed or Simultaneous Instantaneous Speed of

Sianal 8.7 Sensitivity 8.1 Signal 9.1 Signal 10 Analysis 8.1 Siana! Ac:quisition Acquisition Capability Bandwidth Acquisition

Hi&h Signal Instantaneous Simultaneous High Signal High Signal Hi&h Siana! Density 8.4 Analysis 8.1 Signal 8.9 Density 9 Density 7.9 Density

Performance Bandwidth Capability Performance Performance Performance

Instantaneous High Signal Instantaneous Applicability Speed of Analysis 8.3 Density 8.1 Analysis 8.7 to Exotic 9 Signal 7.8 Sensitivity Bandwidth Performance Bandwidth Signas Acquisition

Speed of High Signal Simultaneous

9.4

9.0

8.9

Sensitivity 8.1 Signal 8.0 Density 8.6 Cost 9 Signal 7.7 Dynamic Ranae 8.4 Acquisition Performance Capability

Simultaneous Frequently Immunity Instantaneous Signal 7.9 Resolution 7.1 Sensitivity 8.1 From 8.5 Sensitivity 7.3 Analysis Capability Jamming Bandwidth 84

Frequency 7.4 Re ten lion of Speed of Short Pulse Frequency Resolution Dynamic Range 6.9 Signal 7.9 Signal 8 Width 6.7 Resolution 8.0

Characteristics Acquisition Capability

Simultaneous Short Pulse Frequency Simultaneous Dynamic Range 7.3 Signal 6.6 Dynamic Range 7. 7 Width 8 Resolution 6.6 Sisnal 8.0

Capability Capability Capability

Shon Pulse Short Pulse Frequency Processing Immunity Retention of Width 6.5 Width 6.1 Resolution 7.4 Complexity 7 From 6.4 Siana! 7.1 Capability Capability Jamming Characteristic

Applicability Processing Immunit>· Processing Shon Pulse To Exotic 6.5 Complexity 6.0 From 7.1 Size 7 Complexity 6.3 Width 6.4 Signals Jamming Capability

Retention of Applicability Short Pulse lnstan taneous Applicability Siana! 6.5 To Exotic 5.6 Width 6.9 Analysis 6 Dynamic Range 6.2 to Exotic 6.3 Characteristics Signals Capabiliy Bandwidth Sianals

Immunity Power Applicability Applicability Immunity From 6.1 Requirements 5.3 to Exotic 6.4 Sensitivity 6 to Exotic 6.1 From 5.6 Jamming

~

Signals Signals Jammina

Processina 6.1 Retention of Processing Frequency Processina Complexity Signal S.3 Complexity .. 5.9 Resolution 6 Size . S.8 Complexity 5.5

Characteristics . ~

Immunity Retention of Cost 4.8 From 4.9 Cost 4.6 Dynamic Range Signal 5.7 Cost 4.1

Jammina ·,. Characteristics

Power 4.5 Size 4.9 Power 3.4 Power 6 Power 5.4 Power 3.3 Requirement! Requirements Requirements Requirements Requirements

Retention of Size 4.5 Cost 4.5 Size 3.4 Signal 4 Cost 5.3 Size 2.6

Characteristics

-25- •

For the superhet to complete the collection task, it must first be directed to

the frequency of intercept. The time required for the system to automatically

respond to a new intercept and for the set-on receiver to get to the signal

frequency can be considered a part of the total acquisition time. Minimizing

this response time ensures identification of threats and signals of short

duration.

Because the superhet tuners are also used to scan the environment for

acquisition purposes, the same problem exists for handing off intercepts from

a scanning superhet to a set-on tuner. It would be possible for the superhet . that initially intercepted the signal to stop its scan, !eturn to the vicinity of

the intercept and attempt to center its passband on the sigp~l. However, it

has been found to be more efficient for the scanning tune¥ >to continue its

scan while the intercept circuitry and computers responc:t .t>Y directing a

second tuner to the frequency of intercept. In this manner ;U~can strategies

that are being executed by the system cont~nue uninterll i5ted, and the

tuner control complexity is reduced.

For the halli~ff process ~i be successiif, it ~~ important t!I~ the recorded ·.·.·.·.·.··.·.·.·. ·.·.·.·•·•·••·•·• ·.·. ·.·.·.·. ht is equal~; important that

::/::

0:::::::u:::~ :1111::~::t t:e Jl::::li~d repeateflt return to a

specific fre~a;n%Y/ F;;-p9#f ~tj\;g<1ug~4 lt}the timii A{i11t~;,1t r or during the

set-on proces; 1 ift¢t~i{ii7th~ @&~tim ¥t;ponse tirri~] wit~ i~/ due to the fact

that signals must be close to the center of the set-on tuner's passband

before collection of data. In dense environments where high selectivity is

required narrow bandwidths are frequently used. To return to a frequency

of intercept using a narrowband set-on tuner requires a high degree of

accuracy in recorded intercept frequencies. The set-on tuner must similarly

be able to precisely tune to the recorded frequency of intercept.

To optimize the acquisition process, Watkins-Johnson has developed automat­

ed calibration procedures that correct for the error sources involved in the

intercept process. In addition, the use of synchronized set-on tuners en­

sures that tuners can be accurately and repeatedly directed to frequencies

-------------26--_______ U)J San Jose , California

of intercept. Also, the conditions under which the receivers are scanned are

controlled to eliminate as many of the variables that influence frequency

accuracy as possible. The combination of these approaches to the problem of

minimizing acquisition time results in exceptional system performance.

The factors that affect receiver tuning accuracy shall be discussed below, in

order to understand the effectiveness of the approaches described above.

Each of these factors has been addressed, either through calibration or

control techniques in system designs at Watkins-Johnson.

YIG DHVICHS

Microwave signal sources are required in tuners for the gigtration of local

oscillator signals. Fixed frequency, phase locked oscillators are sometimes

applied, as in the TN-122 dual conversion architecture, b1.1t?tunable oscilla­

tors are required to provide variable frequency coverage] \i!\\ l4any types of

tunable oscillators have been used for super heterodyne tib¢rs, including

var actor-tuned oscillators, voltage-controlled oscillators, ijjid YI G-tuned

oscillators. A~ in the TN fits and TN-1ii ! tunJiil the YI G-ttiri~d oscillator is

frequently ti§kd as the )/!lnable local} dscmi£Bt because !6.£ its high Q, ·=<··-:-:.·-:-·.

construe-multi-octave]@requency tjp;yirage, tion. 9 ......... //

YIG crystals a,;' U/\Il/f;s the 1111111!:~:ent b§ciillators and filters.

The behavior of the YIG material in resonant circuits affects the performance

of both types of devices. The f?SCillator, however, directly contributes to

tuning errors in receivers, which limit the accuracy of recorded intercept

frequencies.

The most commonly used geometry of the YIG crystal is the sphere. The

sphere is easily produced from the YI G material and can be easily oriented

in a magnetic field. The interaction between the YIG sphere and applied RF

magnetic fields creates a resonance phenomenon whose frequency is deter­

mined by the DC magnetic field in which the sphere is submersed. lo The

RF magnetic field and the DC bias field must be orthogonal during coupling,

-27-,__ _____________________ UJ •

however. To produce this relationship between the fields, the sphere is

placed between the poles of an electromagnet as shown in Figure 9. The

sphere is bonded to a rod which allows rotation of the sphere within the

magnetic field. Shown in the figure are the series wound coils and coupling

loops.

The YIG tuning magnet is a simple electromagnet with a single air gap. The

current through the coils ci:eates the required magnetic field across the gap.

The gap region is critical to the performance of the oscillator. Variations in

the magnetic field across the YIG sph_ere must be precisely controlled to .. avoid uncertainty in the resonant frequency of the oscillator.

87.R.1137

Figure 9 . Magnetic St ructure of a YIG Device

-28-________________________ UJ San Jose, Cali f

•

A high permeability material is used for the core of the electromagnet and

serves as a flux conductor. The properties of the core material's conductivi­

ty and hysteresis are important to the tuning characteristics of the oscilla­

tor. The conductivity of the material affects the dynamic response of the

oscillator by allowing the flow of eddy currents in the pole pieces and RF

circuit. The eddy currents cause sweep delays as shown in Figure 10. The

hysteresis effects of the material result in frequency differences obtained

from the oscillator when tuning to a particular current value from both the

low and high ends of the oscillator's range, as indicated in Figure 11. The

thermal expansion of the core material causes a drift in the tuning

characteristics of the oscillator over temperature. Also, the resonant fre­

quency of the sphere itself depends on temperature.

Frequently a second coil is wound on the electromagnet

modulation of the oscillator output, or to allow use of the

chronized application. When used in a fixed tuning mode

er, the frequency output is extremely stable and accurate.

in a syn­

a synchroniz-

1- - --- - - - • - - - - - I

~ u T-------->- ~;';'i ~e:;t5 SWEEP ~ ~o.. ~~~ DELAY LLJ Vlt::j >-V)Ct:: :::, s: o I 8 V'l -- ...,

8: j _____ I

I I I I

I I I

I I I I I CURRENT 1 1-<--SWEEP RANGE-I

87.R.1138 TUNING COi L CURRENT

Figure 10 . Dynamic response of a YIG-tuned Oscillator

N

~ 61----

;_ DUETfRESIS .- ~ I HYST~

a:: LL.

87.R.1140

/

MIDBAND CURRENT

1MIN 1MAX TUNING CURRENT

Figure 11. Hystersis

-29-_____________________ UJ •

However, the synchronizer cannot be used while scanning the receiver, so

the dynamic performance of the open-loop YIG oscillator determines the

system's signal acquisition performance.

DYNAMIC TUNING CHARACTERISTICS

Both long and short term dynamic tuning effects are exhibited by open- loop

YIG oscillators. Both types of effects can be seen after a single step change

in tuning current. Short term effects are noted during the first few

microseconds and _can be attributed primarily to eddy currents flowing in the

core. Figure 12 shows the typical short term response. of a YIG oscillator to

a step change in current. The YIG output frequency does not change

instantaneously with the current. Rather, the output changf}approximates a

simple exponential curve with a time constant of between 20 and 200 micro­

seconds .11

Long term effects can be attributed to self-he·ating of the !1\¥i G itself, and

manifest themselves in thg .Jorm of a slC>\V frequency drift. :&fie drift is due

to the chant§~in temperai§#e dth1·sast1·poactceud•.•.r .. ,i ,'.•, •.•.• , •.• ,•.:,s, .•. •,~.•.·.·.········Yafttehr.•.•e•.•.•.•.•.•,.•, ..• , .•..•• , .• ,.a, •. c •.•. ucrhraenngteflionw •.•.•. 1,1,11.·.··t=." .•..•.•• ·.tl .•.•• :gnitnhgrocuugrhretnhte. Large amoul~~ of power l~e V /> ::,,}\

coils in the ) t"!G. Changej in coil currmn,~ cht]Wf the interj![} dissipation of

the YIG, a}t.ijpjng the ,f.imp~rature gf} the c~fit sphere ,,inj walls of the

structure. Tfii/ ~~P.rnM~#i~~f~ i~!PJ~§ iJiii.t take ':titi, :A9JJii¥ YI G occur over

several seconds 'cjf J:,t:;n min~fJi Kft~r the chang; fif hbii current. This is

due to the substantial thermal mass of the device.

Under repetitive scanning conditions the YIG device will eventually reach a

quiescent temperature point. This temperature is approximately equal to the

temperature that would be established if the YIG were parked at the mid­

point of the scan for several minutes. Once the quiescent temperature ,is

achieved, self-heating effects contribute little to tuning inaccuracies in the

receiver. If the oscillator is tuned to a single frequency for long periods of

time, the oscillator temperature will stabilize at a different point, depending

on the frequency relative to the scan range. When the tuner is subsequently

• _____________ 30_-_______ wu

San Jose, Californ ia

scanned, a finite amount of time will be required to reestablish the quiescent

scanning temperature. As the temperature changes, the frequency will drift.

Changes in ambient temperature also affect YIG devices. Several techniques

have been implemented to minimize the effects of ambient temperature chang­

es, including the use of heater blankets and temperature compensation

circuits . 12 Neither technique is practical in many systems, so other ap­

proaches to the problem have been developed. The approach that will be

described for this system is the use of calibration techniques that can be

quickly and automatically repeated when significant changes in ambient

temperature are experienced.

OSCILLATOR PERFORMANCE

The performance of a YIG oscillator can be specified quantj\~tively under a

combination of static and dynamic conditions. The first pll~ eter used to

specify YIG performance is overall linearity. This param:tit- is measured

under static conditions (after all long term tuning effects l~ve stabilized)

and can be [~1otted as otjltiut frequen¢M ver$tj$ tuning cu##int. A typical

::·~;:• f:)tt• b::;~;,tlll:::· u~.· )tt,.:i:lit:::·~::Y il~:.t;:~::~ NonlinearitieI~t for the YI Q 6$dllators u.$ijd in thi LTN-218 aJl! i¾N-122 tuners

are as great as :~+§ Miii: > <<< ····

t

Ftu

Response of filter center frequency to

a current step at time t = t0 . F 1 is the

filter frequency for a de coil current I 1 and F2 is the filter frequency for a de

current 12 .

Figure 12. YIG Response to Step Change in Input Current

-31-

N I 2 ' - >-u ~ :::, 0 u.J

"" LL

87.R.1139

1

L BEST LINE THROUGH DATA POINTS TO MINIMIZE 6f

TUN! NG CURRENT

Figure 13 . Output Linearity of a YIG Device

r------------------------UJ

•

Hysteresis is defined as the "maximum frequency difference obtained when

tuning to a particular current value from both the low end and the high end

current extremes. 1113 The hysteresis is measured at midband by cycling the

oscillator across its band and approaching the frequency from both direc­

tions. The frequency difference is a measure of the hysteresis. Hysteresis

values can be as high as 15 MHz.

Sweep delay is a measure ;>f the lag between the output frequency of the

oscillator under dynamic conditions and the applied tuning current. It is

difficult to measure sweep delay, alth~ugh measurements have been per-~

formed. Results are shown by plotting deviation versus desired frequency

for various sweep rates. Sweep delay is a function of §weep rate, and

typically remains constant across the band or increases as the oscillator falls

farther behind the tuning current toward the end of the sw~ep. Deviations

remain fairly constant (20 to 40 MHz) across the band wheri\\: irioderate sweep :;:;:::::;:::::;::::

rates are used (less than 40 GHz per second) . 14

linearities iri ltn.e YIG :•.t·_r_:_::•o_•.::e.•_•.-.•. ·_s_ pd<{)y:·-····-n_:._:n•_•_••.•_:_s ___ :a•_•_ •_e_._•m·_•_·:·_.·:·' __ l·_. ·: C_::._--•. s._-._•._._-.. _·.w.-._· -.t __ · __ •._·ue_·_:..:_•_•_e_-.... _•_• _•_·_•_r _•_;:._-_ •.. _.n/gdelayJt{P:,ysteresi~ :ijijd temperature effects all ccirifflp:qJ~> U1 errors !~ .iii~ \~eceiver, Without

calibration or c~~Jl~tJrit contr~f tg&hiii~ues the un~;~tJfi.fi~ in tuned frequen­

cy during scanning operation can be as great as 50 MHz. This level of

performance is unacceptable for rapid signal acquisition and identification.

Techniques can be applied to overcome the problems associated with scanning

receivers, however. To overcome the errors introduced by hysteresis effects

tuners are commonly scanned in only one direction. Typically, scan strate­

gies are implemented that cover a range of frequencies starting with the

lowest frequency and moving upward to the end of the band. When a tuner

reaches the end of its scan the controller commands the tuner to retrace to

the low end of the band. During the retrace cycle, which usually lasts the

'

I

------------32-_______ UJJ San Jose , Calif -

L

1: 1, ,

1: 1,

approximately 20 milliseconds, the microprocessor applies a hysteresis pulse

to the YIG which drives the oscillator well below the bottom of the band.

The YIG then approaches the lowest scan frequency from below to return the

core to the proper point on the hysteresis curve. The techniques of always

scanning the tuner in the same direction and incorporating automatic

hysteresis recovery responses into the tuner itself are commonly used and

effectively eliminate the contribution of hysteresis to receiver tuning errors.

Non-linearities and sweep delay errors can partially be overcome by YIG

driver design. Break points are frequently incorporated in YIG driver

transfer functions to compensate for gross nonlinearities in the device.

Sweep delay, however, is difficult to address for all tuning conditions.

If the step size taken between frequency points during a .scan is large,

drivers can be designed to accelerate the YIG response by): ipplying excess

tuning current during the first few microseconds of the st~~I This is done

to overcome the eddy currents in the device and effecti#~li shorten the

delay time liit~een tuning- ~urrent and tµp.ed {#gquency. Ttl; tUning current

is allowed tdf1eturn to va1Ji \&s u~; YIG approa#lj~s the desired ::;::::=::::::::::

:::·~::~ :;;::~jgq i$ tllgg, JPe ,1, dJi,~~tpµt ~lfnt appears to

overshoot the J&J1i~a v!1ue. ¥H& IM&Gnt or ove;~s6&i i~Jd is a function of

step size. The interaction between eddy currents and tuning currents is

complex, and can only be optimized for a specific step size through careful . adjustment. Adjusting the circuit for the proper amount of overshoot for all

step sizes is virtually impossible. However, if a constant step size is used

when scanning the receiver the technique can~ be used to optimize the YIG

driver for the specific application. The contribution of sweep delay to dy­

namic tuning error can then be reduced.

-33-,...... _____________________ UJ •

CALIBRATION TECHNIQUES

In this particular system, the tuner control software and hardware are

designed to step-scan the TN-122 and TN-218 tuners in a consistent manner

to minimize the tuning errors described above. The tuners always take the

identical step size, which allows optimization of the YIG driver circuits.

The TN-122, which scans at a maximum rate of 40 GHz per second, takes a

step size of 4 MHz. The TN-218 tuners have a maximum scan rate of 16 GHz

per second and step 1. 6 MHz ~ during scans. The tuners receive frequency

commands every -100 microseconds, minimum. Adjustment of the scan rate is

accomplished by varying the time between steps.

Using a constant step size allows optimization of the YIG Ca.rivers in the

tuners for the particular application. Sweep delays are therefore minimized,

but not eliminated. Hysteresis effects are minimized by alw~~ij scanning the

tuners from the low end to the high end of the band. ~k~~inearities and

temperature effects can only be addressed through calibratioti techniques.

Calibration ti¢hniques iajpl¢mented in ttte slijj¢m design i#i intended to

remove the £.~rnaining tunifig, errors. Tij~/ tecljiji~ues that vJ;.11 be described

uansed sci gonntarlsoll~e···· e.·,·~·;···.•·,n, •.•. ·,., ::·.::::e.•.:• .. ,r. na1'tte.d Cwo1.·nt .. ·.• n .. t••••.t·n:··~.•.·.•·: .• :1•••.:.::::::t:···: .. hofe tshyestem ! Ila d~tiltors in the\:)#ideo Processor .. : :.., """' g~Htfratiori :P:~9.cess if?:( ~~~rcised by the

Video Proces~dii lha :c!Ht;~ii~~ intt itt~ell, andf ijiic•t1ij:[¢6ffi;1etely automat-'·"<<:\:::=:::::=::;::::::-:.:-·,· .· . ···<:::::::::;:::::;:;::::::::::>:···· . ··: -:: ::::::::::::::::<.::::::::::::-,•.

ed. Execution time of the calibration procedures is approximately 2 minutes

for the tunable IFM and 1 minute per superheterodyne tuner channel, which

allows the process to be initiated as frequently as necessary without signifi­

cant operational impact.

TUNABLE IFM CALIBRATION

The tunable IFM calibration takes place in two stages. The first stage is -

referred to as the Static-tuned IFM Discriminator Calibration. The second

stage is the Dynamic Tuning Calibration. The purpose of the first stage is

-34- •

-------------~u San Jose. Ca liforn ia

to remove the nonlinearities associated with the discriminators in the IFM

Demodulator and the translation circuitry in the Video Processor and Control

unit. The translation circuitry is part of the preprocessor, and is required

to correlate the coarse and fine discriminator outputs and resolve ambiguities

in measured frequencies.

To begin stage 1, the controller commands the RF Distribution (RFD) to

connect the aux IF signal to the input of the first TN-218. The aux IF input

to the RFD is actually the LO sample from the TN-122. The controller also

connects the IF and video outputs of the first TN-218 to its processing

inputs. This allows the controller to measure the LO frequen<?y of the

TN-122.

The controller commands the RFD to activate the comb gene;.~lpr and discon­

nect the antennas via the input switch unit. The comb gJijlj,ator produces

signals spaced every 100 MHz which are routed to the inpui §f the TN-122.

The narrowband filter is switched into the comb generatorii iutput so that

only one of /jle comb fre.jp:~ncies is p«$.$¢d tdl the TN-122.1Jbis signal, or

picket, is l®Ated at 3 Glti/n:

The TN-122 {ij tuned frqf t 75 GHz ~lil.25111 in 10 Mijlf !Lps. At each

step the coJti-iitw; m2ntti¥ii: lhg ¼P :f#~quenc}I:liiin: th~ ,W:!Jif::1.;2 through the

first TN-218 c~iririil( The t~ri~a t+i4Gency of th; iw:~~ri8h'~onized TN-122 is

adjusted until the LO frequency is correct. When the TN-122 is properly

positioned, the controller records the frequency offset of the picket as . measured by the IFM Demodulator · and preprocessor.

The differences between the measured and actual frequency offsets at each

point are used to build a calibration table for the discriminator. The calibra­

tion table corrects for the nonlinearities in the discriminator/measurement

chain. After the calibration table is constructed by the computer, it is

loaded into memory on the preprocessor board to automatically correct fre­

quency data during operation.

• -35- • I

~---------------~U San .la2.

Once the discriminator has been calibrated, the remaining sources of error in

IFM frequency measurements are primarily the dynamic tuning characteristics

of the TN-122. To remove the tuning errors, the Dynamic Tuning Calibration

stage is performed. This involves sweeping the tuner across its entire

tuning range and recording the results of frequency measurements of the

comb frequencies from the processing chain.

To perform the Dynamic Tuning .· Calibration, the controller commands the

RFD to remove the filter · from the comb generator output, allowing all comb

frequencies -to reach the tuner. The controller does, however, place the IF i

Demodulator in its narrowband mode; so that only one comb frequency is

within the IFM's passband at any given time. The TN-122 is1Jpen placed into

a scan mode from O. 5 to 18 GHz.

The controller enables normal processing of the IFM data ihf:ough at least

one complete scan. The A/D converters in the Video Procesidjj and Control-

\~:[ f ;:~f 1iiJ:\I~;t~i~:1:~?f 1:~)i~::i~:1::Jl~if ~::~~:~ quency of t~~ receiver' .iwnich was latrihid at: \ll~ie instant )~;, signal broke

threshold. Thij PH!P~t jitij lngi.~':.lr•!{ J,e measq!W:f, . f:~9Jtip<iy of the comb

signal.

Since the comb frequencies are known by the controller to great precision ,

the differences between actual and measured frequencies can be attributed to

tuning errors in the receiver. These differences are used to construct a

tuning calibration table which adjusts the digital tuning word that is sent to

the tuner. The control software accesses the table at each frequency step

during scanning to ensure that the tuner follows the correct tuning curve

during its scan.

-36-

SUPRRHETRRODYNR TUNING CALIBRATION

The bank of superheterodyne TN-218 tuners are used to search for signals

in the environment in parallel with the IFM. Intercepted signals cause the

controller to assign an available TN-218 to the task of collecting analysis

data. The handoff process is limited by the accuracy with which the inter­

cept mechanism can record frequencies of intercept from a scanning TN-218.

The intercept mechanism involves monitoring of log video levels with thresh­

old comparator circuitry. Threshold breaks cause the circuitry to latch the

digital tuning word that is currently applied to the scanning tuner. The

tuning word is saved in a FIFO memory for later access by the control

software.

The point at which the signal breaks t hreshold is dependent on IF band­

width , threshold level, signal strength , scan rate , scan djr,~Gtion and step

size. To minimize the variables, the TN -218 tuners are scanh~d in a constant

direction, with a constant step size, usually with minimu~ threshold, and

generally in the 20 MHz bandwidth . If a standard scan Jij\ti is used, the

point at whfoli the comb $$;nals break t}tresh¢}ij is relativJii constant from

;~:.;::;;~it::~ :,~i11~.::;~,,1:n .r!l~n::1:JII:::: :::·1:::

TN-lOO's in a ilitihriif ~imil;f iB th; Dynamic ttiriirlf c;libration of the

tunable IFM becomes very effective.

The Super heterodyne Tuning Calibration is performed as follows. The con­

troller connects the comb generator to t!1e inpu~ of the funer channel. The

filter is removed from the output .of the comb generator to allow all comb

frequencies to reach the tuner. The 20 ~Hz bandwidth is selected in the

MD-100 Demodulator. The tuner is then placed in a scanning mode to cover

the 2 to 18 GHz range.

-37- •

As the tuner encounters each of the comb signals, the intercept circuitry

latches the frequency of intercept as it would in normal operation. The

controller reads the frequencies of intercept from the FIFO. The differences

between actual frequencies and latched frequencies of intercept are calculat­

ed, and are used to create a correction table for the tuner.

During operation, the table is accessed by the control software to correct

each frequency of intercept. When an available tuner is directed to the

signal frequency the corrected frequency of intercept is used. The set-on

tuner's synchronizer is activated to ensure that the IF passband is centered

on the signal.

CALIBRATED PERFORMANCE

Performing the automated calibration procedures described ap:pye significantly

improves the ability of the system to return to intercepted $~liial frequencies

with narrowband receivers. Without calibration or properly :¢on trolled scan

conditions t he errors introduced in the frequency measuretjj~ilt process can

be as greaf ii 50 MHz. <Pi~trolling thJ ijcan #lte and step ~fze, optimizing

the YIG dr~mJ rs, and uii#g hysteresii /recol~#y during J~~#ace eliminates

approximatefy \ 20 MHz of thi overall err&f:} \

The · _>i < > ' ,•,,.,,.'0.•.•.•.:.,.•,...•.•.•.' .•. ·.·,•e1 ....... '•'a''.'.'.•.·.,d·.•.•,'.'.•.• ~.'.·.· .... '•,:r· .. ·.·.·.,e ... ·.·.s ... · .. s .... ·e .. ·:.,·.·.·d .. ··· ... ·.•·· .····.•.','.•.,.•.', •. ',~,• .• ,' ...•. •.•.iy•. /f the ••t•••••u1 .• n1, •. :,.,·1.•,•.•,·n ... •,·'.,•,·.·,• .. •, .. g'·.'. ·.·,',.',.,.,,·,. lid t t<} procedures . remammg ;js~[~ ..... , u u ca . 1;1:fa ion The resultant system p;rfor~Jrid~j iifter execution' 6ft•tw: / ~alibration proce-

dures is better than 5 MHz, RMS. This allows synchronized receivers with 20

MHz bandwidths to return to the intercept frequency without further center­

ing. The reduction in total acquisition time ensures rapid identification of

threats and signals of short duration.

Performance improvements of the magnitude achieved here are possible in

many applications. The techniques require little in the way of additional

hardware, so the recurring cost impacts are minimal. The results realized

through careful consideration of the actual sources of system errors, and

implementation of control algorithms and calibration procedures to eliminate

•

-------------38--_______ UJu San Jose, California ---

those sources of error, required only the integration of distributed proces­

sors. However, these processors are already being incorporated into many

current system designs, as the availability of inexpensive microprocessors

and powerful single board computers improves. Such processing power

provides the intelligence to automatically perform the required control and

calibration functions.

1.

2.

3.

4.

5.

6.

7.

8.

9.

CITED REFERENCES

L. W. Lemley and R. F. Gleason, ed. , RF Receivers for Electronic Warfare, Report 8737, Naval Research Laboratory, Washington, D.C., 1985, p. 37.

Ibid, p. 37.

Ibid, p. 6.

Ibid, p. 7.

Comal Tech-Notes, "YIG-tuned Bulk GaAs Oscillators, 11 Watkins-Johson

~:~ki~:;,~;.~:~ cL;;;:;;~:~i~iJff :! llle~te5~ ~:·It~~~. -oct.

~:;Ldi~: ~!!~~~·-(~~a~~ ~~df t .. ~~-b~:;f ~hnsl (Company

Lemley, 6~Ii~f!•pJ$;.z~~ji········}•••••••••••••···••·••••·•·•••••••U>••••••••·•

C. E. Foster, II and T. G. Simon, "Microwave Frequency Synthesizers, 11

Watkins-Johnson Company Tech-Notes, vol. 2, no. 3, p. 6 (May-June 1975).

10. Ref. 5, Op Cit, p. 3-4.

11. K. D. Gilbert, "Dynamic Tuning_ Characteristics of YIG Devices," Application Note DCN 100343, originated at Watkins-Johnson Company, reprinted in Microwave Journal, June 1970, p. 3.

12. Papp, Op Cit, p. 4.

13. Ref. 5, Op Cit, p. 9.

14. Gilbert, Op Cit, p. 4.

• -------------39--________ LUtJ