What makes a DX receiver great?

Understanding receiver specs

John Eisenberg K6YP

Agenda

• Introduction• Receiver fundamentals• Sensitivity• Linearity• Dynamic Range and AGC Function• Selectivity• Stability• Conclusion

Introduction

• If you can’t hear him, you can’t work him!

• Hearing him depends on:– Is he on?– Is there decent propagation?– Do you have enough antenna?– How much QRM/QRN is present?– The performance of your receiver.

• Today’s talk will focus on receiver performance.

What are you up against?

• Weak signals (CW or SSB or a digital mode)

• Atmospheric and man made noise (QRN)

• Interfering signals (QRM)– Strong signals adjacent to your frequency – Strong signals far removed in frequency

• Fast or slow fading (QSB)

What are your weapons

• Key receiver performance factors– Sensitivity (Weak signal reception)– Selectivity (Bandwidth matched to signal, Ability to

reject adjacent QRM)– Optimum detector for desired signal modulation type– Linearity (Spurious free dynamic range)– Blocking dynamic range (From strong adjacent signals)– Stability (Keep the signal in the pass band)

What this talk will address

• Key Receiver Specifications– What are they?– Why each is important?

• How to read a QST product review.

• I will not address the pros and cons of specific receiver architectures.

Receiver fundamentals

• What must a receiver do?

– Amplify a weak signal delivered to the receiver by the antenna.

– Filter out undesired interfering signals and noise .

– Detect the desired signal, extract its intelligence and present the content in a useful format.

Receiver fundamentals

• What must a receiver not do?– Add additional excess noise to the received

signal (Degrade SNR)

– Generate additional spurious signals or mixer images which corrupt the detection process

– Drift off the desired signal frequency

Simple super hetrodyne receiver

Image RejectFilter

IF RoofingFilter

DetectorRF Amp

IFAmp

IF Pre- Detect Signal Filter

BBAmp

AGCSystem

Pre-SelectFilter

Local Oscillator

Mixer

AGC Line

Antenna

dB’s and dBm’s

• Power ratio in dB = 10log(P2/P1)

• Gain in dB = 10log(Pout/Pin)– 3 dB is a factor of 2, 6 dB is a factor of 4– 10 dB is a factor of 10, 20 dB is a factor of 100– 39 dB is a factor of 2x2x2x10x10x10 = 8000– 39 dB is 3+3+3+10+10+10 dB

• 0 dBm is 1 milliwatt– Thus +13 dBm is 20 mW, -9 dBm is 1/8 mW

Noise power

• Noise is distributed over frequency.• Noise Power is measured “per unit bandwidth”• Example: A noise signal has a uniform power spectral density of -

120 dBm/Hz.

• Noise power increases by 10log(Bandwidth in Hz)

Bandwidth Total Noise Power 1 Hz -120 dBm 10 Hz -110 dBm 100 Hz -100 dBm 1 MHz -60 dBm

Freq

PSDdBm/Hz

1 Hz UniformNoise PSD

Receiver sensitivity

• Noise Figure =

• Noise figure = Input S/N (dB) - Output S/N (dB)

Signal/Noise ratio at RX OutputSignal/Noise ratio at RX Input

Device with NF = 10 dB

Input S/N = 40 dB Output S/N = 30 dB

Best possible receiver sensitivity

• The noise power from a resistor at 25°C (or a matched antenna in signal free environment) is kTB (Boltzmann’s Constant (k) x Temp (°K) x Bandwidth (Hz).

• kTB = -174dBm/Hz This is the noise floor of a noise free receiver at 27 °C .

• kTB = 3.98 x 10 watts/Hz at 27°C or about 0.01 V in a 500 Hz bandwidth.

-21

Minimum detectable signal

• Noise Floor = KTB + NF + 10log(BW in Hz)

• MDS = KTB + NF + 10log(BW in Hz) + 8 dB

• Maybe for OH2BH, MDS = Noise Floor + 5 dB (The 8 dB factor is subjective !)

• Often other problems such as reciprocal mixing further degrade MDS

MDS for CW and SSB signals

CW Filter 500 Hz BW (27dB)

SSB Filter 3 KHz BW (35dB)

Noise Floor = -174 dBm/Hz + 5dB NF +10log(BW)

CW MDS -134 dBm

CW Noise Floor -142 dBm

SSB Noise Floor -134 dBm

SSB MDS -126 dBm

Noise Power PSD is-174dBm/Hz +5 db NFor -169 dBm/Hz

MinimumdetectableCW signal-134 dBm

MinimumDetectable SSB signal-126 dBm

The “standard”S meter

S meter reading Signal level in V Signal Level in dBm

S9 + 60 dB 50000 -13S9 + 40 dB 5000 -33S9 + 20 dB 500 -53S9 + 10 dB 158 -63S9 50 -73S8 25 -79S7 12.5 -85S5 3.13 -97S3 0.78 -109S2 0.39 -115S1 0.20 -121MDS (in a 3 KHz SSB BW) 0.195 -121.2MDS (in a 250 Hz CW BW) 0.056 -132.0

ReceiverZin = 50NF= 10 dB

1 ‘S unit’ = 6 dB

LO phase noise & reciprocal mixing

• Imagine that you are copying a weak signal and all of a sudden a very strong clean carrier pops up 100 KHz from your frequency.

• Nothing happens. It is rejected by your receiver’s battery of filters. Right????

• No! Your receivers sensitivity may be degraded by reciprocal mixing with local oscillator (LO) phase noise.

LO Phase noise

VLO = (A + Nam(t)) sin[LOt + + pn(t)] The phase noise term pn(t) usually dominates the AM noise Nam(t)

LO Spectrum with phase noise

FLO

Im(VLO)

Re(VLO)

+ pn(t)Phase

AmplitudeA + Nam(t)

dBc/Hz

1 Hz

10 kHz Offset

Phase noise is often expressed in:dBc/Hz at some carrier offset

VLO

Receiver

StrongInterferer

WeakSignal

RX RF input signals RX IF Output

IF Filter Bandwidth

Local OscillatorwithPhase Noise

Buried Weak Signal

LO phase noiseon interferer

LO phasenoise onweak signal

Interfererwith LOPhase noise

Reciprocal Mixing Process

Reciprocal mixing

-20 dBm Interferer after 1st mixer

RX IF Output

IF Filter Bandwidth

Desired Signal

RX NF = 15 dB, Gain to 1stIF filter after the mixer =10 dB

A -20 dBm strong interferer is 100 KHz from desired signal

LO phase noise = -110 dBc/Hzat 100 KHz carrier offset

RX noise floor = KTB+NF+G = -174 dBm/Hz +15 +10 dB = -149 dBm/Hz

At 100 KHz away from the -20 dBm interferer phase noise PSD is-110 dBc/Hz -20 dBm = -130 dBm/Hz

Adding noise powers in a 1 Hz bandwidth yields ~ -130 dBm/Hz. Thus theEquivalent RX NF with phase noise = 15dB + (-130 +149)dBc/Hz = 34 dB!

100 KHz

Receiver total gain• The lowest noise receiver still must have enough gain to bring the

input signal strength up to the level the detector requires to process it.

• Both signals and noise are amplified.

• Hopefully the signal is well above the noise.

• A strong interferer can (and often does) reduce total gain through saturation or AGC action

Receiver sensitivity summary

• Noise figure, predetection bandwidth and total gain ideally set receiver sensitivity.

• Predetection bandwidth and the detection process must be matched to the signal characteristics.

• Spurious signals and mixer images generated in the receiver must be suppressed

• LO phase noise in the presence of strong interfering signals can severely degrade receiver sensitivity and usually sets MDS in real world DX situations.

Receiver linearity

• Why worry about linearity?

• Strong signals close to a weak DX signal can saturate your receiver’s front end or its IF amplifiers dramatically reducing total gain.

• Pairs (or multiple) strong interferers can place unwanted intermodulation products on top of that all time new one you are trying to pull in.

• These issues compound the previously addressed reciprocal mixing problem.

Gain compression

Gain (dB)

Receiver Input Signal Level (dBm)

Small Signal Gain

SSG - 1 dB

Input Power@ 1 dB GainCompression

Linearregion

Saturationregion

Nonlinearregion

Intermodulation

• When 2 or more signals are presented to a nonlinear device, the results are harmonics of each signal and sum and difference products of the signals and their harmonics. These sum and difference products are called intermodulation products.

NonlinearDevice

F1 F2

Freq

Power

dc

F1 F2

Even Odd Odd2F1 2F2

Even

Intermodulation

• Odd order products (IM3, IM5 ....) are close to the original signals and can interfere with another weak close in signal.

• Even order products (IM2, IM4 ….) can also cause interference. Usually the receiver’s preselect filter takes care of even order products. (Unless your neighbors are W6YX and W6XX.)

Even order intermodulation

Interfering Signal Pair

F1 F2 IM product = mF1 ± nF2Product order is m+n (1F1+1F2), m=1 n=1, Order is 2nd

7.10 MHz 7.14 MHz

F1+F2=14.240 MHz IM2

A92BR 14.243 MHz

Receiver IF Passband

F2-F1=0.04 MHz IM2

2F2-2F1=0.08 MHz IM4

2F2+2F1=28.48 MHz IM4

W6YX W6XX

Odd order intermodulation

Interfering Signal Pair

F1 F2

F F FF F

F=F2-F1 = 14.2-14.1 =0.1 MHzIM product = mF1 ± nF2Product Order is m+n (3F1-2F2), m=3 n=2, Order is 5th

14.1 MHz 14.2 MHz

2F1-F2=14.0 MHz IM3

2F2-F1=14.3 MHz IM3

3F1-2F2= 13.9 MHz IM5

3F2-2F1 =14.4 MHzIM5

DX0K 14.303 MHz

Receiver IF Passband

W6YX W6XX

IF Output Power(dBm)

RF Input Power (dBm)

FundamentalSignals

LinearRegionSlope=1

InterceptPoint

IM3Slope=3

IM5 Slope=5

Intercept point

Estimating IM levelPower (dBm)

Frequency (kHz)

InterceptPoint (dBm)

Signal Level (dBm)

P OrderIM Level (dBm)

th

(dB)

(P-1) (dB)

Order Order Order Order Order 3 3 5 5 7

53 26 20 31 21 dB

106 52 80 124 126 dB

+40 dBm

-13 dBm

-119 dBm

Is your IP3 good enough?

• Its close in IMD performance that matters.• A great input intercept point without equally great roofing and

predetection filters is worthless!• IIP3 at 5 kHz spacing not 20 kHz counts in a pileup 756 ProIII

(20M/500Hz/No Preamp) -17/+25dBm IC7800 (20M/500Hz/No Preamp) +22/+37dBm Source: Mar. 2005 QST Product Review 756ProIII

• Don’t forget that -30 dBc IM products from a “20 over 9” perfectly clean SSB signal are > S8! So the problem isn’t always your receiver.

Spurious free dynamic rangePower (dBm)

Frequency (kHz)

InterceptPoint (dBm)

Signal Level (dBm)

P OrderIM Level (dBm)

th

=54.66 dB, 3rd Order (P=3)

(P-1)=109.33 dB, 3rd Order (P=3)

+40 dBm

-14.66 dBm

-124 dBm

Noise Floor =-174+15+10log 3000= -124 dBm

SFDR = (-124 dBm) - (-14.66 dBm) =109.33 dB (Noise floor = IM3 level)

SFDR

Receiver gain distribution

• Minimize RF gain ahead of the mixer to just enough to achieve required noise figure. Don’t overdrive the mixers thus degrading the receiver’s spurious free dynamic range. Use high IIP3 mixers.

• LO phase noise level not NF usually sets real world receiver sensitivity.

• Two conversions max! Minimize number of spurs.

• Locate the majority of gain after the roofing filter. Keep IM products out of the IF and detectors.

AGC function

• AGC reduces the gain of the receiver RF and IF amplifiers in the proper ratio to maintain sensitivity and SFDR in the face of rapidly changing signal levels (QSB).

• The analog or DSP detector suite (one for each mode) drives the AGC function. The AGC algorithm should be optimized for each mode.

AGC function

• AGC rate must adapt to the mode in use and if possible to the QSB conditions.– Fast attack to minimize pops and thumps– Adaptive decay matching signal characteristics

• AGC holds the detector input level approximately constant as receiver input signal level varies.

• Modern DSP based AGC systems can offer vastly improved capability.

Receiver selectivity

• Selectivity is determined by the final IF filter– The filter must be matched to the signal characteristics.– Crystal filters are good but they are expensive and can

suffer from ringing and delay distortion.– DSP based filters are generated in code and can be

designed for a wide variety of bandwidths, and shape factors. Thus additional filters are almost free.

– Best of all DSP filters can greatly reduce ringing.

Receiver selectivity

• An excellent receiver has at least 2 crystal roofing filters wide enough to avoid ringing, but narrow enough to reject close in interferers and IM products. For example: 6 - 10 kHz for SSB, 2 - 3 kHz for CW

• These would be followed by a choice of DSP filters optimum for various conditions. For example: 3.2, 2.8, 2.4 and 1.8 kHz for SSB, 500 and 250 Hz for CW

Receiver selectivity

• The set of DSP filters should allow for various operating conditions such as local rag chewing and intense contest or DX situations.

• DSP based filter suites should contain an adaptive notch filter to reduce CW beat notes in the IF pass band (Tuner uppers)

• A variable IF band pass filter with selectable center frequency and bandwidth can also be very useful.

Blocking dynamic range

• How large can a single CW interferer 20 KHz away from a weak signal be, before the desired signal’s detected level drops 1 dB?

• Blocking dynamic range is the difference in level between the weak and strong signals

• What happens as the interferer moves closer to the desired signal? How about many close in intereferers as in a pileup.

Blocking dynamic range

Gain toroofingfilter o/p (50dB)

Gain topre-det’nfilter o/p (100dB)

FoFo-3kHz

Fo-6kHz

Fo+3kHz

Fo+6kHz

Offset Total Signal Interfererfrom Fo Gain Level Level(kHz) (dB) (dBm) (dBm)

01234579111315

100 99 82 50 49 47-47-59-71-83-95

? ? ? -1-0.7-0.3 0 0 0 0 0

Signal -100 dBmInterferer -29 dBmIF Po-1 +20 dBm

AGCAGCAGC+20+19.3+17.7-76-88-100-112Noise

BlockingDynamicRange =71 dB(3 kHz)

Signal-100dBm

Interferer -29dBm

BDR

Receiver frequency stability

• All modern radios employ synthesized LOs.– Make sure tuning resolution meets your needs– Verify that the synthesizer reference source is

stable enough for the digital modes– A 10 ppm TCXO is often a good option to

invest in.– A 10 MHz reference output is also a useful

feature• Most important .... How’s the phase noise?

DX superhetrodyne receiver

Image RejectFilter

IF RoofingFilter

DetectorRF Amp

IFAmp

IF MatchedSignal Filter

BBAmp

AGCSystem

Pre-SelectFilter

Local Oscillator

Mixer

AGC Line

Antenna

Analog DSP

Two complete receivers with Split/Dual Watch capabilitySimple, maximum of 2 conversionsEngineered to minimize IF spuriousJust enough gain ahead of 1st mixer to set noise floorTake advantage of near perfect DSP linearityVery high input intercept pointHigh performance pre-selctor Multiple high performance matched roofing filtersStable, low phase noise DDS/DSP LO

Fast IF DSP (MHz), High resolution A/D & D/AOptimized AGC algorithms for each modeSeveral filter choices for each modeEffective auto notch and dual passband tuningAdaptive Noise reduction and noise blankerSeparate optimum detectors for each modeIntuitive, ergonomic user interface, RTTY built inStraight forward computer interface

Conclusion

• My Priorities1 Close in (5 kHz) phase noise Phase noise usually sets

receiver sensitivity, not noise figure. If you can’t hear him in the pileup, you can’t work him!

2 Close in (5 kHz) input intercept. You still can’t hear him if he is wiped out by IM3 from strong stations.

3 Close in (5 kHz) blocking dynamic range. Analysis has convinced me that long before BDR becomes an issue, reciprocal mixing has buried the new one I am trying to hear.

Conclusion

• Rigs with great dual receivers, terrific specs and good bang for the buck are very important but .......

• Don’t neglect front panel ergonomics, an intuitive user interface and well thought out menus and control functions.

• You will most likely using this radio for many years. Get the rig that is right for you!

• Thanks for coming. See you in the pileups!