A SAW 330 mz-ioops CHIRP GENERATOR*

M. CHOMlKI - F. GENAUZEAU

THOMSON-CSF/DASM - DGpartement des

BP 38 - chemin des CrStes - Sophia

Abstract

A 330 M H z - I O O p s FM linear chirp generator W ~ S

designed with SAW dispersive delay lines for a ra- dar altimeter application. This study was sponsored by European Space .\gency. The B x T product was obtained L y cascading two "RAC"-dispersive dela:) lines with B x T product: of 3300 and 13700 respectively ar? by doubling the frequency of the inpulse response O F the filters. The main results are : Input : TTL ':rigger

Output : 0 dDm peak Amplitude variation : 1 dB peak, phase error 1O"rrIs S/N : 20 dB, pulse to pulse jitter : 50 ps.

1. Introduction

The purpose of the study described here, was to demonstrate the feasibility of a linear FM chir? generator of 100,J.L~ with a bandwidth of 330 MHz. The application was a high resolution ( I O cm) radar altimeter on-board a satellite with lowtransmimssion power. The study was sponsored by European Space Agency for EXS--l nrogram.

To obtain a good reliability we have looked after a solution using a minimum number of parts to achieve the function.

The chirp generator was designed with SAW dis- persive delay lines by cascading two filters (one with B x T product of 3300 and one with B x T nro- duct of 3200) and a frequency multiplication by 2 of the spectrum.These filters are designed with the standard RAC technology using etched grooves on Lithium Niobate. They are a good illustration o f the capabilities of the technology for achievinf very large B x T products with very good amplitude and phase characteristics.

2. Design of the chirp generator

2 .1 . Generals

The main specifications of the chirp genera-

- time dispersion : 100 ,Us - spectrum bandwidth : 330 MHz - linear frequency modulation

tor are. :

Techniques Acoustiques SpGcialisGes

Antipolis - 06561 VALBONNE - FRANCE

- down chirp - input signal : TTL Trigger - frequency repetition : 1 to 4 kHz - output signal

. peak power : 0 dBm

. amplitude variation : < 1 dB peak

. phase error : ( 10" rms

. signal/noise ratio : > 20 dB

. frequency spurious : ,< - 20 dB

2.2. Discussion on the choice of RAC technology

To achieve large relative bandwidth SAW filter it is necessary to use hizh electromechanical cou- pling material : Lithium Niobate LiNbOg. The IO0 p time dispersion cannot be obtained without folding the acoustical path which will be greater than 350 mm.

Tile B x 1' product is too his11 for interdizital dispersive filters capabilities. Reflective array with etched grooves is a best solution [ I , 21. For this application the transducers can be dispersive ones, slanted [3] or phase reversal transducers (PRT) [ S I type. The slanted dispersive transducers would have been adequate to achieve directly 330HHz dispersion bandwidth at 500 MHz center frequency ; it was not yet developed at the beginning of this study [S] .

with PRT and a classical in line RAC technology.

2.3. Design of the dispersive filter

The design of the dispersive filters was made

The main specification to be taken into account in the design of the dispersivefilters is the S/N ratio. The S/N ratio can be calculated as follows : S/N = Pin - I.L. - 10 Log B x T - FxKTB Pin : impulse peak power : 20 dBm is a typical

I.L. : insertion loss of the dispersive delay line 10 Log B x T : expansion loss KTB : noise power in 330 MHz bandwidth F : noise figure of the amplifier chain : 5 dB

S/N ratio was calculated for the fc'i wing confi- gurations : @ Filters with bandwidth B and dispersion ' T

@ Filters with bandwidth B/2 and dispersion r ( T

value

typically.

* Work sponsored by ESTEC

0090-5607/83~0000-0209 $01.00 1983 IEEE 1983 ULTRASONICS SYMPOSIUM - 209

For dispersion smaller than T we cascade several filters, for bandwidth of B/2 a frequency doubling is required.

0 Filter with 330 MHz bandwidth

In this case the center frequency is around 850 MHz. For 1 0 0 ~ s time dispersion on a single filter the insertion l o s s is very large. For insertion loss consistent with the S/N ratio of 20 dB the time dispersion must be smaller than I O p,i.e.,lO filters ought to be cascaded to generate the desired time dispersion ! This solution was notchosen.

@ Filter with 165 MHz bandwidth

In this case a 2 : 1 frequency multiplication is required. The optimum center frequency is around450 MHz : - the relative bandwidth is smaller than 40 % - the frequency multiplication does not generate harmonics in the final bandwidth.

For 100,~s time dispersion on a single filter the insertion loss is too large. For 504s time dispersion the insertion loss is consistent with S / N ratio of 20 dB if the impulse peak power is around 25 dBm. But this peak power is at the 1 dB-compression point of the input trans- ducer admission. (see figure I ) .

+ I I

+6

+ I

-L

- c

- 1 s

/ A

E e: 0 L

2 E a F8 3 !x I- f3

/ P

/'" / INPUT POVER (dBm)

IO I 5 20 25 30 35

Figure 1 : Admission curve of Phase Reversal Transducers measured with a fixed delay line with two PRT

If we want to keep a 10 dB margin with respect to this point the maximum time dispersion on a single filter is around 2 O y s . The first filter of the chirp generator is then a 20 &s time dispersion one.

The residual S o p time dispersion can be done on a single filter without affecting the S/N ratio if the input signal of this filter is the 20,Us time- limited and amplified response of the first filter.

In summary we choose to get the B x T product of 33000 by cascading two dispersive filters of 165 MHz bandwidth : - the first one with 20,l.k time dispersion - the second one with 8Op.s time dispersion and by multiplying by two in the frequency domain the resulting impulse response.

2.4. Design of the chirp generator

The block diagramm of the chirp generator is given in figure 2 .

Input output

1 Impulse generator 2 20 ys DDL 3 Amplifier and gate 4 50 p DDL 5 Amplifier 6 Frequency doubler

Fipure 2 : Block-diagram of the chirp Renerator

The different parts are :

- Impulse generator.

a TTL trigger and amplified to a peak power of 15 dBm. - 20)s dispersive delay line (20 p - DDL)

in figure 3. The main experimental results are : . insertion loss : 4 3 dB . amplitude variation : 3 dB peak to peak . phase error : . 4' rms - Amplifier, gate and limiter

plified, time-gated and limited. The time gatingcircuit suppresses noise and spurious in the absence of the signal. The command of the gate is the detected envelope of the signal itself. - 8 0 , k dispersive delay line (80 p DDL)

function of this filter is given in figure 4. The main experimental results are : . insertion loss : 59 dB . amplitude variation : ( 5 dB peak to peak . phase error : < 4"rms We can notice that the "clip" around 49C MHz is due to a defect of the mask. - Amplifier and 2 : I mutiplier

The frequency doubler is a broadband devicena- deby ANZAC (0-6-4). The nominal input is 13 dBm and the insertion loss of the doubler is 13 dB.

An impulse is generated on the rising edge O F

The transfer function of this filter is given

The impulse response of the 2 0 ) s DDL is am-

The transfer

210 - 1983 ULTRASONICS SYMPOSIUM

M. CHOMIKI - F. GENAUZEAU 3 . Performances of the chirp generator

The main performances are summarized in the table below.

0

-25

-50

-75

-10

2 5

0

.25

I

h

v

SPECIFICATIONS UNIT VALUE

900 > 100 > 330 linear

down chirp TTL/50 0 to 10

0

Center frequency Time dispersion Bandwidth Modulation Slope Input : level

Output : level repetition rate

amplitude var. phase error S I N ratio

Jitter pulse to pulse Breadboard : size

power cons.

MtIz

XHZ P - -

kHz dBrn

d! P.P. rms dB PS dm3 W

L 10

3 20 < 50 0.5

FREQUENCY (W.2)

400 450 500 5 The figure5 s k the transfer function of the cascaded filters. The phase error is 5.33 'rms This error is doubled by the frequency multiplica- tion. The output level is 0 dBm with a variation of 2 dB p.p.ovqthe bandwidth. The spectrum after the multiplication by 2 presents the following spu- rious :

- fundamental feedthrough : < - 15 dB - 3rd harmonic : ( - 20 dB - 4th harmonic (2nd harmonic of the chirp) :

S - 16 dB

The jitter from pulse to pulse is smaller than 50 ps including the noise contribution.

Figure 3 : Phase error and Amplitude response of 2 0 p s DDL

0 !5

0

-25

h I v

-50 t h + -25

1 % - I I

-25

-50

FREQUENCY ( W z ) 0

350 400 450 500 550 -75 Figure 4 : Phase error and Amplitude response

of 8 0 p DDL

- I

FREQUENCY (MHz) 1 - I C

350 Figure 5 : Phase error and Amplitude response of

100 p cascaded DDL : 20 y s DDL + limiter + 80 yS DDL

1983 ULTRASONICS SYMPOSIUM - 211

4 . Conclusion

A chirp generator unit was made to demonstrate the feasibility of a B x T product of 33000 inclii ding dispersive delay lines in RAC technology. The chirp signal exhibitsa S/N ratio better than 20 dB and a jitter pulse to pulse smaller than 50 ps and is a good demonstration of the possibi1;- ties of the SAW technology.

References

C. LARDAT, "Experimental performances of grooved reflective array compressors and re- sonators" 1976 IEEE Ultrasonics Symp. pp 2 7 2 276, Sept. Oct. 1976.

J . MELNGAILIS, R.C.WILLIAMSON, J.MOLTHAM, R . C . M . LI "Design of Reflective-array Surface Wave Devices" Waves Electronics (1976)

B.R. POTTER and C . S . HARTMANN, "Surface Acouy- tic Wave Slanted Device Technology", IEEE Trans. on Sonics and Ultras., SU 26 p 411 (1979)

T.W. BRISTOL, "Synthesis of periodic unapodi- zed surface wave transducers" 1972 IEEE Ultr;. sonic Symposium, pp 377-380

C. WATERKEYN, H. GAUTIER, "Detailled analysis of slanted reflective array compressors" 1983 IEEE Ultrasonic Symposium.

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212 - 1983 ULTRASONICS SYMPOSIUM