WO1991008614A1

WO1991008614A1 - Device generating a frequency modulated signal of the 'ramp signal' type using a surface wave dispersive filter generation

Info

Publication number: WO1991008614A1
Application number: PCT/FR1990/000868
Authority: WO
Inventors: Michel Chomiki
Original assignee: Thomson-Csf
Priority date: 1989-12-01
Filing date: 1990-11-30
Publication date: 1991-06-13
Also published as: FR2655494A1

Abstract

Generator producing a frequency modulated signal of the 'ramp signal' type (20) by means of SAW dispersive filter generation (19). The phase error inherent in the SAW component is corrected by a phase lock loop (22) which receives a prepositioning voltage (31) modulated by the inverse curve of this phase error. This prepositioning voltage is preloaded into a PROM memory (37).

Description

DEVICE FOR GENERATING A MODULATED SIGNAL

FREQUENCY OF TYPE "RAMP SIGNAL", USING

A GENERATION BY DISPERSITIVE WAVE FILTER

AREA

The present invention relates to a device for generating a frequency modulated signal of the kind

"ramp signal", this device using a generation by dispersive surface wave filter, or "SAW dispersive filter".

Generators of this type are used mainly on the one hand in "pulse compression" devices which equip almost all modern radars, and on the other hand in spectrum analyzers called "FOURIER transformers" which put implement the algorithm known as "Chirp Transform". The operation of these devices includes the generation of one or more signals, of fixed duration T, frequency modulated according to a linear law or not, e * that to simplify, we will call in what follows "ramp signals" .

To generate signals of this type, the process which is by far the fastest at present, consists in using a dispersive filter with surface waves, or dispersive filter SAW. It is an analog filter, for example of the "RAC" type, comprising two acoustic channels provided with grooves engraved at 45 degrees. Such surface wave filters are for example described in the article by Messrs RC Willia son and HL Smith "Large time bandwidth product surface wave puise compressor employing reflective gratings", published in the British review "Electronics Letters", volume 8, N ° 16, August 1972, pages 401 and 402.

In attached FIG. 1, a radar installation, today very common, known as "pulse compression", has been shown schematically. The radar pulse 1 is applied in 2 to an electronic unit 3, comprising a dispersive filter SAW, therefore capable of transforming the fine pulse 1 into a signal 5 of duration T linearly modulated in frequency, and which will be conventionally called by the following "ramp signal" (although this designation is actually rather used in spectrum analyzers of the Transformer type of FOURIER), then to the transmitter 4.

This signal 5 is emitted by the transmitting antenna 6 towards the target 7, on which it is reflected to be, in return, received by the receiving antenna 8 of the installation (which can moreover be confused with the antenna 6).

The received signal is then applied to an electronic reception block 9, which includes a SAW dispersive filter complementary to that contained in block 4 (ie with an identical frequency / delay curve but with a slope of opposite sign), and which is therefore able to transform the ramp signal 5 back into a "fine" pulse, the real appearance of which is represented at 10.

The ramp signal generator concerned by the invention is therefore, in this application example, the electronic unit 3 (or, at least, contained in this electronic unit). In these pulse compression systems, the characteristics of the compressed pulse output strongly depend on the residual phase error, i.e. on the difference between the phase of the signal obtained and the phase theoretical desired. In fact, this phase error creates malfunctions at the output, in particular poor reproducibility of the time-frequency law and distortions of the shape of the compressed pulse. In the attached FIG. 2, the multiplication-convolution part of a spectrum analyzer called "FOURIER transformer" with SAW dispersive filters has been represented schematically. The electrical signal to be analyzed is applied at 11 to a multiplier 12 which receives at its other input 13 a ramp signal similar to the signal 5 of FIG. 1 and generated by a fine pulse 14 applied to a SAW dispersive filter 15, followed by an electronic unit 16 comprising (or constituting) the device of the invention. Conventionally, this multiplier 12 is followed by another SAW dispersive filter 17 and by an electronic output unit 18.

In such a FOURIER transformer, the compressed pulse is altered by the phase error of the SAW dispersive filter 15 used to generate the ramp signal from the brief electric pulse 14. This filter has residual phase errors which , after manufacture and acoustic correction (conventionally carried out by depositing on the substrate a metallic film of phase correction), nevertheless remain of the order of one to ten RMS degrees.

It has already been proposed, according to document FR-A-2612711, an electronic correction of these errors by means of an electric filter placed downstream. This solution, if it solves a large part of the problem, remains complicated since it is necessary to carry out the measurements of the errors, then to manufacture the electric filter so that its transfer function corrects these errors. In addition, this technique is ineffective at high frequencies because the manufacturing and synthesis errors of the electric filter become of the same order of magnitude as those to be corrected.

The invention aims to remedy these drawbacks due to the phase error of the SAW dispersive filters. She relates to this effect to a frequency modulated signal generator of the "ramp signal" type, using a generation by dispersive surface wave filter, or "SAW dispersive filter", this generator comprising a phase locked loop whose reference is constituted by the useful signal from this SAW dispersive filter and which receives a pre-positioning electrical voltage which is modulated by an instantaneous voltage representative of the inverse of the phase error, measured in advance, generated by this filter dispersive SAW, so that this phase locked loop reacts by introducing a phase shift which compensates for this phase error.

The invention will be clearly understood, and its advantages and other characteristics will emerge during the following description of two nonlimiting exemplary embodiments, with reference to the appended schematic drawing in which:

- Figure 3 is a block diagram of a first embodiment of this frequency modulated ramp signal generator;

- Figure 4 is a set of three explanatory curves for the operation of this generator; and

- Figure 5 is a block diagram of another embodiment of this frequency modulated ramp signal generator.

Referring first to Figures 3 and 4, this device comprises a conventional device 19 for generating the ramp signal 20, comprising a dispersive filter SAW, driven, to generate this signal 20, by a fine pulse, or pulse of DIRAC, 21, and including conventional electronics (including, for example, an amplifier, a filter and possibly an Automatic Gain Control). According to the invention, the ramp signal 20 is used as the reference signal of a phase locked loop 22 which operates as a phase shifter adjusted to correct the phase error of the SAW dispersive filter contained in block 19.

This phase locked loop includes a Voltage Controlled Oscillator, or VCO, 23, of which (conventionally) part of the output voltage at 24 is taken by a coupler 25 to be applied to a first input 26 of a comparator of phase 27. The second input 28 of this comparator receives the reference signal which is constituted by the ramp signal 20, marred by the phase error to be corrected.

This phase error Δφ is represented, as a function of time t and during the duration T of the signal modulated linearly in frequency 20, in diagram A of FIG. 4.

The error voltage at output 29 of the phase comparator 27 is applied to the first input of an adder 30, the other input 31 of which receives a prepositioning voltage Vp, whose variation curve as a function of time t and during the the aforementioned time period T is represented in diagram C of FIG. 4.

This variation law C is very particular, because it results from the modulation of a sawtooth voltage 32 (in phantom in Figure 4), which is itself capable of controlling the VCO 23 so that its voltage output 24 follows the same linear law of frequency variation as that followed by the ramp signal 20, by a variable voltage V / K, represented in diagram B of FIG. 4, which is equal to the inverse of the error according to diagram A but converted into voltage, K being the sensitivity of the phase comparator 27 in volts / degrees. This law of variation comes from digital data recorded, after laboratory measurements, of the phase error specific to the SAW component used in block 19, in a PROM memory 37, and converted into pre-positioning voltage Ve according to curve C by a digital-analog converter 33. Of course, this prepositioning signal Vp is adjusted as a function of the particular characteristics of the phase loop 22 (slope of the phase comparator 27, gain between the output 29 of this phase comparator and the control input 34 of the VCO 23, relative position of adder 30 and loop filter 36).

As seen elsewhere in the drawing, the output signal at 35 from the adder 30 is conventionally applied, via a loop filter 36, to the control input 34 of the V.C.O.

This device works as follows: Given the sawtooth tension 32, the V.C.O. easily hooks onto reference 20. The phase error of this reference signal at 28 is that shown on curve A.

The V.C.O. being hung, its output frequency in 24 can no longer move. As we impose in 34 a control modulation corresponding to the curve B

(inverse of the phase error), it reacts by shifting its output signal at 24 by a value tending to cancel this modulation, that is to say by an inverse value of the phase error of the reference signal 20 : this output signal at 24 is therefore corrected in phase with the phase error of the reference signal 20. Note that curve B then represents the error voltage at output 29 of the phase comparator 27.

A convenient way to get the preposition signal C is as follows:

. in memory 37 is first loaded the voltage frequency law 32 of VCO 23; . at output 29 of the phase comparator 27 is measured the error error law C of the loop during the time interval T, law which is converted into phase taking into account the sensitivity K of the phase comparator 27; . the loading of the memory 37 is then modified point by point until the law noted 29 reproduces the inverse of the phase error Δφ, possibly smoothed.

Of course, the bandwidth and the parameters of the loop are chosen so as to allow the correction of the phase error in the necessary band: in general only the relatively slow errors (some oscillations during the period T) are to be corrected. It goes without saying that the invention is not limited to the embodiment which has just been described. Thus, for example, according to the variant embodiment shown diagrammatically in FIG. 5, the input (adder 30) of the prepositioning signal can be placed at another location between the output 29 of the comparator 27 and the control input 34 VCO 23, in this case after the loop filter 36 and not before. Similarly, the prepositioning and phase correction inputs can be separated: - prepositioning just before the VCO command input;

- phase correction just after the output of the phase comparator.

As shown, a frequency divider by N 38 can be incorporated between the coupler 25 and the input 26 of the phase comparator 27, so as to obtain the equivalent of a dispersive filter of bandwidth Bp / N followed by a frequency multiplier by N: in fact, the frequency band Bp covered by the ramp signal which is effectively at 24 is then, due to the divider 38, multiplied by the factor N. This is economically advantageous, because a frequency divider is a much less expensive component than a frequency multiplier. In addition, the filter 36 advantageously includes a frequency band widening device to accelerate the repositioning of the VCO 23 during the return (falling edge) of the sawtooth 32, such a device being known per se. The frequency modulated signal generator which has just been described can also be used as a ramp signal generator in the multiplication-convolution part of a spectrum analyzer of the transformer type of FOURIER, as well as as emission signal generator. of a pulse compression device.

Claims

1 - Frequency modulated signal generator of the "ramp signal" type (20), using a generation (19) by dispersive surface wave filter, or "SAW dispersive filter", characterized in that it comprises a loop with phase lock (22), the reference (28) of which is constituted by the useful signal (20) from this SAW dispersive filter and which receives a prepositioning electrical voltage (32) which is modulated by an instantaneous voltage (B) representative of the reverse of the phase error (A), measured in advance, generated by this SAW dispersive filter, so that this phase-locked loop (22) reacts by introducing a phase shift which compensates for this error of phase.

2 - Frequency modulated signal generator according to claim 1, characterized in that this modulated prepositioning voltage (C) comes from values previously measured and entered digitally into a programmable memory (37).

3 - frequency modulated signal generator according to claim 1 or claim 2, characterized in that the phase locked loop (22) comprises, in its return circuit, a frequency divider (38) capable of causing an enlargement of the bandwidth (B _p ) covered by this frequency modulated signal. 4 - Generator according to one of claims 1 to

3, characterized in that the loop filter (36) of said phase-locked loop (22) comprises a device capable of widening the frequency band, in order to accelerate the repositioning of the Voltage Controlled Oscillator (23) of this loop (22) during the falling edge of said prepositioning signal (32).

5 - Method for obtaining the modulated prepositioning voltage (C) for a signal generator frequency modulated according to claim 2 or claim 3, characterized by the following sequence:

. in the memory (37) is first loaded the frequency-voltage law (32) of the V.C.O. (23) of the phase locked loop (22);

. at the output (29) of the phase comparator (27) of this loop (22), the law (C) of error of the loop during the duration (T) of the useful signal (20) is measured, law which is converted into phase taking into account the sensitivity (K) of the phase comparator (27); the loading of the memory (37) is then modified point by point until the noted law (C) reproduces the inverse of the phase error (Δφ), possibly smoothed. 6 - Pulse compression device, characterized in that it comprises a generator (4) of frequency modulated signal according to one of claims 1 to 4.

7 - Spectrum analyzer of the "FOURIER transformer" type, characterized in that its "multiplication-convolution" part (11 to 18) comprises a ramp signal generator (15, 16) according to one of claims 1 to 4 .