GB1605329A - Improvements in or relating to radar receivers - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO RADAR RECEIVERS We, THE MARCONI COMPANY LIMITED. of English Electric House, Strand, London. W.C. 2, a British company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to radar receivers and in particular to the protection of radar receivers from jamming signals.

One known arrangement for the protection of radar receivers from jamming employs two receiver channels one of which is a normal or unprotected channel of the desired bandwidth and the other of which has a noise limiting circuit (a so-called Dicke-Fix circuit or Lamb noise silencing circuit) included therein. A Dicke-Fix circuit greatly reduces the effect of jamming signals in the form of recurrent pulsed signals the pulse lengths of which are short in relation to that of the transmitted radar pulses. However, in the absence of jamming, the Dicke-Fix circuit produces in the channel in which it is included, a degrading of the signal-to-noise ratio in comparison with that in the normal channel. The normal channel is therefore provided, in addition to that containing the Dicke-Fix circuit, in order that the latter may be used when jamming is present and the normal channel may be used in the absence ofjamming. However a decision has to be made either by an operator or by a sensing circuit as to whether or not jamming is present and the desired channel switched in to operation accordingly. To avoid unnecessary poor results when only a small amount ofjamming is present the Dicke-Fix channel is usually not switched in until the jamming is above a certain level. To determine whether or not the jamming is above or below a certain level is virtually impossible for an operator if he has only a single P.P.I. display tube. Even if a separate P.P.I. display is provided for each of the two channels it is extremely difficult in practice to decide which of the two channels should be switched in at any time and incorrect decisions are likely to be made by even the most skilled operator.

Because of the difficulty an operator has in making correct decisions it has been proposed to provide automatic electronic means for making the aforesaid decisions, i.e. for switching in that channel which is best at any given time but known arrangements of this nature have the defect or limitation that the automatic electronic means for determining which channel is to be switched in, operate by choosing the channel to be switched in for the whole of the succeeding radar period. The present invention seeks to overcome this defect or limitation and to provide an automatic electronic switching arrangement wherein the channel switching decisions are made without restricting, for every form of jamming, the switching in of the chosen channel for a whole radar period.

According to this invention a radar receiver comprises two channels to which received echo signals are fed, one channel containing a noise limiting circuit of the Dicke-Fix type and the other channel including a narrow band filter of the desired receiver bandwidth and an inhibiting circuit; an OR gate having two inputs each fed with output from a different one of the two channels and adapted to pass the larger of the two signals fed thereto at any given time; and a sensing circuit actuated in dependence upon the number of false echo signals occurring in said other channel and adapted and arranged to operate said inhibiting circuit to block said other channel for the succeeding radar period if. during the radar period preceding said succeeding period, the number of false echoes exceeds a predetermined value.

Preferably the two channels are fed with received echo signals through a common head amplifier.

Preferably also each of the channels includes a pulse length discriminator adapted to eliminate pulses of greater duration that the transmitted radar pulses.

The sensing circuit preferably includes a charging circuit fed with pulses of one polarity derived from the pulse length discriminator in one channel and with pulses of the opposite polarity derived from the pulse length discriminator in the other channel and is arranged to operate the inhibiting circuit when the voltage in said charging circuit exceeds a predetermined value.

Preferably again each of the two channels includes a logarithmic amplifier.

The invention is illustrated in the drawing accompanying the provisional specification which shows in block diagram form one embodiment thereof.

Referring to the drawing, I is a head amplifier to which the incoming radar signals are fed and which feeds into amplified output to two channels, generally designated 2 and 3. Channel 2 comprises a noise limiting circuit constituted by a Dicke-Fix circuit 4, a logarithmic amplifier 5 and a pulse length discriminating circuit 6.

Channel 3 is a normal receiver channel comprising a narrow band I.F. amplifier or filter 7 of the desired receiver bandwidth, a logarithmic amplifier 8 and a pulse length discriminator 9.

The channels 2 and 3 feed into the two inputs of an OR gate 10, channel 2 feeding directly into one input of the OR gate from the pulse length discriminator 6 and channel 3 feeding into the other input of said OR gate from the pulse length discriminator 9 via an inhibiting gate 11. The pulse length discriminators 6 and 9 also feed into the inputs of a sensing circuit 12 which is employed to control the gate 11. The output from the OR gate 10 is fed to a Pup.1. or other display or utilisation means (not shown).

The Dicke-Fix noise limiting circuit 4 is as known per se and operates in known manner. It comprises a wide band filter followed by a limiting circuit and a narrow band amplifier. the said amplifier having a bandwidth equal to that of the I.F. amplifier 7 in the normal channel. The said Dicke-Fix circuit is designed substantially to minimise jamming in the form of short duration high amplitude pulses. i.e. pulses which are shon in relation to the length of the received radar echo pulses. Such short pulses, if of low amplitude.

will pass with reduced amplitude through the filter of the narrow band amplifier in the Dicke Fix circuit because the restricted bandwidth of this filter prevents fast rise times and the short pulse is lengthened and reaches a much lower peak level. However. if the wide band filter and limiter were not present in the Dicke-Fix circuit large amplitude pulses could cause the filter of the narrow band amplifier to "ring" with the result that the output pulse from the said narrow band amplifier would be of substantial amplitude and increased in length so as to compare with the wanted radar echo pulses. The wide band filter in the Dicke-Fix circuit can cope with the fast rise and decay times of shon. high amplitude pulses and these pass through the wide band filter vçithout being much lengthened. However, the limiting circuit clips off the peaks of these short pulses to such an extent thai they cannot cause "ringing" in the subsequent narrow band filter and they are therefore reduced in amplitude hy the narrow band amplifier. Therefore. by using a Dicke-Fix circuit, shon interfering pulses can be reduced to a very low amplitude so that they no longer adversely affect the display or utilisation means to any substantial extent.

The normal channel 3 has a similar narrow bandwidth amplifier 7 similar to that incorporated in the Dicke-Fix circuit in channel 2 and this reduces the amplitudes of low amplitude shon pulses. However high amplitude short pulses can cause ringing of the narrow band filter 7 and thus result In the protection of pulses of substantial length and amplitude at its output. If the high amplitude short pulses which cause the filter 7 to ring occur at high frequency - and if they are jamming pulses they usually will occur at high frequency - the ringing of the narrow band amplifier or filter 7 will be continuous and a continuous wave output will be fed into the logarithmic amplifier 8. Desired incoming signals will therefore be swamped by this continuous wave output and prevent pulses being detected and fed to the OR gate 10. Low frequency short duration high amplitude pulses will produce at the output of the narrow band amplifier or filter 7 pulses which are comparable with the wanted radar echoes and such pulses, if passed to the display or utilisation means, would be indistinguishable therein from wanted target echoes.

The combination of units 10. 12 and I minimises the number of false signals passed to the display or utilisation means from the output of the OR gate 10. The OR gate 10 is designed to pass to its output the larger of the signals occurring at its two inputs. In the presence of high frequency' short duration jamming pulses of high amplitude only continuous wave output is fed from the circuit 7 to the logarithmic amplifier 8 and no detectable pulses are passed to the OR gate 10 along channel 3. In these circumstances the Dicke-Fix channel will eliminate the short jamming pulses. as described above, and only wanted echo pulses or jamming pulses of comparable duration will be passed by the OR gate 10 to the display or utilisation means since these pulses will be of higher amplitude than the signal appearing on channel 3. The jamming may be coincident with "clutter" signals received for example from hills or cloud. Such signal amplitudes can exceed the limiting level of the channel 2 and thus swamp this channel. As so far described, under these conditions wanted echoes could be passed by channel 3 if the jamming intensity is not too high and therefore these pulses will be passed via the OR gate 10 to the display or utilisation means.

If. however, the jamming pulses are short duration high amplitude pulses occurring at low frequency then false echo pulses can appear on the channel 3 at the input to inhibit circuit ]1.

Only pulses of very reduced amplitude will appear on channel 2 at gate 10 since the Dicke Fix circuit will. as described. substantially eliminate these pulses. In the absence of any further measures these pulses on channel 3 would pass to the display or utilisation means. However.

the sensing circuit 12 which receives inputs derived from the pulse length discriminators 6 and 9, is designed to control the inhibit circuit 11 in these circumstances. This sensing circuit 12 senses the number of false echoes arriving on channel 3 and if this number exceeds. during a radar pulse period. a predetermined value the said sensing circuit 12 inhibits gate 11 for the next radar period. The sensing circuit 12 may. íor example. comprise a charging circuit to which are fed pulses derived from one pulse length discriminator and of one polarity and pulses derived from the other pulse length discriminator and of the opposite polarity. Pulses occurring simultaneously on both channels and therefore representing true echoes will substantially cancel one another out so far as the charging circuit is concerned whereas pulses only occurring on channel 3 will increase the charge stored in the said charging circuit. The charging circuit is used to control the inhibiting gate 11, by "shutting" that gate for the next radar period when the stored charge exceeds a predetermined value.

The pulse length discriminators 6 and 9 discriminate against unwanted jamming noise pulses having a duration longer than the transmitted radar pulse length. The log amplifiers 5 and 8 operate to stabilise the mean noise level.

It will now be seen that in the illustrated arrangement the Dicke-Fix channel 2 is now utilised to prevent high frequency shon pulse jamming only for that portion of a radar period which is substantially free from large clutter signals, whilst the output from the normal channel 3 is passed to the display or utilisation means during other portions of the radar period.

Also the normal channel 3 is inhibited (by gate 11) when low frequency short pulse jamming occurs the Dicke-Fix channel being utilised during such jamming, the presence of such jamming being detected for a short ponion of each period and the detection used to "shut" the inhibiting gate 11 for the succeeding radar period.

Since the head amplifier 1, which will introduce the major portion of the internally produced electrical noise, feeds into both channels the noise in the two channels 2 and 3 will be to a large extent coherent. Therefore the noise signals which are applied to the inputs of the OR gate 10 at the same time as the signals will not summate since comparable noise peaks will occur substantially simultaneously in both channels and only the higher noise peak will be transmitted. If separate head amplifiers were used, one in each channel, the noise signals would be incoherent, and noise peaks in one channel could occur simultaneously with noise valleys in the other and, because the OR gate 10 passes the larger signal present at any instant it could produce effective summation of the two noise signals. However the use of the common head amplifier 1 largely prevents this summation effect.

WHAT WE CLAIM IS: 1. A radar having a radar receiver comprising two channels to which received echo signals are fed, one channel containing a noise limiting circuit of the Dicke-Fix type and the other channel including a narrow band filter of the desired receiver bandwidth and an inhibiting circuit; an OR gate having two inputs each fed with output from a different one of the two channels and adapted to pass the larger of the two signals fed thereto at any given time; and a sensing circuit actuated in dependence upon the number of false echo signals occurring in said other channel and adapted and arranged to operate said inhibiting circuit to block said other channel for the succeeding radar period if, during the radar period preceding said succeeding period the number of false echoes exceeds a predetermined value.

2. A radar as claimed in claim 1 wherein the two channels are fed with received echo signals through a common head amplifier.

3. A radar as claimed in claim 1 or 2 wherein each of the channels includes a pulse length discriminator adapted to eliminate pulses of greater duration than the transmitted radar pulses.

4. A radar as claimed in claim 3 wherein the sensing circuit includes a charging circuit fed with pulses of one polarity derived from the pulse length discriminator in one channel and with pulses of the opposite polarity derived from the pulse length discriminator in the other channel and is arranged to operate the inhibiting circuit when the voltage in said charging circuit exceeds a predetermined value.

5. A radar as claimed in any of the preceding claims wherein each of the two channels includes a logarithmic amplifier.

6. A radar having an anti-jamming receiver substantially as herein described and illustrated in the drawing accompanying the provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. fed pulses derived from one pulse length discriminator and of one polarity and pulses derived from the other pulse length discriminator and of the opposite polarity. Pulses occurring simultaneously on both channels and therefore representing true echoes will substantially cancel one another out so far as the charging circuit is concerned whereas pulses only occurring on channel 3 will increase the charge stored in the said charging circuit. The charging circuit is used to control the inhibiting gate 11, by "shutting" that gate for the next radar period when the stored charge exceeds a predetermined value. The pulse length discriminators 6 and 9 discriminate against unwanted jamming noise pulses having a duration longer than the transmitted radar pulse length. The log amplifiers 5 and 8 operate to stabilise the mean noise level. It will now be seen that in the illustrated arrangement the Dicke-Fix channel 2 is now utilised to prevent high frequency shon pulse jamming only for that portion of a radar period which is substantially free from large clutter signals, whilst the output from the normal channel 3 is passed to the display or utilisation means during other portions of the radar period. Also the normal channel 3 is inhibited (by gate 11) when low frequency short pulse jamming occurs the Dicke-Fix channel being utilised during such jamming, the presence of such jamming being detected for a short ponion of each period and the detection used to "shut" the inhibiting gate 11 for the succeeding radar period. Since the head amplifier 1, which will introduce the major portion of the internally produced electrical noise, feeds into both channels the noise in the two channels 2 and 3 will be to a large extent coherent. Therefore the noise signals which are applied to the inputs of the OR gate 10 at the same time as the signals will not summate since comparable noise peaks will occur substantially simultaneously in both channels and only the higher noise peak will be transmitted. If separate head amplifiers were used, one in each channel, the noise signals would be incoherent, and noise peaks in one channel could occur simultaneously with noise valleys in the other and, because the OR gate 10 passes the larger signal present at any instant it could produce effective summation of the two noise signals. However the use of the common head amplifier 1 largely prevents this summation effect. WHAT WE CLAIM IS:

1. A radar having a radar receiver comprising two channels to which received echo signals are fed, one channel containing a noise limiting circuit of the Dicke-Fix type and the other channel including a narrow band filter of the desired receiver bandwidth and an inhibiting circuit; an OR gate having two inputs each fed with output from a different one of the two channels and adapted to pass the larger of the two signals fed thereto at any given time; and a sensing circuit actuated in dependence upon the number of false echo signals occurring in said other channel and adapted and arranged to operate said inhibiting circuit to block said other channel for the succeeding radar period if, during the radar period preceding said succeeding period the number of false echoes exceeds a predetermined value.

2. A radar as claimed in claim 1 wherein the two channels are fed with received echo signals through a common head amplifier.

3. A radar as claimed in claim 1 or 2 wherein each of the channels includes a pulse length discriminator adapted to eliminate pulses of greater duration than the transmitted radar pulses.

4. A radar as claimed in claim 3 wherein the sensing circuit includes a charging circuit fed with pulses of one polarity derived from the pulse length discriminator in one channel and with pulses of the opposite polarity derived from the pulse length discriminator in the other channel and is arranged to operate the inhibiting circuit when the voltage in said charging circuit exceeds a predetermined value.

5. A radar as claimed in any of the preceding claims wherein each of the two channels includes a logarithmic amplifier.

6. A radar having an anti-jamming receiver substantially as herein described and illustrated in the drawing accompanying the provisional specification.