RECEIVER SYSTEM FOR THE SUPPRESSION OF JAMMING SIGNALS FROM FREQUENCY MODULATED JAMMING TRANSMISSIONS

Abstract: Device for the suppression of frequency modulated jamming signals in the passband of a receiver. The device is characterized by means (A<u1>u, A<u2>u, A<u3>u, A<u4>u, figure 5; 3, 4, 5, 6, 7, 8, figure 7) for sensing and determining parameters from which can be derived the frequency sweep rate of the jamming signal and its power outside the passband, and by means (I1, I2, figure 5; 9, 10, 13, figure 7) for reducing the amplification in the receiver depending on and in relation to the power of the jamming signal, if and when the jamming signal sweeps through the passband of the receiver.

Description: Receiver system for the suppression of jamming signals from frequency modulated jamming transmissions Since the time when radar came into use during the Second World War the technique for counter-measures, i.e. methods and equipment to jam the function of the radar, has also developed. The methods can be passive, for example consist in the use of decoys, or active, implying the use of jamming transmission. In order to reduce the effect of jammers, it is necessary to provide the radar receiver with a jamming protection device, or to design complete radar systems so that they can escape or counteract the jamming transmission, i.e. the defence develops what is called counter-counter measures. There are wide-band and narrow-band jamming transmissions. Wide-band jamming may have a bandwidth of up to a few hundred times the receiver bandwidth of the radar station, i.e. from, say, 20 to 300 MHz, and narrow-band jamming from an unmodulated signal up to, say, 100 times the radar receiver bandwidth. Wide-band jamming has the advantage of covering with greater certainty the frequency of the radar station or stations to be jammed but it is a disadvantage that the power of the jamming transmitter is spread over a wide frequency band. For this implies that only a fraction of the total radiated energy is utilized as jamming energy in the passband of the receiver. And the total energy available for a jamming transmitter is limited, especially for jamming transmitters carried by aircraft which is an important and generally effective way to apply jamming -transmission against a radar station- Therefore the jamming party strives to use narrow-band jamming, and at the same time to avoid that the uncertainty of the radar station frequency and the jamming transmitter stability jeopardizes the jamming effect. Lately the possibility of using narrow-band jamming has increased by improvement of the frequency stability of the jamming transmitter and by providing the jammer with a receiver, which detects and measures the radar frequency and brings the jamming transmitter frequency to agreement. Modern radar jammers are modulated, often both in amplitude and frequency. One type of effective jamming employs a signal which is frequency modulated by a sweeping signal of irregular frequency (jitter) within the 20-100 kHz frequency range, causing a frequency deviation of 2-10 MHz. In order to cover a greater frequency range with jamming of this type the jamming party can use several carriers on frequencies 2-10 MHz apart from each other and modulate these carriers with the same modulation voltage to cover the passbands of radar stations which employ jumping carrier frequency. The type of jamming outlined above is used to cause . jamming pulses of approximately the same length as a target reflected radar pulse in the radar receiver. ■ The carrier of the jamming transmitter is modulated in such a way that it is swept back and forth through the passband of the radar station at a rate adjusted to cause pulses in the receiver of about the same length as proper radar pulses. By controlling the carrier to reverse the sweeping direction just outside the passband of the receiver, a high jamming pulse repetition frequency is obtained. The frequency and amplitude of the jamming transmitter's modulation voltage determine the sweep rate and the frequency deviation of the carrier. The modulation voltage of the jamming transmitter can be sinusoidal, sawtooth or triangle shaped with a jittering frequency. If the jamming party at the same time modulates the carrier's amplitude with noise of very low frequency then each jamming pulse will have a varying amplitude which makes the signal processing in the radar still more difficult. Figure 1. illustrates how the carrier A of the jamming transmitter is swept back and forth -through the passband B of the radar receiver so that jamming pulses C are obtained at the receiver output. As was mentioned in the introduction the defence develops counter-counter measures and provides radar stations with jamming protection devices.- Examples of such jamming protection devices in use with modern radar stations are o Dicke fix receivers o logarithmic receivers The Dicke fix receiver (also called the Lamb noise-silencing circuit) suppresses pulses from jamming transmitters which cause very short pulses in the receiver, viz jamning pulses much shorter than the proper radar pulses. The passband (bandwidth) of the receiver is adjusted to the pulse length of the -radar station. In the- following rule of thumb. OMP 1 . 4/tp ( MHz ) tp signifies the pulse length of the radar station, expressed in μs. If the bandwidth is increased the sensitivity of the receiver will be reduced due to increasing inherent noise, and increasing jamming effect in case of active jamming- If the bandwidth is reduced to less than what is given by the rule of thumb above, then the echo effect of a target will be reduced, the rise and fall time of the echo pulses will increase, and the pulses will be lengthened in the receiver. A Dicke fix receiver is provided with wide-band input circuits in order to amplify short pulses (much shorter than the proper radar pulses) without causing pulse lengthening. By clipping the signal at the noise level and then amplifying it in a narrow-band amplifier (1.4/tp) the short pulses will remain at the noise level, whereas pulses of the correct length or longer will be amplified. Figure 2 shows a block diagram of a Dicke fix receiver and its function compared with an ordinary linear receiver, see figure 3. The signals at points between the blocks - A,B,C,D,E and F,G,H - are shown in the accompanying oscillogram sketches. A logarithmic receiver is characterized by a high dynamic range. It has normal bandwidth data and functions well at high interference levels. The two receivers outlined above are often combined with a pulse length discriminator and/or a correlator. For a complete description of the Dicke fix and logarithmic radar receivers, see Skolnik, pages 555-567. It is an object of the present invention also to "reduce the effect of jamming signals that, cause interfering pulses of approximately the same length as the radar pulse length or longer. This is achieved by providing the jamming protection device of the 'receiver with circuits* enabling the receiver equipment to detect jamming signals outside and close to the receiver passband. The block diagram in figure 4 illustrates the principles applied. The receiver is f ndamentally a Dicke fix receiver to which has been added a selective receiver channel tuned to a frequency just outside the narrow band of the main receiver channel. The signal at point A comes from the mixer circuits where it is transposed from radar frequency to intermediate frequency. The preamplifier F1 consists of the wide-band portion of the Dicke fix amplifier for wide-band amplification of the signal prior to clipping it at noise level in the clipping circuit. After the clipping circuit the signal is devided between the narrow-band amplifiers F2 and F3 the latter being part of the normal Dicke fix receiver. F2 is a selective amplifier similar to F3 but tuned to a frequency by the side of F3. OMPI If we assume that a jamming signal (a carrier) is swept at a constant rate towards increasing frequency,' we obtain pulses at the B and C outputs. F2 is here assumed to be tuned to a lower frequency than F3, and therefore the pulse will appear at output B before it appears at C. By delaying the pulse at B it would be possible to block F3, so that the jamming pulse does not appear at the C output. In order to block the output B pulse, we must know how long this pulse- shall be delayed. If the jamming carrier sweeps fast the delay shall be short, and if the sweep is slow the delay shall be long. There are several ways to find out the correct delay time. One way of measuring the required delay is to measure the length of pulse B. A longer pulse means a longer delay,- a shorter pulse means a shorter delay. A fast digital clock with a counter can be used for determining the delay. In case of an unknown active jamming signal, the sweep may come from either below or above the passband of the main channel. Therefore the receiver in figure 4 must be provided with two or more selective amplifiers tuned to frequencies on both sides of the passband of the main channel. Example 1 In order to obtain a radar receiver with good jamming protection against fast as well as slow jamming sweeps, a Dicke fix receiver is equipped with interference detecting devices in the form of complementary, circuits for the suppression of slow jamming sweeps. Figure 4 shows a type of jamming -protection device consisting of detector arrangements and control circuits. The detector arrangements consist of auxiliary channels tuned to frequencies on both sides of the main channel. Figure 6 shows the location of the bandpass curves along the frequency axis. A1-A4 show the location of auxiliary channels on the frequency axis, M shows that of the main channel. The main channel signal in the Dicke fix receiver is devided after the limiter(ρoint C in fig 2). It branches out partly to the main channel narrow-band amplifier and partly to the four auxiliary channels A1-A4, see figures 5 and 6. The bandpass amplifiers of the auxiliary channels are narrow-band tuned with approximately the same bandwidth as the narrow band of the main channel. After detection the level of the jamming signal is sensed (setting of threshold) by -means of a Schmitt * trigger circuit. The Schmitt trigger circuit switches at a well-defined level and produces a pulse with short rise and fall times. The signal processing after the Schmitt triggers is done digitally. __ f OM If a frequency sweeping signal first enters A1 and its level causes the Schmitt trigger to switch to "1", then the flip-flops F1 and F3 also switch to "1". F1 is always in position "1" if a signal is on its way into the main channel. The task of F3 is to check whether the signal is on its way into or out from the main channel M (figure 6). When the signals travels in the direction towards the main channel and the Schmitt trigger Sm2 has been reset (switched to "0"), the flip-flop F5 switches to position "1". The task of F5 is to switch one of the integrators 11 or 12 into circuit, depending on from what direction on the frequency axis the sweeping jamming signal comes. The 11 and 12 integrators have short time constants and are rapidly charged to the maximum amplitude of each jamming pulse, and are not discharged until flip-flop F6 resets. The task of the integrators is to reduce the amplification in the main channel in proportion to the jamming effect. If an echo were to pass through the main channel at the same time as a jamming pulse, then the echo signal would be superimposed on the jamming pulse and thus pass through the main channel, thanks to the function of the integrators. At the same time as Sm2 resets, F6 is set to "1" and fulfils one of the conditions for energizing the output of the gate D12. D10, D11, and D12 are analog gates, which implies that the output signals are proportional to the output voltages of the integrators. Reduction of the main channel amplification occurs when flip-flop F6 is set to "1". F6 is reset when the jamming signal -has passed through the bandpass filter of the main channel and arrived at A3. When F6 is reset the integrators 11 and 12 are discharged. The signal that brings about the desired decrease of amplification i.e. the output of gate D12, is applied to the amplification stages in the narrow-band amplifier of the main channel (after point C in figure 2). Example 2 Instead of two channels on each side of the main channel, one channel on each side and containing an FM detector, is used. The output signal of the FM detectors is divided in time increments and each sample is amplitude coded giving as many bits as to obtain sufficient resolution. In the system described here 8 bits are used. Depending on the expected form of jamming transmission one can choose between two ways of tuning these two channels, either one side of the centre frequency of the main channel, or both tuned to the same centre frequency as the main channel but with a three times greater bandwidth and overlapping the main channel. A block diagram applicable to both these variations is shown in figure 7. Frequency curves are shown in figures 8 an 9. The function of the receiver system is as follows: An impedance matching amplifier 1 feeds four input filters 2 to four channels. Two of these 9 and 13, 14 represent main receiver channels - one Dicke fix receiver 13, 14 and one logarithmic receiver channel 9 - and the other two are the jamming detection channels. The filter 14 of the Dicke fix receiver is of the wide-band type, while the logarithmic channel filter 9 is adjusted to the pulse length of the radar station. The remaining two filters are adjusted to the desired jamming protection function and amplifiers with FM detectors, designed to produce output voltages with reversed polarities in relation to one-another, follow after the filters. The output voltages from the FM detectors are amplitude coded in A/D (analog to digital) converters 4 and calculation of the frequency change of the jamming signal is accomplished by subtraction of the value of two successive samples in a subtracter 5, but the subtracter does not start working until an amplitude of a certain level has been received from an amplitude detector 6. If, after- the subtraction, the value is negative, i.e. the frequency change moves in the direction towards the centre frequency of the main channel, a digital signal is applied to a correlator 8, a circuit which approves a frequency change towards the centre frequency of the main channel only if the change has appeared in at least n successive time increments, or n increments of m. If such is the case an output signal is delivered to the AND gate 11 the opening of which depends on two more conditions: a) one amplitude condition from a threshold circuit 7 sensing the frequency value after the A/D converter 4 and can be set to a desired frequency distance from the main channel centre frequency in order to supply an output signal, and b) one condition from a threshold circuit at the output of the logarithmic channel which can be set just above the noise level. When these conditions are fulfilled, i.e. when a jamming signal is on its way in to the frequency range of the main channel, a signal is supplied to the AGC (automatic gain control) circuit 12, which, by using the signal from the logarithmic receiver, reduces the -amplification of the AGC amplifier 13 during the time when the jamming signal fulfils the conditions mentioned above. By utilizing the high dynamic range of the signal from the logarithmic receiver even very strong jamming signals will reduce the amplification to a value corresponding to the amplitude of the jamming signal. In this way overdriving the Dicke fix receiver 14 with its narrow band is avoided, and targets can be detected provided they have an amplitude of the same order of magnitude or do not -appear simultaneously with the jamming pulses. A target hit by the radar scan a certain number of times will be detected, for normally it cannot be expected that jamming pulses appear at the same distance during several successive pulse periods. Also, targets relatively near the radar station, at the so-called burn-through distance, can be detected. Frequency curves for the two ways of tuning the jamming detection channels are shown in figures 8 and 9. . Figure 8 shows the broad bandpass curve 1 of the auxiliary receivers tuned to the centre frequency of the main channel with the pertinent discriminator curves 2, and frequency thresholds 3. Figure 9 shows the bandpass curves 1 of the two auxiliary receivers 1 tuned to frequencies on each side of the main channel band, the associated discriminator curves 2, and frequency threshold 3. Reference: 1. Merrill I. Skolnik: Introduction to Radar Systems Mc Graw-Hill

Claims: We cl aim : 1. Device to suppress frequency modulated jamming signals in the passband of a receiver, characterized by means (A1 ,A2,A3,A4, figure 5; 3,4,5,6,7,8, figure 7) for sensing parameters that are related to the frequency sweep of the jamming signal with regard to both the magnitude and the direction of the sweep and to the jamming signal's power outside the passband, and by means (11,12, figure 5; 9,10,13, figure 7) for reduction of the amplification in the receiver in relation to the power of the jamming signal, if and when the jamming signal sweeps through the passband of the receiver. 2. Device according to claim 1, characterized by four auxiliary channels (A-j ,A2, 3,A4, figures 5 and 6) having different passbands situated in pairs on each side of the receiver passband, intended for detection of the frequency sweep rate of the jamming signal and for reduction of the amplification in the receiver, if and when the jamming signal sweeps through the passband of the receiver, as well as means (11,12, figure 5) for detection of the power of the jamming signal outside the passband of the receiver, so that the amplification in the receiver is reduced in relation to the power of the jamming signal (figure 4). 3. Device according to claim 2, characterized by the use of integrators (11,12) as parts of the detecting means. 4. Device according to claim 2, characterized by the fact that the auxiliary channels (A- j ,A2,A3,A4) -include means for detecting the duration of the jamming signal inside the passband of the auxiliary channels. 5. Device according to .claim 1, characterized by the use of FM detectors (3,4,5,6,7,8, figure 7) for detection " of the frequency sweep rate of the jamming signal, and means (9,10,13, figure 7) for reduction of the amplification in the receiver in relation to the power of the jamming signal, if and when the jamming signal sweeps through the passband of the receiver (figures 8 and 9). 6. Device according to claim 5 characterized by the use of two FM detectors tuned to passbands on each side of the passband of the receiver (figure 9). / Ol, 7. Device according to claim 5, characterized by the use of two FM detectors with ' passbands wider than that of the receiver and overlapping the passband of the receiver (figure 8). 8. Device according to claims 5,6, or 7, characterized by means (9 and 10, figure 7) for the detection of high signal levels without amplitude limitation. OMPI

Inventors: RYTTER S, STEFANSSON L
Applicants: RYTTER S, STEFANSSON L, TELEPLAN AB
Application year: 1981
Main classification: G01S7
IPC codes: G01S7, H04K3