RU1841314C - autoheterodyne receiver - Google Patents

Abstract

FIELD: radio interference.

SUBSANCE: invention relates to the technique of reconnaissance and radio interference, in particular to the field of application of autoheterodyne receivers. An autoheterodyne receiving device of a frequency modulated signal, the input of which is connected to the mixer directly and through a phase modulator on a traveling wave lamp with a delay line at the input, contains an amplifier and a frequency detector at the mixer output, as well as a sawtooth voltage generator connected to the spiral of the traveling wave lamp. In this case, the spiral of the traveling wave lamp is connected to the adder switched on at the output of the frequency detector through a compensation channel containing a current-voltage converter, a filter and a phase shifter connected in series.

EFFECT: elimination of parasitic phase modulation.

1 cl, 2 dwg

Description

In the technology of reconnaissance and radio countermeasures, the so-called autoheterodyne receivers are used, with the help of which it is possible to receive one or several signals at once in a wide band of carrier frequencies. Receivers of this type do not have a frequency-tunable local oscillator and at the same time they do not have the bulky structure inherent in multichannel or matrix receivers. To receive FM or FM signals in autoheterodyne receivers, delay lines are used directly on the carrier or at an intermediate frequency. The essence of receiving FM signals is to convert the frequency of time-delayed signals using the original, undelayed ones.

If the converted signal is also shifted in frequency relative to the original, the selection of the FM (or PM) envelope is performed at a difference frequency equal to the shift frequency. Traveling wave tubes are often used to provide this frequency offset. For this purpose, sawtooth oscillations are applied to the TWT helix, with a shift frequency. TWTs are also used in such circuits to amplify signals, and in some cases to delay them (electronic delay lines). The widespread use of TWTs in autoheterodyne receivers provides a large frequency overlap ratio, a relatively high reception sensitivity, and a number of other advantages. At the same time, TWTs have a drawback that makes it difficult to use them in autoheterodyne and other receiver circuits designed to receive signals with a small frequency deviation. We are talking about parasitic phase distortions of the signal that occur in the TWT under the influence of external mechanical, electrical or other factors. So, for example, during vibrations or impact load on the TWT due to mechanical vibrations of the helix, a parasitic modulation of the signal phase is created with a frequency of impacts, which in magnitude can be commensurate with and even exceed the useful modulation.

An increase in the rigidity of the lamp design is possible only up to a certain limit and often leads to a deterioration in its electrical characteristics. Phase distortions can also occur under the action of parasitic electrical signals (for example, the background of power supplies).

The purpose of this proposal is to create an autoheterodyne receiver based on traveling wave tubes, in which the influence of parasitic phase distortions would be significantly reduced.

List of figures

Fig. 1. Block diagram of an autoheterodyne receiver.

Fig. 2. Block diagram of the proposed device.

The essence of the proposal

An autoheterodyne type device based on traveling wave tubes is proposed for receiving FM (PM) signals, in which, in order to eliminate the influence of parasitic phase modulation that occurs in traveling wave tubes, under the influence of external factors, an additional low-frequency channel is introduced, through which a signal that compensates for the specified parasitic modulation is transmitted to the output of the device's frequency discriminator.

Consider the autoheterodyne receiver circuit shown in Fig. one.

The heterodyne channel of the receiver consists of a delay line 1 and a phase modulator 2. The phase modulator receives sawtooth oscillations of the frequency ƒ _pr with an amplitude corresponding to the amplitude of the phase characteristic 2π.

At the output of the converter 3, oscillations of the difference frequency ƒ _pr are distinguished, which are fed to the frequency discriminator (detector) 4.

Due to the fact that the heterodyne oscillation is delayed in time relative to the converted signal, the phase envelope of the useful signal at the output of the converter changes in such a way that instead of the phase function ϕ(t) embedded in the input signal, the function ϕ(t)-ϕ(t-τ) is formed , where τ is the delay time in the line.

Denoting Δϕ(t)=ϕ(t)-ϕ(t-τ) and taking into account that the function Δϕ(t) will be differentiated at the output of an ideal frequency detector, we obtain

The voltage at the output of the frequency detector will have the form (I) if there is no parasitic phase modulation in the receiver channels.

Suppose now that the phase modulator, which was discussed, is made on a traveling wave lamp, and this lamp itself is subjected to vibration or shock. Due to the insufficient rigidity of the lamp components, in particular the spiral, mechanical vibrations occur, which lead to the appearance of parasitic phase modulation with the frequency of the external action.

The depth of modulation depends on the design of the lamp and the exposure mode and can reach a value of the order of several degrees, which in some cases is commensurate with the depth of useful modulation.

As a result of the frequency conversion, the spurious phase modulation is transmitted further into the path and separated by a frequency discriminator. Thus, at the output of the detector, we obtain a voltage in the form

- паразитная составлявшая сигнала.where

is the parasitic component of the signal.

The presence of parasitic modulation can significantly distort the useful signal and even make it impossible to receive it.

The presence of parasitic phase modulation as a result of external influences is also characteristic of other TWT applications (amplifiers, electronic delay lines, etc.).

The elimination of this parasitic modulation is complicated by the fact that the converted signal contains both useful and parasitic modulation at the same time, and in many cases it is impossible to separate them, since the modulation frequencies are close.

The proposal is based on the fact (experimentally confirmed) that there is an alternating component in the coil current proportional to the parasitic effect. As for the useful frequency or phase modulation, it is not observed in the current of the spiral. This makes it possible to isolate parasitic modulation by directly converting the variable current component of the spiral into voltage.

The resulting voltage is the initial for the subsequent formation of a compensating signal.

The main autoheterodyne receiver circuit and the additional compensation circuit must have similar frequency responses. In this case, one should take into account the fact of differentiation of the modulation envelope when separating the signal in the frequency detector. To compensate, it is necessary that the voltage of the additional channel be in antiphase to the parasitic component of the selected signal. An appropriate phase shift is provided by a phase shifter.

The block diagram of the device is shown in Fig. 2. The input FM (FM) signal is fed to converter 3 and a heterodyne channel consisting of delay line 1 and TWT 2. TWT acts as a phase modulator. The frequency shift is carried out by applying sawtooth oscillations from the generator 6 to the TWT helix. The difference frequency oscillations are isolated at the output of the transducer 3, amplified in the amplifier 5 and fed to the frequency discriminator (detector) 4. At the output of the detector, an oscillation consisting of useful and spurious signals is selected. To eliminate the parasitic component, a compensation circuit is used. It includes a voltage generation circuit 8, filters and a phase shifter. The low-pass filter 9 is designed to eliminate unwanted components that fall on the lamp coil, and, in particular, to suppress the sawtooth frequency and its harmonics.

Differentiating stage 10 and shaping filter 12 provide the required amplitude and phase characteristics. The amplification is carried out in the amplifier 11. The phase shift is carried out by the phase shifter 13, from the output of which the signal is fed through the buffer stage 14 to the adder 7.

Claims (1)

An autoheterodyne receiving device of a frequency-modulated signal, the input of which is connected to the mixer directly and through a phase modulator on a traveling wave lamp with a delay line at the input, containing an amplifier and a frequency detector at the mixer output, as well as a sawtooth voltage generator connected to the spiral of the traveling wave lamp, which differs the fact that, in order to eliminate parasitic phase modulation, the spiral of the traveling wave lamp is connected to the adder connected at the output of the frequency detector through a compensation channel containing a current-voltage converter, a filter and a phase shifter connected in series.

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