GB981791A - Improvements in or relating to heterodyne receivers - Google Patents

Abstract

981,791. Radar receivers; reducing interference; frequency changing. MARCONI CO. Ltd. Jan. 25, 1962 [April 22, 1961], No. 14652/61. Headings H3F, H4D and H4L. A radar heterodyne receiver comprises two mixers; means for applying the received signal to both mixers; means for applying local oscillations of different frequencies to the two mixers, at least one of the local oscillations being higher in frequency than the received signal; and means for applying selected resultant beatfrequency outputs from the two mixers to at least one further mixer to derive therefrom a beat frequency which is dependent on the difference between the local oscillation frequencies but independent of the received signal frequency. As applied to an early warning radar system, wherein the received signals may have frequencies within a very wide band width[#s 1 ##s 2 ], the receiver comprises (Fig. 1), two mixers M1 and M2, receiving the signals #s 1 ##s 2 from a transmitter/receiver set TR and fed respectively with local oscillations #0 1 and #0 2 from oscillators O1 and O2. Low-pass filters incorporated in the outputs of the two mixers allow only the difference beat frequencies [#0 1 -(#s 1 ##s 2 )] and [#0 2 -(#s 1 ## 2 )] to pass to a further mixer MF which is also provided in its output with a lowpass filter such that only the difference beat frequency #0 2 -#0 1 appears at its output. The operation is thus such that the mixer/ filters M1 and M2 only pass signals when a signal [#s 1 ##s 2 ] has been received. In a Doppler radar receiver, Fig. 2, wherein the Doppler shift Œ#d is separated from the received signal [#sŒ#d], the received signal is fed to two mixers M1 and M2 fed respectively with local oscillations f0 1 and #0 2 from oscillators O1 and O2, where #s=#0 1 - #0 2 . Filters FD and FS then separate out the difference and sum beat frequencies [#0 1 - (#sŒ#d)] and [#0 1 + (#sŒ#d)] from the output of M1 respectively, whilst FS2 separates out the sum beat frequency [#0 2 + (#sŒ#d)] from the output of M2. The outputs of filters FS and FS2 are fed to a third mixer MF1, whilst the outputs of filters FD and FS2 are fed to mixer MF2. Both mixers have a band-pass filter tuned to #s in their output and give output beat signals of [#sŒ2#d] and fs respectively. These are then fed to a combination circuit CC to give an output of frequency 2fd. In an antijamming radar receiver, Fig. 3, the basic signal fs is received by TR, frequency modulated due to jamming, and is fed to the inputs of four ring modulators 1M, 2M, 3M and 4M, the first two being fed with local oscillations #0 1 from oscillator LO1 and the second two being fed with oscillations #0 2 from oscillator LO2, the relationship #s =#0 1 -#0 2 again holding. Filters FS1, FD1, FS2 and FD2 select respectively the sum, difference, sum and difference beat signals from the four filter outputs. Signals [#s+#0 1 ] and [#s+#0 2 ] are fed from FS1 and FS2 respectively to a mixer MF1 having a filter FD3 in its output, selecting frequency [#0 1 -#0 2 ], i.e. #s, which signal is taken out through channel CH1. Signals [#0 1 -#s] and [#0 2 +#s] are fed to mixer MF2 and filter FD4 to give a frequency output [2#s+ (#0 2 -#0 1 )], which is taken out through channel CH 2 . The term 2#s is however frequencymodulated due to jammings, so that the output of channel CH2 has a frequency of #s plus twice the modulation frequency. The frequency modulation can thus be extracted for antijamming purposes by combining the outputs of the two channels CH1 and CH2.