GB636638A - Improvements in or relating to telecommunication and radar systems - Google Patents

Abstract

636,638. Radio receiving circuits ; radiolocation. BANWELL, C. J. Nov. 5, 1947, No. 29502. [Classes 40 (v) and 40 (vii)] In a double or multiple superheterodyne receiver, continuous variation of the overall band width is effected by varying the frequency by one or more of the local oscillators. The first intermediate frequency f2 is fed to a highpass filter A2 having a sharp out-off at fH below f2, and a falling amplitude above f max. The output from A2 is converted at X2 into a second intermediate frequency f3 which is fed to a low-pass filter A3 having a characteristic shown in full line above A3, giving a sharp cut-off above fL and a falling amplitude below f min. The pass band of filter A2 is thus transposed so as to overlap in part the pass band of filter A3, the degree of overlap being determined by the setting of the second local oscillator LO2. Thus the effective pass band of the receiver is controlled by varying the frequency of the second local oscillator, and can be varied from zero up to the bandwidth f max-fH of filter A2. The output from A3 passes to a detector D and subsequent stages. The above arrangement may be preceded by an additional frequency changer and a wide band filter centred about the first intermediate frequency. Also the final local oscillator is ganged so as to vary at twice the rate of the second local oscillator. This may be done by using as the final oscillator the second harmonic from the second oscillator. The variation of frequency of the local oscillators may be produced by mechanical means, or by varying a control potential applied to a reactor valve forming part of a frequency determining circuit. In the latter case the potential may be derived from rectified noise obtained from an auxiliary amplifier, and used to narrow the bandwidth in the presence of noise. If the ordinary A.V.C. potential is applied to the auxiliary amplifier, the arrangement may be made to maintain a constant signal-to-noise ratio. In radar systems the bandwidth may be made to vary with time so as to give the greatest bandwidth for the reception of the more distant echoes. This is done by applying a suitable voltage wave form synchronized with the transmitter pulse to the frequency determining reactor valve of the local oscillator. To give the same signal-to-noise ratio for all signals it would be necessary for the band width to vary inversely as the fourth power of the time for free space targets such as aircraft, and inversely as the eighth power of the time for targets such as ships. The gain of the receiver may also be variable with time. The circuit may also be arranged to narrow the bandwidth by means of rectified noise voltages. The above arrangements may be combined with means for varving the pulse length inversely with the bandwidth.