GB585907A - Improvements in or relating to methods and apparatus for reducing the effect of interference in the observation of recurrent oscillatory signals - Google Patents

Abstract

585,907. Wireless signalling. BLUMLEIN, A. D., and WHITE, E. L. C. Dec. 1; 1939, No. 31241. [Class 40 (v)] A train of signals having a repeated wave form is freed from random interference which may extend over a wider frequency-band than the desired signal by a process which comprises feeding the desired signals during spaced timeintervals coinciding with the same portion of the signal train or envelope and integrating these elements over a period of time sufficient to ensure a favourable ratio of signal to interference. The invention may be applied to the detection of an aeroplane by analysing the reflected waves from short "bursts" of transmitted energy as described in Specification 581,920. The transmitted signals may be slowly varied over a predetermined range either of phase or frequency in order to minimise the effect of interference of constant frequency. Or a frequency change may be applied to' the received signals in order to balance or offset the apparent change of frequency in the reflected waves due to the Doppler effect. As shown, the transmitter 1 radiates pulses of short-wave signals under the control of a master pulse-generator 6 and a carrier source 11. Signals reflected back from the aeroplane are picked up by an aerial 2 and are fed to a superhet receiver of which the stage 4a is normally blocked until impulsed, as described in Specification 585,906. The unblocking impulse is applied from a multivibrator 5 controlled from the master pulsegenerator 6 through an adjustable phasing- device 7. The output from the pulsed. stages is heterodyned in one or more stages to a frequency of 20 kilocycles in one or more mixers 8 and then passed to a circuit 9 having a band width of Œ200 cycles. As shown the local oscillation is supplied from a mixer 10 fed from the carrier source 11 and from a stable oscillator 12 generating the desired intermediate frequency. The circuit 9 in effect integrates the successive bursts or pulses, after reflection from the aeroplane the amplitude of the built-up oscillations being proportional to the number of oscillations received, though any interference present is attenuated. Means are provided for offsetting any D÷ppler effect on the frequency due to the approach or recession of the aeroplane ; and also for preventing effects due to frequency " drift " in the oscillator stages.