GB595094A - Improvements in or relating to cathode ray tube display systems - Google Patents

Abstract

595,094. Radiolocation. OATLEY, C. W., ELLIOTT, W. S., and PURSEY, H. May 2, 1945, No. 11192. [Class 40 (v)] Recurrent pulse signals of a periodically varying amplitude, which are displayed on a cathode-ray tube display system, in which successive traces are displaced from each other, e.g. a P.P.I. in a pulse radar system, are distinguished from signals of substantially constant amplitude by periodically reversing the polarity of the signals applied to the cathoderay tube at such a frequency that the indications produced by the signals of varying amplitude are striated, in consequence of the low-frequency beat between the amplitude variation and switching frequencies. In a pulse radar system, the echo signals are mixed with a signal of the transmitted radio-frequency, so that echo signals received from moving objects become amplitude modulated due to the Doppler effect. Figs. 1a, 1b show, when one particular type of demodulator is used, the video signals obtained from echoes from stationary and moving objects respectively, and Fig. 1c shows the video signal, corresponding to a moving object, obtained with another type of demodulator. With the latter system, the signal 1c is periodically reversed in polarity at a rate approximating to the modulation frequencies of the desired signals by the switching waveform shown in Fig. 1d, which results in the signal shown in Fig. 1e. The irregular waveform 1e which contains a component shown by the dotted line z is then applied to modulate the beam of a P.P.I. tube, the resulting signal indication appearing as a series of bright and dark striations. This indication will be produced only for signals having a modulation frequency approximating to that of the switching frequency ; a constant amplitude signal will produce an indication of constant intensity unless the switching frequency is low or the aerial scanning rate high. Fig. 2 shows the receiving circuits wherein a receiver 1 of standard form is fed with signals from an oscillator 2 which is locked in phase with the transmitted radio-frequency by signals sent over line 3. The receiver output is applied to the video amplifiers 4, 5, the outputs of which are substantially equal, but of opposite polarity. An electronic switch 6 selects the outputs from these amplifiers alternately and applies them to the cathode-ray indicator 7.