GB1146875A - Improvements in radio systems with means for processing demodulated signals - Google Patents

Abstract

1,146,875. Secondary radar. COMPAGNIE FRANCAISE THOMSON HOUSTONHOTCHKISS BRANDT. 15 Sept., 1966 [15 Sept., 1965], No. 41344/66. Heading H4D. In a secondary radar system, wherein a response signal may be received at the interrogating receiver both directly and after reflection, said reflected signal is removed to enable accurate distance measurements. The interrogation, and response signals are frequency shift keyed signals modulated according to a desired binary code between frequencies f1 and f2. The response signals are received at the interrogator, Fig. 1, and a frequency discriminator 16 produces for a frequency shift from f1 to f2, outputs as shown in Fig. 2. In waveform 20, the direct signal d is very much larger than the reflected signal r, in waveform 21, r/d = 0À1, in waveform 22, r/d = 0.5 and in waveform 23, r/d = 0.9. The position of the direct signal thus becomes increasingly obscure as the reflected signal increases. The rapid fluctuations are removed by a low-pass filter 17 to produce corresponding waveforms of Fig. 3. The waveform of Fig. 3, (33), is shown in full at 43, Fig. 4. Waveform 43 is differentiated in means 18 (waveform 44) and also peak clipped (waveform 45). Waveform 45 is fed to a logic circuit 19 which produces a pulse 46 having a first step coincident with the first positive pulse in waveform 45, and a second step coincident with the first negative pulse. Pulse 46 corresponds to the direct received signal and enables accurate distance measurements to be made. Logic circuit 19 comprises, Fig. 5, a bi-stable circuit 54 triggered at both stage inputs 55 and 56 by positive pulses, diode 51 to pass positive pulses in waveform 45, and diode 52 to pass negative pulses in waveform 45, an inverter converting these passed pulses to positive triggering pulses. Thus, once circuit 54 has been set in its upper state by the first positive pulse, further positive pulses have an effect, and the first negative pulse, via inverter 53, returns it to its lower state.