GB1328558A - Fm pulse compression system for communicat-ons - Google Patents

Abstract

1328558 Digital transmission systems DEFENCE SECRETARY OF STATE FOR 17 Nov 1971 [17 Nov 1970] 54585/70 Headings H4L and H4P A demodulator for upward swept (1) and downward swept (0) chirp signals consists of: two mixers 12, 13 transposing the input signal into two separate frequency bands which are symmetrical about a selected frequency f 3 , a symmetrical dispersive network 16 having a delay-frequency characteristic also symmetrical about f 3 (A, Fig. 8, not shown), and two filters 17, 18 passing frequencies above (1) and below (0) f 3 respectively. The received signals at 11 sweep through a range of 2# in one or other direction about a centre frequency f 0 , The mixer outputs corresponding to an upward sweep of the received signal, both sweep upwards through 2#, one 12 starting from f 3 and the other 13 finishing at f 3 . Similarly, the mixer outputs sweep downwards, to and from f 3 respectively, for a downward swept received signal. Since the dispersive netwrok introduces a delay which is maximum (T, Fig. 8, not shown) at f 3 and which decreases on either side of f 3 , signals whose frequencies are swept away from f 3 in either direction are compressed to give a significant output from the network 16, while signals swept towards f 3 are expanded to give insignificant outputs from the network 16. Thus a significant output in the upper band f 3 to (f 3 +2#) is produced by upward swept received signals (1's) and a significant output in the lower band f3 to (f 3 - 2#) is produced by 0's. These signals are supplied by detectors 19, 20 to a threshold device 21 which regenerates the D.C. 0 or 1 signal. The dispersive network 16 may be one or a chain (Fig. 7, not shown) of second order all-pass networks (Figs. 3, 4, not shown). In what is said to be a known demodulator (Fig. 1, not shown) two dispersive networks are used having linear delay-frequency characteristics, one of which has a delay falling with rising frequency (C R , Fig. 5, not shown) and the other rising with rising frequency. One network then produces a compressed output for upward swept signals and the other network for downward swept signals. A synthesizer for generating the frequency modulating signal at the transmitter includes a shift register (Fig. 9, not shown) whose successively energized outputs are combined through progressively different resistors (R1 to RN) to give a rising voltage at the input of an amplifier (25). The output of the amplifier is applied to one gate (26), and the inverted (at 28) output is applied to a second gate (27). One or other gate is enabled by a sample and hold circuit 29 in response to a data input, to pass the rising or the falling (inverted) voltage to a voltage controlled oscillator 30.