Patent GB-701429-A (GB)
Title: Improvements in or relating to receiving arrangements for electric communication systems
Inventor: EARP CHARLES WILLIAM
IPC: G01S7/28
Filed: 1952-04-25
Abstract: 701,429. Radio navigation; pulse communication systems. STANDARD TELEPHONES & CABLES, Ltd. April 25, 1952, No. 10457/52. [Classes 40 (5) and 40 (7)]. [Also in Group XXXV] In an arrangement for deriving unambiguously an item of information from two waves of different frequency providing respectively different representations of the item of information, af least one representation being ambiguous, e.g. due to one or both waves being phase modulated the frequencies of one or both of the waves are translated to the same frequency by frequency multiplication and a control voltage derived from the phase difference is retroactively applied to eliminate or reduce spurious phase errors in one or both of the first-mentioned waves due to interference. In Fig. 1 two phase modulated received waves y=a sin(m#t+m#+p) and y=b sin(n#t+n#+q) of angular frequencies m # and n# where the variable phase 0 denotes the information and p and q the spurious phase modulation are multiplied in frequency by n and m respectively in harmonic generators 7, 9 and applied to a phase difference detector 11 whose D.C. output voltage is applied to reactive elements 12, 13, e.g. reactance valves forming part of filter networks 3, 4 for the received waves and so eliminating any variation in phase difference from the outputs of the harmonic generators and therefore the phase errors p and q in the received waves due to interference. The unambiguous item of information may be derived from the difference frequency (m-n) # if (m-n) is small, e.g. 1, or from either of the frequencies m# or n# if unambiguous. As applied to the pulse system described in Specification 673,354 in which two phase modulated short wave-packets of frequencies 500 kc/s and 450 kc/s corresponding to a sample of a speech wave are radiated at 100 Á sec. intervals, (so that m=10 and n=9), the difference frequency 50 kc/s is applied through two paths, one producing (at 15) a delay at 100 Á sec., to a phase discriminator 14 producing an output pulse of amplitude proportional to the phase difference between successive wavepackets. The output pulses after passing through a low-pass filter 16 and integrating network 17 reconstitute the speech wave. As applied to the radio beacon system of Specification 694,549 in which a wave of frequency Fo + F1, is radiated by a beacon with a single lobe field rotated at a low frequency 50 c/s, and a wave of frequency Fo +F2 forming a "daisy" field of m lobes also rotated at 50 c/s and a uniformly radiated field of frequency Fo amplitude modulated by a frequency F3 itself frequency modulated at 50 c/s, the three waves F1, F2 and F3 which may be 3 kc/s, 4 kc/s and 5 kc/s are filtered from the receiver demodulator and applied to detectors 24, 25 and a discriminator 28 to recover waves of frequency 50 c/s, 400 c/s and 50 c/s respectively. The latter gives a wave of reference phase and the phase of the other waves is variable in accordance with the position of the receiver relative to the beacon. To compensate for site errors the 50 c/s wave of variable phase is multiplied in frequency by eight and applied with the 400 c/s wave to a phase difference detector 32 which retroactively applies a D.C. control voltage to a phase control member 33 associated with a filter 27 for the 50 c/s wave to eliminate phase errors due to site error and interference. The output of this filter and the reference phase are applied to an indicator 30 to denote the direction of the beacon. Specification 673,355 also is referred to.