Patent GB-1253761-A (GB)
Title: Radio direction finding system
Inventor: CLEAVER RICHARD FRANCIS
IPC: G01S3/52
Filed: 1969-06-06
Abstract: 1,253,761. Radio direction finding. STANDARD TELEPHONES & CABLES Ltd. 6 June, 1969, No. 28696/69. Addition to 991,365. Heading H4D. Specification relates to an improvement in or modification to a Doppler radio direction finder of the type described in parent Specification. In such a direction finder, Fig. 1, the output from one of the aerial systems AE 1  (i.e. either the rotating aerial or the stationary aerial as in Fig. 1 of said Parent Specification, or either one of the diametrically opposite rotating aerial as in Fig. 2 of said Specification) is fed to a frequency shift unit to shift said output by a predetermined amount (4KH 3 ) and pass on only the resultant upper side band to IF amplifier and detector receiver stages 6a and 6. The aerial output is amptitude modulated in modulators 1 and 2 by respective 4HH 3  signals in phase quadrature. The modulator outputs, containing both upper and lower side bands are combined in amplifier 4, that from modulator 2 being fed via a (-90‹) phase shift 3. The lower side bands should substantially cancel out, such that at the output combining network 5 (receiving the outputs of amplifier 4 and the other aerial system AE2, whereby the carrier frequency of the wiring wave is cancelled, leaving only the doppler frequency modulator signal frequency shifted by 4KH 3 ), the amplitude ratio of the lower and the upper side bands is a number p << 1. If however, due to inaccuracies of frequencies, the signal passing through the IF amplifier is centred above the centre of its pass band, the large upper side band can be so attenuated relative to the small lower side band (which will be well within said pass band) that the lower side band will become dominant and, as it contains doppler phase information at 180 degrees to that in the upper side band, will cause frequency discriminator 10 and its associated circuitry to indicate a direction opposite to true. The present invention thus seeks to detect such shifting of the IF signals outside the IF amplifier pass band and cause and alarm and/or removal of the direction indication. In the embodiment of Fig. 4, the phase of the 4KH 3  signal at modulator 2 is alternatively shifted through 180 degrees at a rate of 2H 3  by means of a phase splitter and switch S3. This causes the lower side band to be alternately made dominant at the amplifier 4 output. With no frequency shift, both side bands pass through the IF amplifier and due to their containment of direction phase information in antiphase would give rise to alternating direction indications. To obviate this, switches S1 and S2 operate in synchronism with S3 to alternate the aerial inputs AE1 and AE2 to the modulators and combining network. The aerial inputs contain Doppler phase information in antiphase, whereby the alternating of the direction indicator is counteracted and a constant indication is given. In the presence of a frequency shift, however, the lower side band is dominant in both positions of the switches, whereby an alarm indicating alternation of directional indications is produced. The lower and upper side bands alternately dominant in the case of no frequency shift have approximately equal amplitudes Fig. 3A. With a frequency shift however one dominant lower side band is very much less than the other dominant lower side band by the factor P, Fig. 3B. A frequency shift can thus be detected by monitoring the IF amplifier output for the presence of much increases in amplitude. Thus the arrangement of Fig. 5, switch S2 applies the 4KH 3  signal to modulator 2 in antiphase for lms every 40 ms whereby the opposite direction phase information fed to the frequency discriminator (there being no switching of aerial inputs) is too short for an indicator to respond. The IF amplifier output is detected and the shift indicating pulsed output integrated at 57 and 59, to actuate and indication disabling switch 50 after a predetermined number of pulses have been detected.