GB1509595A - Differential phase correcting arrangement - Google Patents

Abstract

1509595 Phase shifting circuits; modulation LGT LABORATOIRE GENERAL DES TELECOMMUNICATIONS 21 Oct 1975 [22 Oct 1974] 43229/75 Headings H3T and H3R A phase shifting circuit, e.g. for a TV transmitter, has an input terminal E 1 and means 27 for forming a signal in phase opposition to the input signal, the two signals passing through parallel channels to an adder 10, 15 having an output terminal S, which is the output of the circuit, one channel including a phase shifting device 1 comprising an oscillatory circuit 6, 7, a voltage divider 2, a resistor 4 and a variable resistance element 5 connected substantially as shown, element 5 being responsive to a control signal at E 2 to vary the phase shift produced by 1. The variable resistance element 5 may be a PIN diode, and it is shown in the Specification that variation of the resistance of element 5 does not affect the voltage output of the phase shifter provided that R 4 =(n-1)Rp where Rp is the parallel effective resistance of the tuned circuit 6, 7 including its load, and the voltage at the top of the divider 2 is 1/n of the input voltage. At the resonance frequency (W 0 ) of the tuned circuit the emitter voltages of 15 and 10 (V 1 , V 2 respectively), Fig. 5 (not shown), are 180 degrees out of phase producing a resultant (V 3 ) in phase with or 180 degrees out of phase with the input signal. At a neighbouring frequency (W) the phase of the resultant (V 3 <SP>1</SP>) is shifted. The phase shifter may be used to correct for phase distortion in a modulator (21), Fig. 2 (not shown), which is supplied with IF and VF signals, the modulator output being connected to the input terminal E 1 . The VF signal is also applied to a branch comprising a delay line (23) which compensates for the delay time of the modulator, and clipping circuits (24, 25) which select and pass those portions of the VF signal above and below two threshold voltages (V 1 , V 2 respectively). The outputs of the two clipping circuits are connected to an adder (26) whose output is connected to the terminal E 2 . In another application the phase shifter is used to correct for phase shift in an output amplifier (33), Fig. 3 (not shown). The output of an IF modulator is connected to the input terminal E 1 , and the output feeds a frequency converter (32) the output of which passes to the output amplifier (33). To the input of the latter a branch feeding a phase comparator (39) is connected, which includes a phase shifter (34), a modulation detector (35) and a delay line (36). At the output of the amplifier (33) is connected a branch consisting of an attenuator (37) and a modulation detector (38) which also feeds the phase comparator (39). The phase comparator output is connected to the terminal E 2 , supplying a signal whose amplitude is proportional to the phase difference between the two signals at the input and output of the amplifier.