Patent GB-587714-A (GB)
Title: Improvements in and relating to adjustable electrical phase-shifting networks
IPC: H03H7/20
Filed: 1945-01-09
Abstract: 587,714. Phase-shifting networks; frequency modulation. WIGAN, E. R. Jan. 9, 1945, No. 700. [Classes 40 (iii) and 40 (v)] In a phase-shifting network having shunt and series arms, each containing resistances and reactances, the frequency at which the phase-shift is zero is varied by means of an additional impedance element connected between a tapping on the series arm and a tapping on the shunt arm. By choosing the tapping points according to certain conditions, the zero phase-shift frequency may be varied by means of the additional impedance, but the voltage transfer ratio at this frequency kept constant. The network may be used in the feed-back circuit of a phase-shift oscillator and provides a convenient method of frequency modulation. The invention is shown in generalized form in. Fig. 3, a tapping on the series arm comprising impedances 10, 1 1 being connected to a tapping on the shunt arm comprising impedances 12 ... 15, by means of the variable impedance 16. Impedance 15, if used, is normally a resistance r1, and has the effect of increasing the possible frequency range. A similar effect may be obtained by including a series resistance r2 in impedance 14, if it is normally reactive, or a series reactance if it is normally resistive. In one embodiment, Fig. 2, a resistance capacitance network has tappings on its series and shunt resistances connected by a variable resistance and in another embodiment, Fig. 7, a tapping from the series capacity 18, 19 is connected by a variable condenser 23 to a tapping from the shunt capacity 21, 22. The condensers 18, 19, Fig. 7, may be replaced by a delta formation of condensers, Fig. 9 (not shown), with the variable condenser connected to the apex of the delta and similar substitution may be made for condensers 21, 22. A similar substitution may be made where the impedances concerned are resistances or inductances. Additional impedances Z3 and Z4, Fig. 10 (not shown), may be introduced as desired in the series and shunt arms without affecting the zero phase shift frequency or the voltage transfer ratio (L) provided Z3/Z4=(I-L)/L. In the oscillator, Fig. 11, a resistanceinductance phase-shift network is utilized, having tappings from the resistive elements in the series and shunt arms connected by the variable resistance 44 and the shunt inductance 39 forms part of the step-up transformer in the feed-back -circuit from cathode to grid. The variable resistance 44 may be a carbon microphone which by variation of the zero-phaseshift frequency of the network, frequency modulates -the oscillations of the valve 30. Resistances r1 or r2, as described above, may be inserted to vary the frequency-resistance characteristic of the oscillator to compensate for non-linearity in the microphone. Other networks as described above, may be used. Alternatively, one of the networks may be incorporated in one of the couplings in the usual type of resistance-reactance oscillator having two valves, each grid coupled to the anode of the other. Frequency modulation by remote control may be effected by varying the potential applied to a rectifier, Fig. 12 (not shown), or rectifier bridge Fig. 13, which is then used as thevariable impedance in the feed-back network -here shown as of the resistance-capacity type. Instead of the rectifier bridge an indirectly heated thermistor may be used. The Provisional Specification also states that the invention may be applied to frequency bridges and that for frequency modulation the variable impedance may be provided by a thermionic valve device controlled by the modulating voyage.