Patent GB-490625-A (GB)
Title: Improvements in radio-frequency control systems
IPC: H03J7/04
Filed: 1937-07-05
Abstract: 490,625. Valve circuits. PHILCO RADIO & TELEVISION CORPORATION. July 5, 1937, No. 18653. Convention date, July 31, 1936. [Class 40 (v)] A system of control of oscillation frequency comprises an oscillator and a resonant circuit associated therewith, a control voltage being derived from the oscillator circuit in phase quadrature with the voltage across said circuit and applied to a control valve which injects into the resonant circuit a reactance variable with the mutual conductance of the control valve, and means for applying to said control valve an in-phase voltage to oppose the effect of an undesired resistive component introduced in said resonant circuit. Fig. 2 shows a frequency discriminator network by which automatic frequency control biassing voltage may be derived from the I.F. circuits of a superheterodyne receiver. The circuits 1, 2 are tuned to the I.F., the phase quadrature relation between their voltages at the resonant frequency varying when the input frequency departs from its true value thereby producing a resultant rectified voltage across the series load resistances R1, R2 of the diodes D1, D2. The connection of the diode cathodes to the inductance L prevents leakage current paths through the cathode-heater circuits from changing the differential A.F.C. bias. The discriminator shown provides two differential biassing voltages built up across R3, R4 the common point of which is earthed ; if one bias only is required, this earth point is opened and one end of either R3, R4 is earthed. A.V.C. potentials are taken from the anode of amplifier V1 through rectifier D3 and network 6. Fig. 3 shows a method of controlling the frequency of an electron-coupled oscillator V2 the anode (second grid of V2) of which is back-coupled by coil L2 to the tuned grid circuit 7. The output circuit of V2 includes a capacity C, the A.C. potential across which is fed back to the control grid of a control valve V3 serving as a thermionic reactance across circuit 7. This potential is in phase-quadrature with that across circuit 7 which is in phase with the anode-cathode potential of V3 ; the conditions for the latter to act as a variable reactance are thus fulfilled. Due to the control gridanode capacity Cgp, and other causes, an inphase component is also fed to the grid of V3 by virtue of which V3 presents appreciable resistance across circuit 7. This is overcome by feeding a compensating voltage in phaseopposition to that through Cgp, by means of condenser C2. Alternatively the control reactance C may be modified to include a resistive component, e.g. a variable resistance r, small compared with the impedance of C. This also balances the in-phase resistive component owing to 180‹ phase shift between grid and anode circuits of V2. The phase quadrature voltage may be derived from a circuit, comprising a capacity, inductance and resistance. between the oscillator control grid and earth. The circuit is tuned to the midfrequency of the oscillator tuning band, the junction of condenser and inductance being connected to the thermionic control reactance, Fig. 4 (not shown). Fig. 6 shows an arrangement employing two differential A.F.C. bias voltages applied to a balanced pair of valves V5, V6, acting as a control reactance across circuit 7, the resistive component being thereby compensated without the use of condenser C2 of Fig. 3. The oscillator comprises a triode V7 and hexode V8 with their input grids connected, the anode of V7 being the oscillator anode ; this arrangement gives a more powerful oscillator. Interstation muting is obtained by biassing off the first grid of V8 from the circuit D4, D4, R7 across the A.F.C. bias leads. This also prevents " locking-in " of frequency on a strong signal. The phase quadrature voltage may be applied to the control grid of the thermionic reactance from the output circuit of a gain control valve to which the A.F.C. bias is applied, and the output circuit of the reactance may be inductively coupled to the controlled oscillator circuit 7, Fig. 7, (not shown). Or separate quadrature voltages may be applied in phase opposition to the input circuits of a balanced valve pair whose anodes are connected together and to the oscillator tank circuit. The quadrature voltage circuit comprises resistance, capacity and inductance in series, the latter being centre-tapped and connected to the control grids of the two valves, Fig. 8 (not shown). The use of two valves as the control reactance avoids variation in frequency due to variation of the slope of the valve due to ageing &c. The quadrature voltage derivation circuit may have as its control reactance a capacity, which reflects a negative inductance across the controlled circuit or an inductance which reflects as a negative capacity ; in such case the control sensitivity varies with frequency as shown in curves I and II, Fig. 11, respectively. To make the sensitivity more nearly uniform over the oscillator tuning band, e.g. 1À0-2À0 mc., the control reactance may be a series inductancecapacity circuit resonant at a frequency above the highest oscillator frequency, Fig. 13 (not shown) ; this gives curve III, Fig. 11. Or the reactance may be a parallel circuit tuned to a frequency below the lowest oscillator frequency, Fig. 14 (not shown), which gives curve IV, Fig. 11. The arrangement may be adapted to frequencymodulation of an oscillator by supplying the modulation frequency to the grids of the balanced pair of control valves, e.g. in Fig. 6 and in Figs. 5 and 8 (not shown) by an input circuit balanced about earth, linear frequency modulation being thereby obtained.