Patent GB-360891-A (GB)
Title: Electrical frequency control systems
IPC: H03L7/02
Filed: 1930-08-01
Abstract: 360,891. Valve circuits. STANDARD TELEPHONES & CABLES, Ltd., Columbia House, Aldwych, London. Aug. 1, 1930, No. 23216. [Class 40 (v).] High-frequency oscillations are synchronized by a " reactance control " method in which the effective reactance of a network is varied by variation of a resistance shunted upon or coupled to a part of the reactance of that network. This method of synchronizing is applied to frequency-dividers and multipliers and to single-side-band systems with varying carrier frequency. The controlled frequency may be equal to the controlling frequency or may differ from it by a fixed difference, or may be a multiple or submultiple of it. Two controlling frequencies may be employed simultaneously as a precaution against fading. A system in which output amplitude-variations depend on input frequency-variations but are independent of input amplitude-variations is described also (Fig. 8). Frequency multiplication and division. An oscillator A, Fig. 4, is tuned approximately to a high harmonic of the low-frequency standard S and is synchronized exactly with that standard as follows; it then serves as a standardized source of high frequency. By means of an aperiodic frequency-divider F, of the kind described in Specification 296,827, [Class 40 (v), Wireless signalling &c.], the oscillations from A are made to yield in the circuit 1, 2, a subharmonic frequency comparable with the frequency of the standard S. Both frequencies are applied to the grid of rectifier B, and the voltage-drop across the latter's platecircuit resistance R constitutes the grid-bias of absorber valve C. If the subharmonic frequency derived from A is too high as compared with that of the standard S, the negative grid-bias and consequently the plate-circuit resistance of C both increase. Coil D, which is wound on the same iron-dust core as the coils of oscillator A, has now a smaller shunting effect, so that the effective inductance of the oscillator increases and its frequency is reduced. A frequency-error in the opposite direction is similarly checked. In a frequency-divider for use in frequency measurement, Fig. 5 (not shown), the two frequencies applied to and compared by the valve B are (1) the high frequency which is to be measured and (2) a harmonic derived from a lower-frequency generator, which is pulled into synchronism with (1) in the manner described with reference to generator A, Fig. 4. The frequency of this generator is measured with an aperiodic frequency meter, and the required frequency is a known multiple of it. Synchronization at varying frequency. In a short-wave single-side-band system in which the carrier has to be reinserted at the receiving station, the local generator at that station is synchronized with the average frequency of the received pure carrier, which may be partially suppressed so as to have small intensity as compared with the sideband, and may fade for short intervals without disadvantage since its average frequency is unaffected by the fading. Long-period variations in the frequency of the received carrier are, however, followed by the local transmitter at the same frequency or at an interval (=the example described) of 4000 -. In this example the speech frequencies, covering a side-band up to 3000 #, are displaced upwards by 1000 # and inverted before transmission, while at the receiving station they are received by means of an intermediate frequency, 4000 # above the carrier, generated by the local oscillator A, Fig. 6, and passed to the receiver (not shown) over leads S. A synchronizing frequency, derived from the received carrier applied via terminals 5, 6 to screened-grid amplifier As and serves to control the frequency of the local oscillator A at an interval of 4000 #. To this end the outputs of A and As are both applied to the grid of rectifier C and the resulting beats are applied, via leads 7, 8, to valves H, K, in parallel. The latter are loosely coupled and so tuned (by simple tuned circuits or band pass filters) that their resonance points are spaced equally on opposite sides of the dlesired beat frequency (which is 4000 # in the present example). So long as the latter holds its correct value, the d.c. outputs of valves H, K are equal and points F, G are at the same potential and the grid bias of the absorber tube C is that of battery J, which brings the working point to the lower bend of the characteristic. If now the carrier frequency received at 5, 6 varies, the d.c. outputs of the unequally tuned valves H, K become unequal, the grid bias of valve C changes, and the resistance of that valve, in series with coil D, changes so as to correct the effective tuning inductance of the local oscillator A and to keep the latter's frequency spaced by 4000 # from that of the carrier. Gain control with frequency stabilization; frequency-demodulation. Signals applied to the rectifier B, Fig. 8, are passed on to output valve L through an arrangement such that variation in the input amplitude does not affect the output amplitude whereas variation in the input frequency does so. The latter variation may be used in the manner above described to stabilize the frequency. The rectified output from valve B is applied through transformer T to the valves H, K, in parallel, so that a change of amplitude in the received signals causes similar changes in the potential drops across resistances R1, R2' and consequently in the potential of the point Q, which is fed back through choke CK to the grid of rectifier B so as to keep the output of this rectifier independent of changes of input amplitude. Owing, however, to unequal tuning of the circuits D, E, a change in the input frequency produces opposite changes in the outputs from valves H, K, and a consequent change in the grid potential of valve L, that of point Q meanwhile remaining substantially independent of small variations of frequency. In a superheterodyne system, Fig. 9 (not shown), the local intermediate-frequency oscillator has its frequency stabilized by a reactance control device of the above type in which the amplitude of the input to the reactance-control device is checked by a gain-control device. Fading, preventing. In the arrangement shown in Fig. 6 (see above) the local oscillator A is synchronized with incoming carrier radiation received at As. The time-constant determined by resistances N, P and condenser Q is high enough to ensure that the carrier-frequency-measurement be,averaged over a sufficient long period to provide against the effects of fading. Or the transmitting station may send out two synchronizing waves, one slightly above and the other slightly below the limits of the side-band. These are both applied, Fig. 10 (not shown), through separate circuits of the kind shown in Fig. 8, to control the tuning of a local oscillator, so that if one of the pair fades the other still functions.