GB554836A - Improvements in and relating to frequency variation response network - Google Patents

Abstract

554,836. Frequency discrimination ; automatic frequency control. BRITISH THOMSON-HOUSTON CO., Ltd. Jan. 16, 1942, No. 660. Convention date, Jan. 17, 1941. [Class 38 (iv)] [Also in Groups XL and XXXVI] Deviations in the frequency of a carrier wave are indicated or measured by applying the carrier across one diagonal of a bridge circuit and connecting a circuit or crystal, resonant to a standard frequency, across the other diagonal ; the bridge arms are unbalanced, one pair being constituted by reactances, and the other by rectifiers, and the magnitude and polari y of the difference ot the rectified current is indicative of the frequency deviation. The circuit may be used as a demodulator of a frequency-modulated wave, or as an error detector for the regulation of the tuning of a radio transmitter or receiver, or for the measurement of frequency. The F.M. detector shown comprises an amplifier 10 feeding the carrier across a point between two condensers 14, 15, and an earth point connected to the cathodes of the rectifiers 16, 17. The condensers 14, 15 and the rectifiers 16, 17, form the arms of a bridge circuit, the horizontal diagonal of which includes the circuit 13 resonating at the standard frequency. The carrier is thus applied directly to the rectifiers in phase agreement, and owing to the unbalance of the bridge the circuit 13 is energized and applies voltages in phase opposition to the rectifiers, these voltages being in quadrature with those directly applied when there is no frequency deviation, but varying in either sense from the quadrature relation when the frequency differs from the standard. Voltages taken at lead 23 across the joint lead resistances 18, 19, are a measure of the frequency deviation. The bridge may be unbalanced by connecting a variable inductance 22 across one or both rectifiers, or by using condensers 14, 15 of unequal capacity, or by using diodes of unequal size. The input from valve 10 may be to a point on the inductance of the circuit 13 displaced from the mid-position. Amplitude modulation may be detected by a rectifier 27 associated with a tuned circuit 24, 25, 26 coupled to the circuit 13. The Specification describes a circuit for the control of the frequency of a radio transmitter, Fig. 2 (not shown) in which a piezo-electric crystal replaces the tuned circuit 13 and in which the voltage on the lead 23 is smoothed out and used to correct the frequency of the transmitter by applying it to the grid of a reactance valve associated with the oscillation generator. The reactance valve may also be used for applying frequency modulation to the generator from a microphone or other signal source. The piezo-electric crystal may be operated in the range in which it exhibits either series-resonant or parallel-resonant characteristics ; or two crystals operating respectively in these ranges may be used in parallel; or a parallel resonant crystal may be connected in parallel with a series-resonant circuit.