Patent GB-760906-A (GB)
Title: Pilot regulator
IPC: H04B3/10
Filed: 1954-12-21
Abstract: 760,906. Automatic volume control. AUTOMATIC ELECTRIC LABORATORIES, Inc. Dec. 21, 1954 (Dec. 22, 1953], No. 36928/54. Class 40 (4). A pilot wave, which is transmitted continuously and at substantially constant amplitude to control the regulation of an associated communication channel, is frequency modulated to convey auxiliary signalling pulses pertaining to the channel. The invention is described as applied to a single-sideband multi-channel carrier system, the receive equipment for one channel only being shown. The channel sub-carriers are assumed to be uniformly spaced at 4 kc/s., the lower 3200 cycles of each channel width being used for message frequencies and the signalling being effected by shifting the frequency of the pilot from 3400 c.p.s. to 3550 c.p.s. The incoming message and pilot waves of a channel are applied via band-pass filter 1 to the transformer 3, the secondary of which is centretapped to form two arms of a bridge circuit. The other arms consist of a fixed resistor 7, and a thermistor 9 in series with a blocking condenser 11, respectively. The impedance of the latter arm shunted by resistor 12 is slightly less than that of resistor 7 when minimum control current is supplied to the thermistor 9, and the unbalance output of the bridge is applied to transformer 13. Increase in the control current decreases the impedance of the thermistor, unbalances the bridge further, and increases its transfer coefficient. The mixed message and pilot waves are demodulated at 14, the demodulation products passing via low-pass filter 17 (cut-off 4 kc/s.) and matching transformer 19 to amplifier 21. Secondary 39 of output transformer 35 provides negative feed-back of about 12 db to stabilize the amplifier 21; secondary 43 feeds low-pass filter 45 which cuts off sharply between 3200 c.p.s. and 3400 c.p.s., and the output of which is connected to the usual voice frequency terminal apparatus; and secondary 47 feeds a narrow band-pass filter 49 which attenuates 3400 c.p.s. and 3550 c.p.s. equally, and the output of which is connected to the pilot amplifier 51. One secondary winding 67 of the output transformer 65 of this amplifier is connected to a discriminator 69 which demodulates the frequency-shifted signals and feeds them to the differential relay 71. A second secondary 73 is connected between A.C. ground (via blocking condenser 75) and the control grid 55 (via biasing resistor 53) to provide negative feedback of about 20 db. The gain of this amplifier is stated to be constant to within a fraction of 1 db irrespective of the individual characteristics of the specific tube used and of the changes in anode current through which regulation is effected. A third secondary winding 77 supplies a rectifying circuit 79 having an integrating condenser 81 in series therewith, a resistor 83 in series with a resistor 85 having a variable top 86 being connected across the condenser 81. A clamp circuit 87 connects tap 86 to ground, and tap 86 is also connected via resistor 89 to the low potential end of winding 73. The potential divider 91, 93 connected between the anode supply 37 and ground applies a comparison voltage (15.2 volts) between one side of condenser 81 'and ground. With no current flowing in the rectifier circuit (i.e. no pilot), a second path to ground is available via resistors 83, 85, tap 86 and the clamp circuit 87 which reduces the drop across resistor 93 to form 11.5 to 12.5 volts depending on the setting of the tap 86. The tap 86 and hence the grid 55 are at ground potential and the tube 51 is biased by cathode resistor 59 alone to give maximum normal tube current. The voltage across resistors 83 and 85 backbiases the rectifiers 79, so that no current flows in the rectifier circuit until the pilot signal in winding 77 exceeds this voltage. When this occurs, the voltage drop produced by the rectifier current will become equal to that produced by the current from source 37 across resistor 93, whereupon the clamp circuit ceases to conduct. The regulator then becomes effective: further increase in pilot amplitude increases the potential drop across resistors 83 and 85, thus depressing the potential of tap 86 below ground and applying a negative bias to the control grid 55 so as to reduce the anode current of tube 51. The integrating circuit has a time constant less than 0.01 seconds, which is adequate to remove substantially all pilot frequency components from the rectified signals. The change in mean anode current has negligible effect upon the amplification, but is used to control thermistor 9. The major portion of the D.C. component of the anode current flows through lead 95<SP>1</SP>, choke 97<SP>1</SP> and thermistor 9; resistor 97, which serves to adjust the current flow through the thermistor to a desired working range, being of relatively high value.