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126
BRITISH KINEMATOGRAPttY
Vol. 18, No. 4
the built-in loudspeaker and a proportion of this is also available for the 600 ohm line output circuit.
Muting Circuit
The valve V7, which provides A.V.C. for valves VI, V3, V5 and V6, also works in conjunction with V9 and Vll which constitute the carrier operated muting circuit (Fig. 4). The latter functions as follows: —
A D.C. voltage proportional to the incoming signal plus noise is smoothed and applied as a negative voltage to the grid of the D.C. amplifier V9. This causes an am
SIGNAL FP?OM Z.F. STAGES
noise plus signal at the grid of V9. The magnitude of this positive voltage can be adjusted to give resultant negative voltage at the grid of V9, which is proportional to signal alone. By this means RL1 is made to operate by an incoming carrier only, and is unaffected by variations in noise level, provided that the noise is fairly evenly distributed within the receiver pass band.
Fixed Station Transmitter
The design of this part of the equipment follows standard A.M. practice and incor
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NEGATIVE VOLTAGE DUE To NOI5E PLUS SIGNAL
H.T.+
R 24 AND R5.9 PORM POTENTIAL Divider MOLDING CATHODE V 7 AT ABooT ■+-2Q 'VOLTS D!
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^af SPEECH
AND NOISE
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SIGNAL. PL.USNOI5E HT.
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OUTPUT
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POSITIVE VOLTAGE PROPORTIONAL TO NOISE ALONE
AOJUST MUTE OPERATION &f BALANCING VOLTAGES X &'B
Pig. 4
carrier operated muting circuit (skeleton)
plified positive voltage to be applied to the grid of Vll, thus increasing the anode current of Vll and tending to operate the relay RL1 which connects the receiver output to the loudspeaker. A.F. voltage from the demodulator diode is fed to the grid of the A.F. amplifier V7 via a high pass filter. The filter removes all components of speech frequency, leaving an A.F. signal which is approximately proportional to noise alone, and is added to the negative voltage due to
porates a crystal oscillator to whose anode circuit is tuned (in the case of the 156-184 Mc/s) to the third harmonic of the crystal frequency. Two doubler circuits follow to provide a voltage at 12 times the crystal frequency for driving the grids of the double tetrode power amplifier (832).
The transmitter modulation amplifier comprises a first voltage amplifier followed by a pair of phase-splitting valves which in turn feed the grids of the two beam tetrode power