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April, 1951
Iayton: business radio
125
The Fixed Station Receiver is a crystal controlled double superheterodyne, of sensitivity such that for a signal input of 1 microvolt (modulated 30% at 400 c/s), an output of 50 milliwatts is delivered to the built-in loudspeaker. The R.F. arrangement is as follows (Fig. 3): —
The input signal of frequency /o is fed from the aerial circuit via the R.F. amplifier VI to the grid circuit of the first mixer V3. A voltage from the crystal oscillator is multiplied through V4 and V2 and is also injected
oscillator voltage at 16.Y Mc/s for V3. Thus for spot frequency reception at 172.9 Mc/s the crystal frequency will be
172.9-2.9
=10 Mc/s
17 and the oscillator frequency for V3 will be 16X=160 Mc/s. This frequency beating with /o will produce a first I.F. of 12.9 Mc/s, which itself will beat with the crystal fundamental in V5 and provide a second I.F. of 12.9-10 = 2.9 Mc/s.
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into the grid circuit of V3. The difference frequency or 1st I.F. is such that when fed to the second mixer V5, it beats with the crystal fundamental frequency X (also fed to V5), and produces a second I.F. of 2.9 Mc/s. Interpreting this procedure for frequencies within the high band (156-184 Mc/s), the required crystal frequency is found from the formula
/o-2.9 Mc/s,
X =
17 and the circuits of V4 and V2 multiply this frequency four times each to provide an
For low band operation (70-100 Mc/s) the multiplication factor is reduced by half, and calculation of the crystal frequency is modified accordingly.
The second I.F. chain consists of two valves and eight tuned circuits in the form of four over-coupled band-pass transformers. A series-shunt diode noise limiter is operated by the audio signal from the demodulator valve, and this has the effect of suppressing impulsive noise of the type due to car ignition systems.
An audio output of 2 watts maximum provided by the 6V6 GT output valve is fed to