Page 131 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

capacitor is connected to the main primary winding of T7, whereas the secondary

winding of T7 provides RF signals to the mixer transistor Q2 via its base terminal.
And for automatic volume control, one side of the secondary winding of T7 is
connected to the AVC voltage source at CS. Figure 8-7 shows the alternative
oscillator and antenna coil circuit, including part of the mixer circuit Q2 and first IF
amplifier circuit Q3. This circuit can be "dropped in" or substituted for the oscillator
and tuned RF section circuits of the eight-transistor radio circuit shown in Figure
8-68.

Figure 8-8 shows another type of oscillator, a differential-pair transistor oscillator.
As stated earlier, the loop antenna is connected to the low-side tap of T7
(42IF100).

It was found that an IF transformer (the 42IFI04) has about 140 ~H from the
low-side tap of its primary winding. Because of interelectrode capacitances of the
oscillator transistor, it is undesirable to connect this low-side 140-~H tap directly to
a transistor. Instead, therefore, the stepped-down secondary winding is used. At
resonance, resistance "looking" into the 140-~H tap is very high, on the order of
100 kV or more, but looking into the secondary, this 100 k

gets divided by N x N, where N is the turns ratio from the low-side tap to the

secondary winding. For the 42IF104, N is in the range of about 14, which means
that at resonance the equivalent resistance across the secondary winding is about
500

(100 k

2
divided by 14 ).
The differential-pair transistor oscillator is formed by Q1A and Q1B. The collector of
Q1A is connected to the secondary winding of the 42IF104 IF transformer, which is
like a 500-

load. To ensure oscillation by positive feedback, the collector of Q1A is connected

to the base of emitter follower transistor Q1B, where its emitter is connected back
to the emitter of Q1A. With sufficient DC collector currents set for both transistors,
there will be enough gain to sustain a reliable osciUation. And the oscillator's signal
is taken from the collector of Q1A and added in series with the RF signal via the
secondary winding of T7.

It should be noted that the mixer circuit is modified slightly by grounding the
emitter of Q2 with a 0.15-~F capacitor and summing both osciUator and RF signals
into the base of Q2. Again, the oscillator signal is large enough to cause a
modulation or multiplying effect on the RF signal from the secondary winding of T7.