Page 123 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

at the emitter of Ql, which is typically less than 500 V to avoid degrading the Q of

the parallel resonant tank circuit, consisting of the inductance in the primary
winding of Tl and capacitor Vel_Osc. The base of Ql is biased by diodes Dl and
D2 to provide a DC bias voltage at the base of Q1 of about 1 volt, which also sets
the DC collector current for Q1. R2 is chosen to set a sufficient emitter current
(e.g., fV200 IJA) to provide enough voltage gain in transistor Q1 to reliably produce
oscillation.

Because the secondary winding of Ll provides the RF signal to the base of oscillator
transistor Ql, multiplication of the oscillator signal with the RF signal occurs. And
the output signal at the collector of Ql includes an amplified frequency-translated
signa:1 version of the RF signal to 455 kHz.
Note that T2, the first IF transformer's primary, via its tapped winding is in series

with the secondary winding of Tl, the oscillator coil. By this series connection, the
4SS-kHz (IF) signal is extracted from the collector of Q1.
The secondary winding of T2 is also stepped down to allow connection to the base
of the IF amplifier's transistor Q2, which has a moderate input resistance (e.g., in
the few thousands of ohms) so as not to degrade the Q at the primary of T2.

At the output of the IF amplifier's transistor Q2, the collector is connected to the
second IF transformer. The AM signal from second IF transformer's secondary is
demodulated via diodes D3 and D4. One may ask why two diodes in series? The
second diode performs the function of level shifting up the voltage by about 0.5
volt DC, which is needed in an automatic volume-control system with transistor Q2.
Thus diodes D3 and D4 perform envelope detection, and through biasing resistor

RS, the two diodes also level shift the detected lower half of the AM envelope to a
voltage of about 1 volt DC when the RF signal is zero or weakly received. This
1-volt signal is further filtered by R4 and C4 and applied to the secondary winding
of T2 to bias the IF transistor Q2. When a signal is received, the demodulated
signal at C6 is sitting at 1 volt DC with a negative-going AC audio signal. Low-pass
filter R4 and C4 filter out the AC audio signal and provide a DC voltage that is 1 volt

minus the average carrier level of the received signal. The stronger the received RF
signal, the more the average carrier level is "subtracted" from the l-volt DC signal
at C4. By lowering the biasing voltage for Q2 depending on how strong the
received signal is, Q2 performs an automatic gain or volume control for the
demodulated audio signal. Note that if there were only one diode instead of D3 and

D4, maximum DC voltage would be 0.5 volt, which is insufficient to turn on the
base of Q2 (e.g., Q2 requires at least 0.6 volt on the base).
It should be noted that the second IF transformer here or, in general, the Ilast IF
transformer before diode detection usually has a lower turn ratio from primary to
secondary windings compared with any of the other IF transformers. The reason is
to provide more IF signal to the diode detector, which also generates sufficient AVC

voltage. For example, if T3 is changed from a 42IF103 part to a 42IF10l or a