Page 138 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

radio-frequency (RF) signal from the secondary winding of L1. Many antenna coils

come with a secondary winding. But some antenna coils come with just a primary
winding. Thus a secondary winding can be made by winding about 10 to 20 turns
of wire near or on the primary winding of the antenna coil.
It should be noted that the secondary winding can be made with more turns than
usual because the input resistance to the mixer transistor Q3 is higher than usual
owing to the lowered collector current. For example, most antenna coils come with

a primary-to-secondary turns ratio of about 10 to 15: 1. For this project, a turns
ratio of 4 to 5: 1 will provide more RF signal into the mixer transistor while still
maintaining the high Q of the antenna coil.
With the RF signal added to the 170 mV of oscillator signal, mixer transistor Q3 is
driven into gross distortion such that there is a multiplying effect of the RF signal

and the oscillator signal. Thus the collector of Q3 includes a signal that is a
frequency-translated (455-kHz) version of the RF signal.
IF transformer T2 works as an inductor capacitor tank circuit tuned to 455 kHz. The
collector of Q3 is fed to the low-side tap of T2 so as to form a lower-impedance
load. At the primary, the turns ratio from the whole winding to the low-side tap is
about 3: 1. The equivalent parallel resistance across the whole winding of the

primary winding is about 500 k
n

. Thus the resistance at resonance at the low-side tap is 500 k
n

divided by 3 x 3 = 9, or about 55 k

By C16, the low-side tap of T2 is connected to the input of the first IF amplifier
transistor Q4, which has an input resistance of greater than 100 k
n

-

-
Note

The input resistance to a common or grounded emitter amplifier is the current gain

divided by (DC collector currentjO.026 volt). Assuming a current gain of 50 and a
DC collector current of 12 ~A, then the input resistance is about 50/0.000046mho =
108 k

IF amplifier Q4 also has a gain control, VR1, that provides a gain reduction of 8:1.
When VR1 is adjusted for maximum gain (VR1 = 0
fl