Page 168 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Sending such a nonsymmetric waveform to the mixer will cause leakage of the RF

signal to the output of the mixer, which is not preferred in a balanced-mixer
configuration.
in Chapter 10 concerning the one-transistor superheterodyne radio, it was
mentioned that having a huge-amplitude local oscillator signal added to the IF
signal hampers proper envelope detection of the IF signal. And thus multiple IF
filters are needed to sufficiently remove or attenuate the oscillator signal.

In this inductor-Iess radio, there is only one IF filter, so great care must be taken at
the design stage to provide a mixer/converter circuit that outputs essentially no
local oscillator signal nor the RF signal. This is the reason for using a balanced
mixer.

The output of the balanced mixer is fed to a second low-pass filter at around 1,600
kHz to remove the very high-frequency components from the balanced mixer owing
to the square-wave switching. The output of this second low-pass filter is fed to
amplifiers U6A and U6B. VR2 serves as a variable gain control to adjust the IF gain..
The output signal from U6B then is band-pass-filtered via R24, resonating capacitor
C29, and gyrator (active inductor) circuit U7A and U7B that has an inductance of
[(C30 x R28 x R25 x R27)/R26] = L. R24 sets the Q of the parallel tank circuit to

greater than 50 at a resonant frequency of 600 kHz. The 600-kHz IF signal from
the output of the band-pass circuit is further amplified via U8A and U8B before
being demodulated by 01. Audio signals via C35 then are connected to a crystal
earphone or to an audio amplifier. Also, a power switch may be connected in series
with the battery or power source to conserve energy.

Note that the inductor-Iess superheterodyne radio is simple to use. There are no
coils to adjust, and there are no mUltiple-gang variable capaCitors to deal with. And
this radio uses very common parts that are more available than the antenna coils,
IF transformers, ceramic filters, and/or variable capaCitors that were used in
previous superheterodyne radio designs.