Page 115 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Chapter 8

Superheterodyne Radios

In Chapter 7 on regenerative radios we found that positive-feedback systems can
enhance the selectivity and sensitivity for improved performance in a radio. Also,
we found that for a regenerative radio, best performance was achieved when we
added as much positive feedback or regeneration as possible, but just below the
threshold of oscillation. When oscillation occurs in a regenerative radio, we lose

performance.
The regenerative radio performs much better than "regular" tuned radio-frequency
(TRF) radios, but it still has limitations because the selectivity is determined by the
amount of regeneration added. For strong signals, the selectivity is less than when
a weaker signal is tuned. The reason is that the weaker signal will require mlore
regeneration, and thus more Q multiplication occurs for the weaker signal versus

the stronger one. Thus the selectivity is variable in a regenerative radio.
So is it possible to design a radio in which the selectivity can be determined by
some other means and such that the selectivity is "constant"? Yes, it is with a
superheterodyne radio (Figure 8-1).

In the figure, the superheterodyne radio has a two-gang variable capacitor. Signals
from the radio stations via the tuned filter (VC_RF) and a local oscillator (VC_Osc)
are combined into a frequency-translation circuit (aka mixer circuit) that produces
an amplitude-modulated (AM) signal at an intermediate frequency (IF). Essentially,
the mixer circuit "maps" the incoming RF signal's frequency to a new frequency
such as 455 kHz, an IF frequency. The IF circuit includes a band-pass filter
centered around the IF. This filter passes signals at or near the IF while rejecting or

attenuating all signals outside the IF band. Demodulation of the AM signal at 455
kHz from the output of the IF amplifier is done with conventional envelope or
power-detector circuits.
In a superheterodyne receiver, there are four new elements, a two-ganged (or

more) variable capacitor, a local oscillator, a mixer, and an IF filter/amplifier.
Demodulation is still done with envelope or power detectors. Now let's 90 over
these four elements briefly:
1. A ganged variable capacitor has a common shaft to turn the rotor plates of two
or more sections. The two sections may be identical, or more commonly, the two
sections are not identical. One section serves as the RF tunable filter (e.g., 540 kHz
to 1,600 kHz), whereas the other section resonates with an OSCillator coil and varies

from about 1 MHz to 2 MHz for a 455-kHz IF.