Page 25 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Circuit Description of a Superheterodyne Radio

The tunable RF filter is provided by variable capacitor VCRF and a 240-~H antenna
coil (Lp,;mary) with a secondary winding (LSecondary). The converter oscillator circuit
includes transistor Q1, which is set up as an amplifier such that positive feedback

for deliberate oscillation is determined by the inductance of Osc Transf 1 and
variable capacitor VC OSC. In superheterodyne circuits both the VC RF and VC OSC
variable capacitors share a common shaft to allow for tracking. At the base of Q1
there is the tuned RF signal, and at the emitter of Q1 there is the oscillator signal
via a tapped winding from oscillator coil Osc Transf 1. The combination of the two

signals at the base and emitter of Q1 results in a mixing action, and at the collector
of Q1 is a signal whose frequency is the sum and difference of the tuned RF
frequency and the oscillator frequency.
A first IF transformer (Tl IF) passes only the signal with a difference frequency,
which is 455 kHz in this example. The secondary winding of Tl IF is connected to
Q2's input (base) for further amplification of the IF signal. The output of Q2 is
connected to a second IF transformer, T2 IF. The secondary winding of T2 IF is

connected to the input (base) of the second-stage IF amplifier, Q3. It should be
noted that in most higher-sensitivity superheterodyne radios, a second stage of
amplification for the IF signal is desired. The output of Q3 is connected to a third IF
transformer, T3 IF, whose output has sufficient amplitude for detector D2 to
convert the AM 455-kHz signal into an audio signal.

Software-Defined Radio Front-End Circuits

A software-defined radio (SDR) is a superheterodyne radio in which there is a
minimum of hardware components that allow a computer or dedicated digital logic
chip to handle most of the functional blocks of the superheterodyne radio. So, in a
typical SDR, the front-end circuits mix or translate the RF channels to a very low IF
(e.g., <455 kHz, such as 5 kHz to 20 kHz). This very low IF analog signal then is

converted to digital signals via an analog-to-digital converter. The digital signal then
is processed to amplify and detects not only AM signals but also
frequency-modulated (FM) signals, single-sideband signals, and so on.
Fortunately, building a front-end circuit for a hobbyist's SDR is not too difficult. It
involves a wide-band filter, a mixer, a local oscillator, and low-frequency amplifiers
(e.g., bandwidths of 20 kHz to 100 kHz).

In the preceding description of a superheterodyne radio, a tuned filter preceded the
converter oscillator or mixer. The tuned filter passes the station frequency that is
desired and rejects signals from other stations to avoid interference. However, this
tuned filter also rejects an "image" station that has a frequency twice the IF
frequency away from the desired frequency to be received. Thus, for example, if

the tuned station is 600 kHz, and if the tuned filter does not sufficiently attenuate
an "image" station at 1,510 kHz (2 x 455 kHz 1 600 kHz = 1,510 kHz), the image
station will interfere with the 600-kHz station.