Page 120 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

The converter circuit in this figure essentially is an oscillator circuit. Normally, the

oscillator circuit has the negative (-) input terminal connected to a voltage source,
which is an alternating-current (AC) ground. For now, let's take a look at just the
oscillator section. The output of the amplifier is connected to a secondary winding
of the oscillator coil/transformer (e.g., 42IF100). The primary winding of the
oscillator cOil/transformer has the required inductance with a variable capacitor to
span a range of about 1 MHz to 2 MHz for the oscillator signal. The inductance in
the primary winding is tapped down to a smaller voltage, as denoted by the K, a

scaling factor of less than 1. The tapped-down signal then is connected to the
positive input terminal of amplifier to form a deliberate positive-feedback system so
as to provide oscillation.
With the circuit oscillating, the transistor is driven with a relatively large oscillating

signal voltage at the positive (+) terminal, usually greater than a 200-mV
peak-to-peak sine wave. This large signal also causes the transistor amplifier to
distort and produce harmonic distortion at the output of the amplifier. When a
low-level RF signal from the antenna coil is combined with the amplifier via its
negative (-) input terminal, the combination of an RF signal and an oscillating
signal produces intermodulation distortion signals at the output of the amplifier. A
couple of the intermodulation distortion signals happen to have frequencies that are

the sum and difference frequencies of the RF signal and the oscillating signal.
These intermodulation distortion signals are attenuated (e.g., ignored) by the
oscillator's tan k circuit and do not affect the oscillator. But one of these
intermodulation distortion signals is extracted via an IF filter tuned to the difference
frequency or the IF frequency. And the output of the IF filter then is connected to

the input of an IF amplifier (not shown).
A Four-Transistor Radio Schematic

In 19S5, the Regency TR-1 was the first commercially manufactured
superheterodyne radio. It used four transistors for the following functions: the
converter, the first IF amplifier, the second IF amplifier, and the audio amplifier.
This radio ran off a 22.S-volt battery, which was common back then and up to the
1970s but is rare today.

For our first superheterodyne radio, we shall use a l.S-volt to 3-volt source instead
a 22.S-volt battery as well as four transistors, but as the converter, the IF amplifier,
the audio driver, and the audio power amplifier.
Parts List

• Cl, C4, Cl0: 33 IJF, 16 volts
• C2,C6, C9, C13: 0.01 J.JF

• C3, C7, C8: 1 IJF, 35 volts
• cs: 0.15 IJF
• Cl1, C12: 220 IJF, 16 volts