Page 61 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Similarly, a second test oscillator is shown for tuning the 455-kHz IF amplifiers of

superheterodyne radios. Figure 4-4 presents a block diagram of the test oscillator
system.

ANTl
WIRE ANTENNA
Pulse-Am plitude
535 KHz T
Pulse Modulator
Generator AM Signals at 535 KHz and 1605 KHz
535 KHz

1 KHz

Generator AM Signals at 455 KHz and 1365 KHz
ANT2
WIRE ANTENNA
'\V
455 KHz AM Signal

Pulse-Am plitude
455 KHz
Pulse Modulator 455 KHz Filter
Generator
455 KHz

FIGURE 4-4 A block diagram of a test generator.
In this block diagram, there are two CW oscillators at 535 and 455 kHz. Both CW
oscillators produce pulses, not sine waves. Pulses also generate harmonic
frequencies of 535 kHz and 455 kHz. Thus, using pulse generators avoids the need
to build extra oscillators at other frequencies. For example, the third harmonic of

535 kHz is 1,605 kHz, and the third harmonic of 455 kHz is 1,365 kHz.
Each pulse generator is fed to a pulse modulator, which changes the amplitude of
the 535-kHz and 455-kHz pulses via a l-kHz audio generator. The output of the
modullators then provides amplitude-modulated (AM) signals at 535 kHz and 455

kHz and their respective harmonics.
A simple wire may be placed near the radio to confirm the tuning range of the radio
at the low end, 535 kHz, and at the high end, 1,605 kHz. The output of the
455-kHz modulator is connected to a 455-kHz band-pass filter to provide only an
amplitude-modulated (AM) signal at 455 kHz for IF amplifier alignment in
superheterodyne radios.

The entire test oscillator is shown in Figure 4-5. However, each functional section
will be covered separately.