Page 70 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

to pin 2 of U2A to provide an output signal via a short-wire (e.g., <12 inches)

antenna to a radio for testing.
Similarly, the 455-kHz CW oscillator circuit consisting of U1F, C3, R13, and R12
produces a pulse waveform. Variabl:e resistor R12 is adjusted to 455 kHz with a
frequency counter connected to pin 12 of U1F or RI!. The 455-kHz CW pulse
waveform is connected to inverter U2F's input pin 13, and the output of U2F's pin
12 is connected to a load or pull-up resistor R7. A modulating triangle waveform at

1 kHz provides a varying-voltage source to R7, and thus the output of U2B pin 12
generates a pulse-amplitude-modulated waveform at 455 kHz, 910 kHz, and 1,365
kHz with a modulating frequency of about 1 kHz. The output is also connected via
capacitor C6 to a short-wire (e.g., <12 inches) antenna.
A parallel resonance band-pass filter formed by R8, L1, C2, C4, R9, and R10 is also

connected to pin 12 to provide a sinusoidal 455-kHz amplitude-modulated (AM)
signal at R10. The signal from R10 will be used for aligning the IF amplifiers for the
superheterodyne radios later.
If a frequency counter is not available, an alternate way of adjusting R12 would be
to place the short-wire antenna from C6 near a radio (with a digital readout) that is
tuned to 910 kHz. Then adjust R12 until a l-kHz tone is heard loudest on the radio.

Similarly, R2 is adjusted by placing the wire antenna connected to C7 near a radio ..
Tune the (digital readout) radio to 1,070 kHz, which is twice 535 kHz. Adjust R2
until a 1-kHz tone is heard loudest on the radio.

Alternate Circuits
With a little more complexity, the 455-kHz and 535-kHz circuits can be done
without having to adjust for the correct frequencies. Crystals and ceramic

resonators are used instead, and these components are quite stable and accurate
in frequency generation.
Figure 4-12 shows that the Schmitt trigger oscillators have been replaced with a
crystal oscillator and digital dividers and a ceramic resonator oscillator. The l -kHz
modullating or audio oscillator stays in this design but can be replaced with a digital

frequency divider to produce modulating tone near 1 kHz.