Page 72 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

• U4: 74HC393
• Yl resonator: 455 kHz
• Y2 crystal: 10.738 MHz
• Ll: 330 ~H
The generator circuit in Figure 4-12 produces waveforms at 455 kHz and 537 kHz
along with their harmonics by using precision resonators in place of the RC Schmitt

trigger oscillators from the previous design. Because of the precise frequency
generated in this design, no adjustment is necessary with a frequency counter or
with a radio. Also, the frequencies 455 kHz and 537 kHz are stable even if the
supply voltage varies by plus or minus 20 percent. After this circuit is built, one can
confirm that there are AM signals at 455 kHz, 910 kHz, 1,365 kHz, 537 kHz, and

1,611 kHz.
The 455-kHz signal is generated by U1A, an inverter biased up as a high-gain
amplifier via resistor R6. In order for oscillation to occur, there must be a
lS0-degree phase shift from the R7-C9 low-pass filter, the 455-kHz ceramic
resonator, and C7. The R7-C9 low-pass filter has a cutoff frequency at about 100
kHz, which ensures by 455 kHz that there are at least 77 degrees of phase shift

including the propagation delay of the inverter gate. The ceramic resonator acts like
an inductor to resonate with its internal series capacitance within the ceramic
resonator and with external capacitor C7. Thus the ceramic resonator with C7 forms
a phase-shift network capable of 90 degrees of phase shifting at resonance (455
kHz) and more than 90 degrees just slightly off resonance because of its very high

Q.
The output of the 455-kHz oscillator (U1A pin 2) is connected to the input of the
pulse-amplitude-modulator circuit U2A pin 1, which has an open-drain output at pin
2 of U2A (see Figure 4-7 and its description for an explanation of a pulse amplitude
modullator circuit).
To produce a precise frequency at the low end of the AM band, a 10.7386-MHz

crystal was chosen to eventuallly produce a 537-kHz signal. An 11.00-MHz crystal
can be used in place of the 10.7386-MHz unit as well, but the resulting frequency,
when divided down, will be 550 kHz instead, a little higher than 537 kHz.
The crystal oscillator works simi:larly as the ceramic resonator circuit. An inverter,

U1C, is biased as a high-gain amplifier via resistor R1. R2 and C2 are for a low-pass
circuit with a phase shift of about 70 degrees at 10.738 MHz. The inverter itself has
a propagation delay of about 8 ns, which is about 30 degrees of (lagging) phase
shift at 10.738 MHz. The remaining (80 or so degrees) of phase shift is produced by
the crystal's internal equivalent inductance and series capacitance inside and with
external capacitor Cl. The output of the inverter is connected via UID to a decade
frequency divider U2B (74HC390). The 74HC390 decade divider integrated circuit

has two divide-by-10 sections. And each divide-by-10 section has a divide-by-2 and
a divide-by-5 circuit. The 10.7386-MHz signal is divided by 10 to provide 1.0738