Page 167 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

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FIGURE 11-68 Inductor-Iess radio schematic diagram part 2-gyrator,
amplifiers, and detector.
The local oscillator signal is generated by a hysteresis oscillator circuit much like the
one seen in Chapter 4, which was used as an RF generator. Since the hysteresis
oscillator in Chapter 4 is sufficiently stable in frequency for generating an RF signal

to test a radiO, it is also stable enough to be used as a local oscillator in this
inductor-Iess receiver.
With an IF frequency set at 600 kHz, the desired local oscillator signal's frequency
to the balanced mixer is S40 kHz 1 600 kHz to 1,600 kHz 1 600 kHz, or 1,140 kHz

to 2,200 kHz. The hysteresis oscillator U3A must generate twice the frequency
because a divide-by-two circuit, U4A, halves the frequency of the oscillator.
Therefore, the output signal frequency of U3A must be adjustable via VRl to range
from 2,280 kHz to 4,400 kHz.
You may ask why not skip the divide-by-two circuit and connect U3A directly to the
mixer USA. It turns out that by dividing the oscillator's frequency by two via U4A, a

perfect square wave is used as the commutating signal for mixer USA. The square
wave is symmetric, which then allows the mixing or multiplying effect to be
balanced such that the RF signal from the low-pass filter does not appear at the
output of USA. The hysteresis oscillator does not generate a perfectly symmetric
waveform. That is, the logic high duration is not equal to the logic low duration.