Circuit Description - Ramsey Electronics FR146 User Manual

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connections.
FR146 CIRCUIT DESCRIPTION
VHF signals from the antenna are amplified through the tuned input circuit (L1,
C3 and L2,C4) by Q1, a microwave bipolar transistor. Q1's output is fed to the
input of the SA602 IC, an efficient single-package (8-pin DIP) mixer-product
detector-oscillator. The tuneable oscillator section of the SA602 is aligned to
operate at 10.7 MHz higher than the signal fed and amplified by Q1. For
example, to receive 144-148 MHz signals, the oscillator must tune 154.7 to
158.7 MHz in order for the SA602's mixing capability to produce a steady 10.7
MHz output signal to the rest of the circuit. The oscillator frequency is
determined by L3 and its associated capacitors, and varied by the varactor
tuning network using D1 and varied by R1.
The output from pin 4 of the SA602 passes through a ceramic 10.7 MHz filter,
amplified by transistor Q2 and applied to input pin 18 of U2.
Q3 provides AFC (Automatic Frequency Control) by keeping the local oscillator
of U1 from drifting away from an incoming signal. This is accomplished by
tuning the varactor circuit in the direction opposite the drift.
The MC3359 IC has an internal oscillator controlled by the 10.24 MHz crystal.
The 10.24 MHz signal is mixed with the 10.7 MHz input from U1 to convert
down to the low IF of 450 KHz. The 450 KHz IF is filtered by FL2 and then
amplified by a limiting amplifier in U2. Audio demodulation takes place in the
quadrature detector, with L4 adjusting the detector.
The audio modulation is detected in U2, and the background noise is used to
control the squelch. U3 amplifies the audio output from pin 10 of U2 to a
practical level for speaker or headphone operation. Pin 16 of U2 will ground the
input of U3 when the squelch is closed.
L4, a 450 KHz IF coil, permits adjustment of the 90-degree voltage-current
phasing ("quadrature") of FL2's output to the FM detector demodulator (pin 8 of
the MC3359).
FR146 • 5