Frequency Generation Circuit
4.2
Voltage Controlled Oscillator (VCO)
(Refer to the Voltage Controlled Oscillator schematic diagram)
4.2.1
Receive VCO
The receive VCO is a Colpitts type design and using two active devices in parallel, Q202 and Q204.
The oscillator is powered off of the 4.3 volt super filter supply when the AUX3 line goes low. The
oscillator operates from 139 to 152 MHz for range 1 and 145 to 160 MHz for range 2. The frequency
is tuned by varactor diodes CR201 and CR202.
4.2.2
Transmit VCO
The transmit VCO is a Hartley type design with active devices Q203. The oscillator is powered off of
the 4.3 volt super filter supply when the AUX2 line goes low. The oscillator operates from 29.7 to 42
MHz for Range 1 and 35 to 50 MHz for Range 2. The frequency is tuned by varactor diodes in U203.
Note that the values of the inductive tap, L208 and L209, and the capacitor C215 which couples the
varactor to the oscillator tank vary between the ranges.
4.2.3
Buffer
Both the receive and transmit VCO are fed to a buffer amplifier Q201. This is a BJT amplifier that
boosts the signal levels to +4 dBm and provides reverse isolation to the oscillators. The amplifier is
powered off the 4.3 volt super filter supply and the feed network is combined with the transmit filter.
4.2.4
Diplexer and Output Filters
The output of the buffer drives a pair of parallel filters. One filter is a lowpass filter in the TX path that
passes 29.7 - 50 MHz signals for the transmitter into the power amplifier while rejecting the receive
LO injection signals at 139 - 160 MHz. This filter is comprised of L204, L211, L212, C230 and C231.
The other filter is a highpass filter which passes 139 - 160 MHz signals for the receive LO into the
mixer while rejecting the transmit injection signals at 29.7 -50 MHz. This filter is comprised of
C228,C229,C235 and L215.
4.2.5
Prescaler Feedback
The prescaler input signal for receive and transmit is tapped off the outputs of each filter by resistors
R234 and R238. This signal is routed to the buffer amplifier consisting of components C287, Q288,
R287, R288 and R289. The output of this buffer feeds U205 pin 32. After frequency comparison in
the synthesizer, current is transferred in the loop filter and a control voltage is generated at the
output of the loop filter to adjust the frequency of the VCO. This voltage is a DC voltage between
3.5V and 9.5V when the PLL is locked on frequency.
2-7
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