Diplexer/Output Filters; Prescalar Feedback - Motorola PR860 Detailed Service Manual

Theory of Operation: Voltage Control Oscillators (VCO)

2.19.4 Diplexer/Output Filters

The output of the buffer drives a pair of parallel filters forming a diplexer. One filter is a lowpass filter
in the TX pass 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.

2.19.5 Prescalar Feedback

The prescalar input signal for receive and transmit is tapped off of 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.
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