Motorola MTR2000 T5544, Инструкция / Руководство по обслуживанию на месте

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VHF Receiver Module
68P81096E19-B
3
09/30/05
The following theory of operation describes the operation of the receiver circuitry at a functional level. The infor-
mation is presented to give the service technician a basic understanding of the functions performed by the module
in order to facilitate maintenance and troubleshooting to the module level. Refer to Figure 2 for a block diagram
of the receiver module.
Synthesizer and VCO Circuitry
Introduction
The synthesizer and VCO circuitry generates the first LO injection signal
for the first mixer in the receiver front end circuitry. Functional operation
of these circuits is as follows.
Phase-Locked Loop
The phase-locked loop (PLL) IC receives frequency selection data from the
Station Control Module microprocessor. Once programmed, the PLL IC
compares a 2.1MHz reference signal (from Station Control Module) with
a divided-down feedback sample of the VCO output. Depending on
whether the feedback signal is higher or lower in frequency than the
2.1MHz reference, correction pulses are generated. The width of these
correction pulses is dependent on the amount of difference between the
2.1MHz reference and the VCO feedback.
The up/down pulses from the PLL IC are fed to a charge pump which out-
puts a dc voltage proportional to the pulse widths. This dc voltage is then
low-pass filtered and fed to the VCO as the CONTROL VOLTAGE.
If a frequency change is requested by the microprocessor, the low-pass
loop filter is momentarily bypassed to accelerate the frequency change.
VCO
The dc control voltage from the synthesizer is fed to dual VCOs which
generate the first LO injection signal. One VCO generates signals in the
upper half of the band, while the other VCO generates signals in the lower
half of the band. Only one VCO is active at a time. Selection of the active
VCO is provided by a BANDSHIFT signal from the PLL IC.
The active VCO responds to the dc control voltage and generates the ap-
propriate RF signal. This signal is fed through a buffer amplifier and im-
pedance matching and output to the first LO injection amplifier in the
receiver front end circuitry. A sample of the injection signal is returned to
the PLL IC (via a feedback buffer) to serve as a VCO feedback signal.
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FUNCTIONAL THEORY OF OPERATION
Note