VHF Exciter Module
68P81096E22-C
3
09/30/05
The following theory of operation describes the operation of the exciter 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 exciter module.
Synthesizer and VCO Circuitry
Introduction
As mentioned previously, the exciter module generates a low-level modu-
lated RF signal which is input to the power amplifier module. The RF car-
rier is generated by a frequency synthesizer consisting of synthesizer
circuitry and VCO circuitry. Exciter module control signals, monitoring,
and audio processing are handled by the Station Control Module. Func-
tional operation of the exciter circuits is as follows.
Phase-Locked Loop
The phase-locked loop (PLL) IC receives frequency selection data from the
SCM microprocessor (via the SPI bus). Once programmed, the PLL IC
compares a 2.1MHz reference signal (from the SCM) 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, up/
down correction pulses are generated. (The width of these correction puls-
es depends on the quantitative 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 SCM microprocessor, the low-
pass loop filter is momentarily bypassed to accelerate the frequency
change (via a SYNTH ADAPT signal from the SCM).
VCO
The dc control voltage from the synthesizer is fed to dual VCOs which
generate the RF carrier signal. Within each band, one VCO generates sig-
nals 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.
3
FUNCTIONAL THEORY OF OPERATION
Note
Содержание MTR2000 T5544
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