Motorola MTR2000 T5544 Instruction / Field Service Manual page 165

VHF Exciter Module
Data Communications
4
The active VCO responds to the dc control voltage and generates the ap-
propriate RF signal. This signal is fed through impedance matching, am-
plification, and filtering and is output to the RF Switch Circuitry. A sample
of the output is returned to the PLL IC to serve as a VCO feedback signal.
Modulation
The active VCO receives an audio/data modulation signal from the Sta-
tion Control Module via a splatter filter and a low-pass filter/attenuator.
This VCO modulation signal modulates the active VCO to produce a mod-
ulated low-level RF carrier signal.
Low-frequency modulation signals (below the loop filter corner) tend to
be interpreted by the PLL as VCO frequency error. A modulation com-
pensation signal is added to the PLL control voltage to cancel out this ef-
fect and allow for low frequency modulation.
RF Switch Circuitry
The modulated RF signal from the VCO is fed through a buffer/attenuator
to an RF switch circuit. Signal TX ENABLE from the SCM controls the
switch. The RF signal is output to a BNC connector mounted in one corner
of the module, just outside the module cover. A short coaxial cable con-
nects the exciter output to the power amplifier module.
Communications with Station Control Module
Data communications between the exciter and the Station Control Module
microprocessor (μP) is performed via a Serial Peripheral Interface (SPI)
bus. This bus allows the SCM μP to send data to the synthesizer PLL IC
(to select frequency) and to read the A/D Converter IC (next paragraph).
A/D Converter Circuitry
Analog signals from various strategic operating points throughout the ex-
citer board are fed to an A/D converter, which converts them to a digital
signal and, upon request by the Station Control Module, outputs the signal
to the Station Control Module via the SPI bus.
68P81096E22-C
09/30/05