User's Manual

APPLICANT: MOTOROLA EQUIPMENT TYPE: ABZ89FC3795
User / Operational Manual
Functional Description / Operation of Modules (Draft)
EXHIBIT D1-6
Functional Theory of Operation
The following theory of operation describes the operation of the Exciter circuitry at a functional level. The
information 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 a block diagram of the
Exciter module.
Functional Overview
Synthesizer and VCO Circuitry
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 reference frequency signal with a divided-down feedback sample of
the VCO output. The reference frequency is selected depending on the frequency band of operation, and will
range from 3.36 MHz to 16.8 MHz. Depending on whether the feedback signal is higher or lower in frequency than
the reference, up/down correction pulses are generated. The width of these correction pulses depends on the
quantitative difference between the reference and the VCO feedback.
The up/down pulses from the PLL IC are fed to a charge pump which outputs 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.
Voltage Controlled Oscillator (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 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 VCO_SELECT
signal from the PLL IC.
The active VCO responds to the DC control voltage and generates the appropriate RF signal. This signal is fed
through impedance matching, amplification, 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 Exciter uses dual-port FM modulator consisting of Digital Signal Processor (DSP), PLL low-port and high port
blocks, fractional-N PLL, loop filter and the VCO. The modulator is calibrated once during factory test and its
values are stored in the EEPROM. The modulator does not require any further tuning or maintenance.
Amplifiers
The amplifiers consist of a driver amplifier and the final amplifier. The driver amplifier operates from a nominal 5V
supply and the final amplifier operates from a nominal 10V supply. The driver amplifier amplifies the VCO output
signal and provides the output signal to the final amplifier. There are fixed attenuators at the output of the driver
and final amplifier to set the Exciter output power level. The final amplifier amplifies the signal and provides a
constant signal level to the PA.
RF Switch Circuitry
The modulated RF signal from the VCO is fed through a buffer, driver, and attenuator and finally to an RF switch
circuit. Signal EX_ENABLE from the SCM controls the switch and final amplifier. The RF signal is passed through
a LPF and to a BNC connector mounted in one corner of the module, just outside the module cover. A short
coaxial cable connects the Exciter output to the power amplifier module.