Service manual

24
active mixer coupled by transformers. Injection is pro-
vided by the VCO through an injection filter. See Table
14 for local oscillator (LO) and first IF information.
The frequency generation function is performed by
three ICs and associated circuitry. The reference oscil-
lator provides a frequency standard to the
synthesizer/prescaler IC, which controls the VCO IC.
The VCO IC actually generates the first LO and trans-
mit-injection signals and buffers them to the required
power level. The synthesizer/prescaler circuit module
incorporates frequency-division and comparison cir-
cuitry to keep the VCO signals stable. The
synthesizer/prescaler IC is controlled by the microcon-
trol unit through a serial bus. Most of the synthesizer
circuitry is enclosed in rigid metal cans on the RF
board to reduce microphonic effects.
The receiver back end consists of a two-pole crys-
tal filter, an IF amplifier, a second two-pole crystal
filter, and the digital back-end IC. The two-pole filters
are wide enough to accommodate 5kHz modulation.
Final IF filtering is done digitally in the DSP-support
IC.
The digital back-end IC consists of an amplifier,
the second mixer, an IF analog-to-digital converter, a
baseband down-converter, and a 2.4MHz synthesis
circuit to provide a clock to the DSP-support IC on the
controller board. The second LO is generated by dis-
crete components external to the IC. The output of the
digital back-end IC is a digital bit stream that is current
driven on a differential pair for a reduction in noise
generation.
The transmitter consists of an RF PA IC that gets
an injection signal from the VCO. Transmit power is
controlled by two custom ICs that monitor the output of
a directional coupler and adjust PA control voltages
correspondingly. The signal passes through a RX/TX
switch that uses PIN diodes to automatically provide
an appropriate interface to transmit or receive signals.
Antenna selection is done mechanically in the control
top.
F. Controller Board Basic Theory of Operation
The controller board contains the radio’s micro-
control unit with its memory and support circuits,
voltage regulators, audio, DSP, and power control cir-
cuits. Connected to the controller board are the
display board, RF board, and control top.
The microcontrol unit controls receive/transmit fre-
quencies, power levels, display, and other radio
functions, using either direct logic control or serial
communications paths to the devices.The microcontrol
unit executes a stored program located in the FLASH
ROM. Data is transferred to and from memory by the
microcontrol unit data bus. The memory location from
which data is read, or to which data is written, is
selected by the address lines.
The support-logic IC acts as an extension of the
microcontrol unit by providing logic functions such as
lower address latch, reset, memory address decoding,
and additional control lines for the radio. The micro-
control unit controls the crystal-pull circuit to adjust the
crystal oscillator’s frequency on the microcontrol unit,
so that the E-clock’ s harmonics do not cause interfer-
ence with the radio’s receive channel.
The regulator and power-control circuits include
an unswitched +5V discrete circuit and the
regulator/power-control IC. Switched +5V is used for
all circuits on the controller board except the audio PA,
which is sourced from 7.5V. The regulator automati-
cally provides 5V when the radio is turned on. The
regulator’s power-down mode is controlled by the
microcontrol unit, which senses the position of the
on/off switch.
The DSP performs signalling and voice encoding
and decoding as well as audio filtering and volume
control. This IC performs Private-Line
®
/Digital Private
Line™ (PL/DPL) encode and alert-tone generation.
The IC transmits pre-emphasis on analog signals and
applies a low-pass (splatter) filter to all transmitted sig-
nals. It requires a 33MHz crystal to function. An 8kHz
interrupt signal generated by the DSP-support IC is
also required for functionality. It is programmed using
parallel programming from the microcontrol unit and
the DSP-support IC.
The DSP-support IC performs analog-to-digital
and digital-to-analog conversions on audio signals. It
contains attenuators for volume, squelch, deviation,
and compensation, and it executes receiver filtering
and discrimination. The IC requires a 2.4MHz clock to
function (generated by the digital back-end IC) and is
programmed by the microcontrol unit SPI bus.
LO Frequency Range 181.15-223.15MHz 329.65-446.65MHz 732.65-796.65MHz
First IF Frequency 45.15MHz 73.35MHz 73.35MHz
VHF UHF 800MHz
Table 14. Local Oscillator and First IF Frequencies