Service manual

July 1, 2002 68P81076C25-C
2-2 General Overview: Analog Mode of Operation
2.2 Analog Mode of Operation
When the radio is receiving, the signal comes from the antenna/antenna-switch on the power
amplifier board to the front-end receiver assembly. The signal is then filtered, amplified, and mixed
with the first local-oscillator signal generated by the voltage-controlled oscillator (VCO). The resulting
intermediate frequency (IF) signal is fed to the IF circuitry on the RF board, where it is again filtered
and amplified. This amplified signal is passed to the digital back-end IC, where it is mixed with the
second local oscillator to create the second IF at 450 kHz. The analog IF is processed by an analog-
to-digital (A/D) converter, where it is converted to a digital bit stream and divided down to a baseband
signal, producing digital samples. These samples are converted to current signals and sent to the
DSP support IC. The digital-signal-processor-support IC digitally filters and discriminates the signal,
and passes it to the digital-signal processor (DSP). The DSP decodes the information in the signal
and identifies the appropriate destination for it. For a voice signal, the DSP will route the digital voice
data to the DSP-support IC for conversion to an analog signal. The DSP-support IC will then present
the signal to the audio power amplifier on the command board, which drives the speaker. For
signalling information, the DSP will decode the message and pass it to the microcomputer.
When the radio is transmitting, microphone audio is passed to the command board limiter then to the
DSP-support IC, where the signal is digitized. The DSP-support IC passes digital data to the DSP,
where pre-emphasis and low-pass (splatter) filtering are done. The DSP returns this signal to the
DSP-support IC, where it is reconverted into an analog signal and scaled for application to the
voltage-controlled oscillator as a modulation signal. Transmitted signalling information is accepted by
the DSP from the microcomputer, coded appropriately, and passed to the DSP-support IC, which
handles it the same as a voice signal. Modulation information is passed to the synthesizer along the
modulation line. A modulated carrier is provided to the power amplifier (PA) board, which transmits
the signal under dynamic power control.
2.3 ASTRO Mode of Operation
In the ASTRO mode (digital mode) of operation, the transmitted or received signal is limited to a
discrete set of deviation levels, instead of continuously varying. The receiver handles an ASTRO-
mode signal identically to an analog-mode signal up to the point where the DSP decodes the
received data. In the ASTRO receive mode, the DSP uses a specifically defined algorithm to recover
information. In the ASTRO transmit mode, microphone audio is processed identically to an analog
mode with the exception of the algorithm the DSP uses to encode the information. This algorithm will
result in deviation levels that are limited to discrete levels.
2.4 Control Head Assembly
This section discusses the basic operation and components of each control head assembly.
2.4.1 Display (W3 Model)
The control head assembly for a W3 model has a two-line, 14-character liquid-crystal display (LCD)
with eight Status annunciators.
2.4.2 Display (W4, W5, and W7 Models)
The control head assembly for W4, W5, and W7 models has an 8-character, alphanumeric, vacuum
fluorescent display. The anodes and the grids operate at approximately 34 Vdc when on and 0 Vdc
when off. The filament operates at approximately 2.4 Vac. The voltage for the display is generated by
a fixed-frequency, variable duty-cycle controlled “flyback” voltage converter. The switching frequency
is approximately 210 kHz. The internal microprocessor controls the voltage converter, which
provides approximately 37 Vdc to the vacuum fluorescent (VF) driver and approximately 2.4 Vrms to
the VF display.