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MX920 Base Station & Repeater
Technical Manual
The FX828 (IC29) is a flexible predictive or non-predictive, full duplex CTCSS /
Selcall encoder / decoder, DCS decoder, which is under the control of the
microprocessor.
5.2.2 TX Signal Processing
TX audio may be sourced from a number of different paths. These include Talk
through audio, microphone and tone generators (ie Selcall, DTMF, DCS, CTCSS
under micro controller control.
The microphone input is feed into a selectable compressor Via the built in menu
system, with 30dB dynamic range. Which is processed through a preemphasis,
summing amp which also serves to combine the audio from the talk through path.
Then pass through a HPF filter chain into a gain controlled latch circuit which is under
control of microprocessor. The tone generator (Morse code / Selcall) signals from the
microprocessor are applied to the input of the limiter amp. Following this the TX VF
low pass filter. The standard Bessel filter has a 3.4 kHz cut off frequency. A second
summing amplifier IC29B follows which combines the reference oscillator centre
frequency adjustment and CTCSS level adjustment. The CTCSS signal is fixed to
500Hz deviation for Wide Band channels and 250Hz deviation for Narrow Band (10%
of maximum deviation). This level is under microprocessor control (IC24C). The
output of this final stage is fed to two microprocessor-controlled digipots, which serve
to adjust the modulating signal level to the VCO and the reference oscillator.
Adjustment of these levels is by way of the built-in radio configuration menu system
using the Channel Edit screen.
The MX920 uses the technique of calibrating all Transmit Modulation level
alignments in the Wide Band mode (25kHz channel spacing) for maximum accuracy.
Gain setting resistor arrays switched under control by the microprocessor
automatically scale the levels so as to set the correct modulation depth as required for
Narrow Band channels.
5.2.3 RX Signal Processing
Raw Discriminator audio to the main ASIC VF processor FX828 which includes a
300 Hz Highpass and 3 kHz Lowpass filter. This output is followed by an additional
Lowpass filter and is used for the Talk Through Repeater (TTR) audio path and
followed by a de-emphasis network for final amplification to the speaker. This TTR
switch is under the control of the front panel Repeater Button via the microprocessor.
Audio Muting / Squelch is also done by the FX828.
As the Discriminator audio is already connected to the FX828, it can also perform
CTCSS / DCS and tone decoder functions.
Discriminator audio is also fed to the mute detection circuit. This comprises a high
pass filter followed by the front panel Mute / Squelch adjust POT, which sets the main
mute detect threshold. Following amplification by the op amps, full wave rectification
converts the noise level to a proportional DC voltage. A comparator, determines when
the detected signal passes a fixed threshold voltage. The output of the comparator is
fed to the logic and microprocessor.
Like the Transmitter Signal processing circuitry, the MX920 uses the technique of
calibrating all Receive Signal level alignments in the Wide Band mode (25kHz
channel spacing) for maximum accuracy. Gain setting resistor arrays switched under
control by the microprocessor automatically scale the levels so as to set the correct
recovered audio level as required for Narrow Band channels.
2003 Revision
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