MX800 Transceiver Technical Manual
© SPECTRA ENGINEERING 2006 Revision 4.2.3
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the radio along with the mute and PTT signals. In addition, provision is made
to inject an Analog CTCSS tone into the transmit leg of the link and a 300Hz
low pass filtered CTCSS signal is available at the RX leg of the link to
recover the tone. Hence the Analog CTCSS tone can be transferred (or cross
banded) from one MX800 to another. On an RF path with good signal to
noise ratio, this enables CTCSS synchronization across the link in the
situation where multiple tone group operate at two different sites. Spectra
Engineering plans to upgrade this function with a digital 4 bit interface such
that the operating CTCSS group is decoded at the repeater and re-encoded or
cross encoded across the link, and the process repeated in reverse at the
remote end.
7.5.3 Data transceiver
The MX800 transmit audio path is user configurable to a very large degree.
TX modulation signals can be injected with or without processing depending
upon the individual application. The transmitter low pass filter (the deviation
limiter which sets the modulation bandwidth and ensures compliance with
regulatory requirements) is a Bessel filter with linear phase characteristics.
Fast transmitter key-up time and mute action mean reduced signalling turn
around overheads for data messages and better throughput especially in a
Simplex system with predominantly short messages.
7.5.4 Paging transmitter
Due to the two point modulation method employed in the MX800, the
transmit modulation frequency response can go down to DC as correctly
required for POCSAG or other FSK based modulation systems. In addition it
has a DC coupled FM modulator input biased to a voltage of 2.5 volts for
carrier frequency Fo. A square wave input signal of 0 to 5V injected on this
point drives the modulator to the positive and negative extremes of the
deviation limiter. In this case set to +/- 4.5kHz. This means that it can
transmit typical POCSAG paging signals. The modulation rise time is
controlled by the frequency response of the transmit audio path low pass filter
and is suitable for data rates of up to 2400 baud.
A typical system arrangement is shown in drawing
SC004-1A
.
SYSTEM ALIGNMENT
The FSK link deviation should be set to 60% of maximum deviation.
The paging transmitter alignment is similar to the standard alignment in
paragraphs 5.1.9 and 5.1.10 in the MX800 technical manual. In 5.1.9
“Procedure” item 3 set the peak deviation to +/-4.5kHz (instead of +/-
4.8kHz). In item 5 set the TX modulation frequency to half the data rate of
the POCSAG data eg 600Hz for 1200-baud data etc.
Other than these changes the TX alignment is as per the technical manual.
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