User's Manual

Table Of Contents

Technical Manual Appendices

7.7.2 Link transceiver

In the UHF bands (400 to 520 MHz) the MX800 is available in 1W and 50W.

The 1W version being intended for link transceiver application. Transmit and

receive audio is conveniently brought out to the D15 connector on the rear of

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.

7.7.3 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.

7.7.4 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 e.g. 600Hz for 1200-baud data etc.