MX940 Base Station & Repeater
Technical Manual
© SPECTRA ENGINEERING
2022 Revision 1.0.0
6
1.2 Module Functional Description
1.2.1 RF Module and Main Controller Assembly
The MX940 consists of a full duplex RF module with its own shielded metal housing
and a Main Controller board integrated on a single PCB. Using advanced yet
simplified circuit designs, the size and complexity is reduced. This affords a number
of advantages including;
Cost reduction
Reduced number of components improves reliability and MTBF
Consistent and improved manufacture
Elimination of connectors and cabling
Reduction of human error
Faster maintenance or swap out
The Exciter module generates the low level, on frequency, RF transmitter signal
which is later amplified to nominal output power level by the Power Amplifier
module. The exciter consists of a Voltage Controlled Oscillator (VCO) and associated
main RF board, which, in conjunction with the reference oscillator and the PLL
circuitry, forms a two-point modulation programmable frequency synthesiser.
Frequency programming data is received from the Micro Controller via an 3 wire
serial data bus.
The exciter circuitry features a modulation bandwidth from DC for accurate CTCSS
and DCS generation and a wide RF switching bandwidth which covers the entire sub-
band. The average maximum RF output power is > +24.7 dBm / 300 mW. Normally
no adjustments are required, however should the carrier freq need re-alignment for
future maintenance, the TCXO reference oscillator frequency can be adjusted
manually or electronically adjusted via software settings.
The fractional N synthesiser provides ultra low spurii while still maintaining fast lock
times. The frequency step size is auto determined at 5.0 kHz or 6.25 kHz.
The receiver section accepts the low level RF input signal and amplifies, filters and
conditions the signal prior to detecting the wanted audio component. The Receiver
features the same advanced synthesiser and wide bandwidth as the exciter. The front
end bandpass filter uses very high tolerance components to minimise production
spread variations has a wide bandwidth and eliminates the need for future alignments.
The bandwidth is equal to the band allocation (refer to Section 5.4 for details of the
band allocations). The VCO has low phase noise and covers the full sub-band.
The receiver has high sensitivity while maintaining excellent Intermodulation
immunity and adjacent channel rejection. A dual first IF filter provides excellent
rejection to common known spurious responses. High blocking of over 100 dB typical
ensures that strong interfering signals do not desensitise the receiver when receiving
weak signals.
The Micro Controller section is physically located towards the centre on the main
board and controls all signal connections (apart from the RF connections). It controls
the operation of the RF sections and acts as the interface between the user controls,
indicators and the RF sub sections. Together with the VF DSP chip, processed
