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MX920 Base Station & Repeater
5. Technical Description
The internal design of the MX920 is of a modular nature allowing for simple
configuration and maintenance while ensuring minimal downtime.
5.1 Exciter Section
RF from the VCO is at a nominal level of +3 dBm is applied to the fractional-N
synthesiser main divider input. This signal is compared with the reference oscillator
frequency and the correction voltage from the synthesiser's charge pump output is
filtered then amplified by a non inverting low noise op amp. This correction voltage is
fed back to the VCO to maintain loop lock as well as being fed to the Micro
Controller. A lock detect signal is also fed to the Micro Controller. The op amp uses a
15 volt power supply from the DC to DC converter so as to provide a wide tuning
range voltage to the frequency control varicaps located in the VCO section. Frequency
programming data for the exciter is sent to the synthesiser chip from the Micro
Controller via a serial data line under the control of the Clock and Strobe lines.
A second RF output from the VCO also at +3 dBm is used as the main transmit RF
amplifier signal source. This main signal is first buffered by a very high isolation
circuit consisting of a cascode MMIC amplifier. The signal is further amplified by a
variable gain wide band bipolar amplifier with 40 dB control range and power output
of a nominal 300 mW. The drive power of this stage is used to set the output power to
the main power amplifier under the control of the DC voltage from the Micro
Controller board.
The VCO section and synthesiser circuits are the similar for the exciter and receiver
sections. The VCO consists of a common gate FET LC tank circuit. The power supply
to the VCO uses an 8 volt regulator and active filter for maximum noise rejection. For
standard modulation, transmit audio is fed to the conventional point of the VCO
varactor. For 2 point modulation, audio is also fed to the voltage control pin of the
VC-TXCO, this in effect cancels out the PLL error that would otherwise have
occurred for low audio frequencies, and hence resulting in a flat VF response.
5.2 Receiver Section
The receive signal from the antenna enters on the RF input connector and goes to a 3
section Bandpass filter which provides the initial filtering for the front-end amplifier.
The front-end amplifier is a wide band low noise bipolar amplifier for high
performance with a nominal gain of 18 dB, noise figure of 3 dB and low
intermodulation distortion. This is followed by a second 3 section BPF, and a high
level double balanced mixer.
The receiver uses high side local oscillator injection for bands below 350 MHz and
low side injection for all other bands. RF from the VCO main output on is buffered
and amplified to +19 dBm by the LO bipolar driver and injected in the high level
mixer which down converts the signal to the first IF frequency of 90 MHz. This IF
signal from the mixer is terminated by a bi-directional constant impedance network
and is then filtered by the first 2 pole crystal filter then amplified by a discreet bipolar
cascode amplifier with high gain. This provides a high degree of intermodulation
rejection for the receiver. This stage is followed by a 4-pole crystal filter with its
associated matching networks. The signal is fed into the main IF demodulator chip
with a second IF frequency of 455 kHz. The resulting audio is passed out for filtering
Technical Manual
2003 Revision
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