PAGE 36 TECHNICAL DESCRIPTION
4.3 RECEIVER AND TRANSMITTER MODULES
4.3.1 Frequency Bands
The Blade is supplied with a matched pair of radio sub-assemblies fitted.
The designated frequency bands are:
66 - 88 MHz E0 Band 335 - 375 MHz R1 Band
136 - 174 MHz AC Band 400 - 450 MHz TK Band
174 - 208 MHz K1 Band 400 - 480 MHz TU Band
208 - 245 MHz KM Band 440 - 500 MHz UW Band
310 - 350 MHz R0 Band 470 - 530 MHz WR Band
The following text refers to the components and circuits within the RF sub-assemblies
only. For additional information, refer to the SRM9000 Service Manual [1].
The Rx and Tx Modules are equivalent and interchangeable for a given power
classification (25W or 100W). They are connected to the Control Board via a 26-way
Ribbon Cable that carries analogue audio, digital audio and control signals.
Power connections are made via a 15-way, D-type connector and RF connections are
made via BNC sockets. RF connections are taken to the rear of the Blade via double-
screened cables terminated with female N-types.
4.3.2 Receiver
Refer to Figure 3.5.
4.3.2.1 Front-end Filters and Rx Front-end Amplifier
The receiver input signal from the antenna passes through the antenna filter comprising
L10, L11, L12 and associated tuning capacitors. With a transceiver module in receive
mode, diodes D3, D4 and D5 in the antenna switch are reverse biased allowing the
receiver input signal to be coupled through to the front-end with minimal loss. The overall
insertion loss of the antenna filter and switch is approximately 0.8dB.
Front-end selectivity is provided by varactor tuned bandpass filters at the input and output
of the RF amplifier.
Front-end tuning voltages are derived from the alignment data stored in the radio. The
DSP processes this data to optimise front-end tuning relative to the programmed channel
frequencies that may be changed at any time without re-aligning the radio.
To achieve the required varactor tuning range an arrangement of positive and negative
bias power supplies is used to provide a total bias across the varactors of up to 14.0VDC.
A fixed 2.5V positive bias derived from the 5.0V supply and voltage divider R425/426 is
applied to the cathodes of the varactor diodes. The negative bias supply originates at the
DSP/FPGA as a composite digital tuning signal (FE TUNE) containing the data for the four
front-end tuning values TUNE 1 to 4 for the particular channel frequency selected. The
level is dependent on channel frequency and tuning and varies between +0.1 and +3.0V.
This signal then passes through buffer U901A and level translator Q900 to Q903 where it
is converted to a high level (-0.5 to -11.5V) negative equivalent of the original signal.
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