16
The RF section has a common RF input and output port and separate audio input for Tx and output for Rx.
A double superheterodyne system is used, with a first IF of 45 MHz and second IF at 455 kHz. Three
oscillator injection frequencies are used, LO1 at (Carrier frequency + 45 MHz), LO2 at 45.455 MHz and LO3
or the Carrier Insertion Oscillator at 453.4kHz on USB and 456.6kHz on LSB. There is a separate 45 MHz
receiver to generate gating pulses for the noise blanker.
The audio section of the RF/Audio PCB implements some frequency tailoring. The audio is further
processed and filtered as required by the digital signal processor or DSP before being fed to the audio
power amplifier. Both of these are located on the microprocessor section of the PCB.
First mixer
Signals from the antenna are fed to the 30 MHz low pass filter, L14, L15, L41 and associated Capacitors.
This minimises the image response in both receive and transmit directions. It also serves to limit LO1
breakthrough. High frequency transformer T2 splits the signal into two phases (0 and 180degrees) to feed
the high level balanced mixer U47, which is in fact a high speed analogue MOS FET bus switch. The
centre tap on the transformer also facilitates the application of the bias required by the mixer. The action of
mixer U47 under control of the first local oscillator signal (LO1 a CMOS level square wave) is to switch pin 4
of U47at the local oscillator rate between its pins 3 and 1, thus switching a two phase sample of the input
signal from transformer T2 to the mixer output at pin 4. This results in sum and difference products which
are fed to the first of two noise blanker switches U51. The operation of the noise blanker is described in a
later paragraph. The local oscillator signal is suppressed by the first mixer. The mixer output passes
through a direction reversing transmit receive (TR) switch U53 to the post mixer amplifier Q5.
Post mixer amplifier Q5
The purpose of the post mixer amplifier is three-fold. Firstly, it supplies a broad band termination to the first
mixer so as to minimise unwanted products from it. Secondly it supplies some gain to overcome the loss of
the first mixer but only sufficient to minimise its overload in the presence of strong unwanted signals.
Thirdly, in combination with the attenuator formed by resistors R142 to R144, it provides a flat impedance
match to the 45 MHz roofing filter F1, which follows another TR switch U54. Note that the TR switches and
the mixers are all of the same part type, a versatile CMOS analogue bus switch NCSB3157. This is an
analogue change over switch for signals in the range of zero to +5 volts. It will also switch digital signals in
that range.
First IF filter F1, F2
The first IF filter removes the unwanted high frequency products generated by the first mixer. This filter has
a bandwidth of about 6 kHz, and consists of two matched crystal filter units F1 and F2. These are matched
to 50 ohms in and out by L networks C183, L20 and C182, L19 respectively. These filters are tuned in the
factory before being linked to their input and output ports by soldering across links SP1 and SP2. Test
points TP1 and TP2 are used at the factory to connect the filters to a spectrum analyser and tracking
generator for tuning.
The 45MHz crystal filter is terminated via T-R switch U50 by the input of amplifier Q3. The stage gain is
about 12dB and is kept low because although the 45MHz filter removes most of the unwanted first mixer
products, it may still allow signals to pass which are close to or adjacent to the wanted signal. If any off
these are very large, they may still overload this amplifier if it had too much gain. This amplifier also isolates
the 45 MHz filter which is sensitive to its termination impedance.
Second mixer U48
This is a duplicate of the first mixer, but in this case converts the 45 MHz down to 455 kHz. The conversion
oscillator signal is LO2 which is on 45.455 MHz. Transformer T3 provides the phase splitting for the input
signal and R39 provides the mixer bias via the transformer centre tap. This mixer, like the first also
generates unwanted output products. The mixer output impedance (and thus voltage) is stepped up from
50 ohms to 1800 ohms by the 36:1 ratio transformer T1. This matches the characteristic input impedance
of the 455 kHz filter. Immediately preceding this filter is the second noise blanker switch U40. The noise
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