3.2
DETAILED TECHNICAL DESCRIPTION
3.2.1
Receiver
3.2.1.1
GENERAL
The double conversion receiver design is intended for narrow band frequency modulation
systems only. Figure 3.2 shows the simplified block diagram of the receiver excluding the final
stages of audio processing.
3.2.1.2
FRONT END
3.2.1.2.1
UHF Section
The UHF front end filter utilizes five ceramic coaxial resonators which are effectively quarter-
wave transmission lines with one end short circuited to ground. A bandpass filter is formed by
capacitively coupling two resonators together, and varicap diodes are used to electronically
tune the resonant frequency of the filter. Referring to the circuit diagram, X400 and X401 are
the resonators in the first input filter pair, and D400 to D407 are the varicap tuning diodes.
Capacitors C403, C404 and C405 form the main signal coupling path for the filter when tuned
to the high frequency end of the required band. Conversely, capacitors C402, C406 and C408
sustain coupling at the low frequency end. This technique helps maintain a constant filter
insertion loss and bandwidth over the tuning range. The tuning of the varicap diodes is
effected by a control voltage coupled from the synthesizer via resistors R401 and R403.
The remainder of the front end consists of a low noise transistor preamplifier in TR400,
followed by a tuned bandpass filter using three coupled resonators, X402, X403 and X404.
This three-section filter operates in a similar manner to the first pair already described.
Conversion to the first IF frequency of 21.4MHz is achieved with the double balanced ring
diode mixer U400.
3.2.1.2.2
VHF Section
The VHF front end filter is realized by two pairs of coupled helical resonators. The first input
bandpass filter FL400 comprises two helical resonators which are arranged in an inter-digital
configuration. This is achieved by grounding the opposite ends of each coil and reversing the
winding direction of one coil.
The coils are mounted horizontally on the printed circuit board and are positioned in such a
manner as to achieve mutual coupling. A tin-plated brass shield is placed over the top of the
coil pair. Each coil is mechanically fine tuned with a copper tuning core.
Varicap diodes D400 to D407 are placed across the coils for electronic tuning. Resistors R400
to R403, and R413 couple the do bias voltage from the synthesizer to control the tuning.
The remainder of the VHF front end consists of a low noise transistor preamplifier in TR400,
followed by a second pair of helical interdigital bandpass filters FL401. The four coils
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