A novel design of VHF transceiver
The following specification describes a VHF transceiver using
an analogue technique to provide continuous tuning for SSB, FM
and CW.
General specification:
Output 144 to 146 MHz
In 20 switched bands 144.000 to 144.100;144.100 to 144.200…….
145.900 to 146.000
Each of the 20 bands is continously tuneable between low and
high
The equipment employs a combination of digital and analogue frequency
display.
The basic design is that of a single superhet transmitter and
receiver with an IF at 10.7MHz
The local oscillator is a VCO (voltage controlled oscillator)
tuning between 133.3/135.3MHz
This is phase locked to a VXO (variable crystal oscillator) running
at a basic frequency of 10.240MHz
The VCO output is fed to a pre-scaler where it is divided by
5 and then to a programmable counter where it is further divided
by one of a sequence of 20 numbers whose codes are held in a
ROM ( consisting of a diode matrix) selectable by a switch. The
numbers are 21,328; 21,344, 21360…etc.
The total VCO selectable divisions are 106,640; 106,720, 106,800
etc…resulting in an output to the coincidence detector of
1250Hz.
The VXO is divided by the fixed number of 8,192 also resulting
in an output of 1250Hz which is fed to the second input of the
coincidence detector..
The output from the coincidence detector is smoothed by a pair
of "bridge T" filters set to 1,250Hz and 2,500Hz and
after a further low pass filter is fed to the VCO as a DC control
voltage.
The VXO is designed to have a linear output from 10.215MHz to
10.365MHz or +/- 25KHz on 100KHz enabling the dial to be calibrated
with uniform divisions of 1KHz. The main tuning element is a
twin gang 500pF air-spaced capacitor.
In CW mode the output of the VCO is mixed with a 10.7MHz crystal
to provide 144 to 146 MHz
In FM mode frequency modulation is derived by driving a varactor
diode, connected to the VCO, with audio and heterodyning with
the 10.7MHz crystal to 144/146MHz
In SSB mode the 10.7MHz heterodyne signal is derived from a single
sideband generator.
In receive mode the incoming 2 metre signals are amplified and
heterodyned to 10.7MHz using the VCO.
In FM mode, after limiting, a NBFM detector is used and in SSB
mode the 10.7MHz crystal is used in a product detector to recover
the audio.
For FM repeater use there are extra ROM divide codes providing
transmission at a preset increment to the received frequency.
Tone burst is generated from a circuit triggered either by the
microphone pressel switch or by a separate push button.
A built-in linear amplifier provides about 100watts of power
for SSB and by mode switching runs in more efficient Class C
for FM or CW.
Implementation:
Key elements of the circuit are as follows:-
VHF prescaler...…………...Plessey SP8621
Programmable counter…...Plessey SP8922
Digital synthesiser………....Plessey SP8921
The latter two chips were designed in
1977 for Citizens Band use but when the UK Government introduced
a new equipment specification the following year for the UK the
designs were made obsolete overnight.
I built a transceiver using the above
design in the late 70's following a previous design based on
discrete TTL chips which worked satisfactorily but which was
too complicated. The new design was superior to commercial frequency
synthesised equipments because it had a proper analogue tuning
dial rather than the now customary digital steps whilst still
retaining crystal stability. The design had two key elements
which were entirely novel. One was the use of a variable reference
oscillator and the second was the design of the VCO lock filter
which allowed virtually instantaneous frequency locking with
extremely low noise sidebands but permitted audio modulation
for narrow band FM. It was never patented.
