True Story No34

 

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.

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