The heart of the G3TSO modular transceiver is a modified version of the well known G4CLF board. This was based upon the Plessey communications ICs and described fully in my earlier article A 30W SSB Transceiver for 160 metres (RadCom July/Aug 85). The current design uses a 9MHz i.f and could easily be substituted by any other 9MHz unit such as the G3ZVC board or a commercial MLX board. The modules to be described could easily be used in conjunction with the G30GQ transceiver (RadCom April 83) or the PW Helford to add to the original two band design.
The choice of a 9MHz i.f enables a single vfo operating from 5 0 to 5-5MHz to provide cov
erage of both the 14MH2 and 3-5MHz bands with a minimum of switching. Local oscillator injection for multiband operation is achieved by mixing the 5MHz vfo signal With the output from a switched xtal oscillator, which is then filtered and amplified to provide the +7dbm required by the ring mixer on the G4CLF board. The new WARC bands have not been included in the design as the primary mode of operation was to be ssb, but their inclusion would not be particularly difficult and the pcb's layouts could be expanded to accommodate the extra filters and xtals.
Availability of suitable PA transistors for home construction is erratic, particularly if they are to be obtained at a sensible price. With this in mind a suitable low cost alternative was sought, resulting in the use of a kit pa currently obtainable from Cirkit Holdings. The kit is designed to give 10W continuous output over the frequency range 1-5 to 30MHz and employs a pair of 2SC1945 PA tranistors. They are well capable of providing up to 20W output for ssb or cw operation with less than 2mW of drive. This is ideal for low power operation as a pa in its own right or it can be used to drive a solid state or valve linear amplifier to considerably higher output.
Bandswitching has been achieved by using low capacitance switching diodes; signals are able to pass through those diodes that are biased on from a 13V supply and are confronted by a high impedance path from those diodes that are biased off. This is a technique used in almost all modern commercial equipment and enables bandswitching to be effected with a single wafer bandswitch linked by ribbon cable to the various modules that require switching. In practice, a second wafer is used on the bandswitch to enable selection of the appropriate oscillator xtal. The low pass filters used in the transmitter output path are switched using miniature relays as much larger currents are present in this part of the circuit.
An swr detector is included in the design. This provides both forward and reverse ale voltages used to control the output power on transmit and provide final protection under high swr conditions. Meter indications of Fwd and Ref power are displayed on a combined S-meter.
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