G3TSO

Designing a Transceiver

"a round about story of evolution"

 

In 1980 I purchased a FT707 for mobile use; it did not contain 160 metres, my favourite mobile band. The solution was either to modify the FT707, or to build a transverter. Yaesu advised against modification, so the transverter option was chosen; the 24MHz band formed the tuneable IF; a 30 watt broadband PA module was included in the design.

FT707 and Transverter Transverter  30-Watt PA and Driver Module

This project was successful, but cumbersome for mobile operation; the next stage of evolution was to use the driver and PA stages as the basis of a single band 160 metre transceiver. The addition of a suitable exciter and VFO were now required. The exciter is based on a kit designed by G4CLF using Plessey SL1600 series ICs with a 10.7 MHz IF. After considerable modification, involving a redesigned PCB, the exciter eventually functioned correctly. The VFO unit operates from 8.7 - 8.9 MHz to provide direct conversion to 160 metres; a mechanical slow motion drive was employed with a LCD frequency readout, driven directly from the VFO. ALC was not incorporated in the Transverter, and was not used in the initial Transceiver design, but was added at a later date.

 8.7 - 8.9 MHz VFO Unit   1.8 MHz Band Pass Filter and 2 Watt Driver
 10.7 MHz Transceiver Unit 1.8 MHz Low Pass Filter & SWR Unit

The End Result

G3TSO MKI Top Band Transceiver Version1


After many hours of successful mobile operation on 160 metres with the new transceiver, there was a desire to take things a stage further. The long term desire to build a solid state HF transceiver was now looking possible. By adding a series of Xtal oscillators, Band Pass and Low Pass filters, multiband operation of the existing design was possible. Rather than build the transceiver all in one go, a modular approach was adopted, building each module as a stand alone unit which was aligned and tested prior to final assembly. A 20 Watt PA kit was now commercially available using relatively cheap CB transistors which represented better value than the earlier PA unit.

The exciter module was based on the same G4CLF design, but with a 9 MHz IF permitting two band operation with direct conversion from a 5 - 5.5 MHz VFO.

 

9MHz Transceiver Unit   Foil side of PCB

The VFO was built into a larger housing, and employed a larger diameter coil and a higher quality variable capacitor, in an attempt to achieve commercial level stability.

 

 The 5.0 - 5.5 MHz VFO Unit

 

Direct conversion to 80 and 20 metres is possible using the VFO directly with the 9.MHz IF however, reverse tuning and sideband inversion occur on 80 metres. A switched xtal oscillator and mixer unit was added to provide oscillator injection on the other amateur bands; 160; 40; 15; and 10 metres. No attempt was made to include the WARC bands at this stage. It was decided to conduct the Band Switching with low capacitance diodes using a switched DC supply, rather than to employ ganged rotary switches.

 

 The Xtal Pre-Mix Unit  The original hand drawn PCB

A sophisticated Band Pass Filter unit is required to filter the output of the exciter unit, and to provide front end selectivity on receive. The filter which operates on both transmit and receive is also switched using switching diodes. Each filter is tuned to give a fairly flat response across an entire amateur band; the bandwidth is achieved by using a lower L to C ratio than that normally used in a tuneable filter.

 

 6 Band BPF Unit  Pre-Driver Add-On  BPF PCB "Pretty" Construction

When the transceiver was finally assembled and tested, the output from thr BPF was found insufficient to drive the new combined driver and PA stage. Using an identical circuit to the MKI transceiver, a SL610/SL1610 was added as a Pre-Driver; this provided an immediate solution to controlling Tx gain using an ALC system.

The PA Stage was purchased as a kit and provided both PA and driver stages on one PCB. It was easy to construct and replicate without having to purchase a further kit. Output was sustainable at 20 watts over the entire HF spectrum. To remove harmonics, a series of Low Pass Filters are used; these are relay switched for each amateur band; the LPFs also act as a roofing filter on receive.

 

 20 Watt HF PA Unit  LPF and SWR Unit

The Drake TR7 employs a very simple ALC unit. After reading the manual I decided to try a similar approach in the Modular Transceiver. A single chip comparator samples the forward and reverse voltage generated by a current sampling SWR bridge. The outputs are amplified and fed directly to the SL610 pre driver stage using the agc input port. Whilst this is a "bang bang" type system it was found ideal for providing forward and reverse ALC as well as permitting continuous adjustment of transmitter power output from zero to full power.

 ALC Unit

When all of the PCBs were completed, they were tested and aligned using a signal generator, an oscilloscope and a multimeter. DC switching was tested to ensure correct operation of the band switching. All modules were then laid out on a flat wooden board and connected together. Immediately after switching on, a local amateur G8ND was heard on 80 metres.

I now adopted the two fingered approach! All the PCBs were based on a 3 inch x 4inch standard size; boards were laid out as they would be in the final project and two fingers were run around the outside to allow space for hardware, wiring and a percentage for those things that might have been overlooked. A chassis was then constructed to house the modules. The overall target was to be smaller than the FT707.

A number of auxiliary units were added including a very complicated "mixing" Digital Display Unit; reproduction of this unit is not an option as the dedicated IC is no longer available.

The End Result

 Above Chassis  Below Chasis

G3TSO MKII Modular HF Transceiver 

Schematics
Block  Block Schematic

 View
Module 1 Exciter Unit

 View
Module 2 and 3 VFO and Pre-Mix

 View
Module 4 BPF and TX Amp

View
Module 5 Low Pass Filter

 View
Module 7 PA Unit

 View
Auxillary Auxillary Modules

 View

But what happened to the FT707?