Thursday, 7 January 2016

Nano: A Minimalist's CW Transceiver

Lately I felt the necessity of an 80 meter portable rig for use during traveling and camping that proved to be the main impetus behind the development of this little transceiver for portable use. As the design was incepted I zeroed my choice for my favorite direct conversion technique for its simplicity added with the incentives of no-nonsense high performance (that of course SPRAT has always been for...!). After all real flavor of QRP advocates doing more with the least possible and I abide it. The design presented here is the final result of the efforts done.


I kept the transmitter section as simple as possible and to do that I used the CD 4069 gates for the oscillator and the driver sections of the transmitter chains. The oscillator uses a common 3.58 Mhz. ceramic resonator VXO that has a wider tuning range than a similar X-tal unit and covers a larger portion of the CW segment of the band. The VXO exhibits excellent stability. A VN10KM provides the final RF amplification and gives some good QRP power for real milli-watting..!! However you can power the final RF stage from a separate 12 volt power source, if you expect more output. This way you can get about 2.5 Watts out. If you opt to do so mount an LM7806 regulator to the enclosure side to provide a regulated 6 volt to the rest of circuit. So there is considerable degree of flexibility for the varying QRP needs. I have included L5 a miniature molded RF choke to the circuitry that along with a polyvaricon capacitor forms a simple on-board antenna matching unit for long wire aerials. The idea actually came from an old issue of Lo-Key magazine of CW operators club, Australia. You just need a simple field strength meter to tune the transmitter to get on to air. It is simple enough. I have not included any LPF at the output but it can be incorporated if desired. The two spare gates of CD4069 are used to provide a simple type of side tone.


The receiver is a simple direct conversion type. L1 and L2 form simple pre-selectors. However this simple arrangement proved sufficient to eliminate broadcast breakthrough of the close by local commercials. The stage following it is a famous W7ZOI RF feedback amplifier that ensures a linear input-output impedance and good dynamic performance. I used a simple single, common source JFET as mixer stage. Despite its simplicity it performed extremely well. The output of mixer is amplified to a reasonable level by the proceeding AF stage. A simple CW filter constituted of C1-L6 provides some degree of CW filtering. The idea came from G3RJV. Both these parts sit directly on the volume control terminals on the front panel. And of course my ever favorite TDA7052 provides the final AF amplification. It is chosen because it is quieter, louder and has lesser number of bulky components around it. Consequently, with a smaller foot print it is in-dispensable for QRP designs. The receiver, despite being simple and small; is very hot and sensitive. Even with a small piece of wire as an antenna it is capable of pulling in some good DX stuff.


I have assembled of the project using VK3XU patchy board construction technique. It is easy to give it a go that way. However a suggestive PCB template is given for those who wish a real commercial look. My version of assembled rig is housed in a cabinet made of double sided PCB laminate pieces. I used polyvaricons from old transistor radio sets for various tuning controls as shown but they can easily replaced with varicap diodes for portability or in case of non-availability of the former.

Schematic & PCB Template


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