Back in March I wrote about my experiments with a minimalist transceiver design that was published in Sprat earlier this year. Today I received an e-mail from Claude, W5FYI, who enquired about this work. He wrote:
I, too, am interested in building G0EBP’s FET transceiver. One thing that puzzles me is reference in the Sprat article to the 560pF C5. Tony says it is for the final filter. Is he referring to the FET’s capacitance, or his value for the pi filter’s capacitors.
When you get your schematic ready, please let me know.
I thought I would share my reply as others may be interested:
Thanks for the e-mail.
Yes I wondered about that reference to C5 too. I presumed it was the 100nF cap connected to the drain and the inductor on one side and the filter on the other.
I have done quite a few mods to this circuit and still tinkering trying to get a 700Hz freq. offset on transmit, so avoiding issues if someone is zero beat. My first attempts were poor, using an idea in Solid State Design for the Radio Amateur, which plays with some feedback in the oscillator circuit (Fig 6 p36 if you have that book). I see a brief freq. shift and then it seems from my freq. counter the oscillator locks back again. Perhaps I need to switch in the capacitance with the crystal which I think will work better. Been too busy recently with work to finish this experimentation.
Back to my main changes that I did.
1. I changed the oscillator to a FET based Colpitts with a J310. Better waveform, but lower output than a 2n2222 and hence only about 1/4W out with 9V.
2. I have used a different muting process. On keydown I put +Vsupply to pin 7 of the LM386. That mutes the audio op-amp. See LA3ZA, Sverre’s notes on this at http://www.qslnet.de/member/la3za/Pixie_mute.htm
3. Rather than have the key in line with the supply I included another transistor (PNP BJT) so key is connected to ground for transmit. This helps if you mount an un-isolated socket for the key jack in a metal chassis.
I need to finish off the experimenting with the offset and then write up the changes for SPRAT.
I looked in my notes and see I blocked out the basic circuit but have not added any component values, so I attach it here, to help you. The offset circuitry is not included. I think you will be able to work out the component values from the original diagram. If you need the calculated values for the Colpitt’s oscillator, let me know and I can supply those.
Hope the above helps you.
By the way I have started to call this transceiver “The Bay” after Morecambe Bay where G0EBP lives and coincidentally where I was born and grew-up.
The draft circuit diagram is above. It is unfinished but still gives a good idea of what I have done with Tony’s, G0EBP circuit.
When I get time to return to this circuit I will report findings and updates here on the blog.