My friend Pete G1INF from across the pond (England) recently built a 20m QRP (1 watt) Transmitter. I asked him for a schematic which he hand drew for me. I used my CAD program and drew it. Here are the details:
The conventional Colpitts vxo is buffered and keyed with a pair of NOR gates, which drive a pair of VN10KM FETS as a final. The pi-filter is a 3-pole, using a ferrite toroid (I'm willing to discuss...) and the output is via a Belling-Lee socket.
The unit gives one watt out at 15V input, at 300mA.
The pulling inductor 'L' is 19 turns of 40SWG ECW on a reclaimed Toko yellow 10mm coil. I've chosen 8.2V as the stabilising zener because I plan to use the tx with between 9V and 20V.
The design is as old as the hills; it's a favourite of mine, the Colpitts. The transistor is a BC547B, and the controlling capacitor is a recycled polyvaricon.
CW20M Driver / Power Amplifier.
This was taken from Tim Walford G3PCJ's 'Kilmot' design. It's so simple, and could be expanded easily to use four FETS, as there are four gates on the buffer / keyer / driver chip. Oh, yes, the chip is a 74HC02 quad NOR gate array. I've left out the power supply arrangements for the 74HC02, as they should be common knowledge among OMs. Suffice to say, I've decoupled close to pin 14 with 100nF, and fed the chip through a 1K resisitor past a 5.6V zener. I've deliberately gone for a higher voltage to keep the propagation delays short.
The FETS are VN10KM, and their drain tabs are soldered to a 10mm x 20mm scrap of copperclad, which serves as a small heatsink and commoning block.
I've used a ferrite toroid in the pi-filter (shock, horror) because it's what I had to hand. In mitigation of this choice, I have experimented with this particular grade (Mullard 4C65), and found that positive and negative temperature excusions from 20 C will both _increase_ the AL value of the core. Thus, the filtering gets more aggressive as the core is heated or cooled.
I wind the pulling coil and adjust the turns to suit the individual rock. For a 14MHz crystal, a good ballpark figure is between 5 and 10uH. The coils itself is a Toko 10mm yellow, stripped of its original windings and 19t of 40SWG enamelled copper wired does the trick for my crystal, which is an ordinary HC49 unit with 30pF load capacitance.
The pi-filter coil is 500nH, and here I've used a 9mm Philips / Mullard 4C65 toroid with 4t of 30SWG ECW on it.
Preliminary tests are good. Clean output, and the signal (via a low dipole) was picked up by the WebSDR at Enschede, Netherlands
Copyright Disclaimer: Feel free to show this off and promulgate - I don't claim any copyright on this design; definitely Public Domain!
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