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Tuesday 13 October 2009

TR switch for use with a SDR RX and main TCVR


After Simon Brown's SDR talk at RSGB Convention this year I have been looking at ways of using a SR with a 'normal rig' and I found this circuit. The eLAD device he mentioned was not available and I couldn't read the circuit diagram. There are probably a zillion ways to do this... and I claim absolutely no originality for the idea/design. This circuit is from a Ten Tec Argosy, and handles that rig's 50 watts with ease and I found it was fine for my use, cw only right now.

The circuit appears broadband enough for HF use, not sure about use up to 6m though. The notes are that this requires 20 to 40 mA of total diode current.

The paralleled 330 ohm resistors. 330 ohms, 1/4 watt may be ommitted if there are no breakthough problems from local AM stations.

The IRF510 MOSFET is used as a switch and I think most qrp homebrewers have at least one spare of these. If not try GQRP sales, I also tried it with a IRF630, either way if this doesn't switch very fast then the capacitor (.47 uF a 100V box capacitor from the junk box) that holds the switch closed on TX would need to be larger, I think you can modify the qsk delay at this point. There are probably smaller MOSFETs available for such low-current switching and I think a 2N7000 might even be used? 1N4007's are 1 kV rectifier diodes that I happen to have in the junk box, I also have some MPN3404 and MPN3700 from W8DIZ but the 1N4007 seemed to do fine and I didn't need the PIN diodes. They appear to operate fine. I used 1N4148 instead of the 1N914's and at 5 watts, I have also had satisfactory results with them.

The chokes are small molded ones bit like fat 1/2 resistors.

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