[Laser] Pre-amp design etc

[Stuart]

Steve,


It seems I have a number of points to clear up about my circuit.


The 33 ohm resistor is to save the charge in the electrolytic capacitor eroding the relay contacts (or even worse, welding them!) when the rig goes to TX which grounds the capacitors, which are there to decouple the 43V negative bias supply used on receive. There is 43V on both capacitors there, so I thought it a wise precaution. I have had that circuit working for over a year now and I am happy to report no problems with it, or need to replace the 43V bias battery as it is isolated when the rig is unpowered, that is the reason for the odd ptt switching, it actually rests in the TX position with the 12V supply disconnected.


You mention the size of the gate coupling capacitor, I use 0.001uF which is plenty, mostly my rig gets used at around 20kHz- 25kHz since my group use sub-carrier FM and SSB mostly, although it does work to baseband rigs.


The bias on the FET, I believe that the FET has a lower noise figure with quite some current through it. I am sure you have read Clint's pages about this. Even so there is so much variability between FETs even from the same batch that I have used a quick "fix" of a 10k ohm pot from drain to 0V to trim the drain voltage. Several who have built this design in the UK have reported this necessary and have told of the sensitivity increase at the correct operating point.


Sensitivity, my circuits were used at both ends of the current record two-way contact distance in the UK, 117.6km. Signals were clear both ways using 20kHz LSB. There is a recording of Rob, M0DTS made at my end with a microphone over the FT817 loudspeaker which fell off after the first few seconds! (that's why it goes quiet....), this is again in the files area of "UKNanowaves" Yahoo group. I have been told this would have been judged by ear to have been 5/8 in a contest. Now there's a thought...


Stuart