EMRFD Message Archive 3806

Message Date From Subject
3806 2009-11-25 13:37:05 w7loz Question: VXO 20 meter QRP transmitter from EMRFD
I've just finished putting together the 20 meter VXO transmitter from EMRFD (first edition) page 5.19. Everything tunes up normally and seems to be working OK, except that I can't get anywhere near 4 watts out of the rig with a 12 volt supply. I have to raise the supply voltage to 14.7 volts for the transmitter to generate that much output. With 13v input, I get around 2.7 watts out. Let's assume my wiring is correct, since I've double and triple-checked it.

The schematic of the vxo section of the circuit on P 5.19 shows a resistor at the emitter of Q5 labeled "R1-Sel." Nothing in the text mentions what this means. . . but I did play with the value. Reducing it to 6.8 ohms gave me a little more drive. Probably raised the harmonic output level as well, though I don't see much difference with the scope.

If the drive control is set far enough open to give a reasonable output, I notice some distortion on the positive peaks of the IRF-510 gate drive signal. This is viewed across the 51 ohm resistor going to ground (via a .1uf capacitor) from the output tap of the 2N3866 driver transformer (T2). If I reduce the drive enough to eliminate this distortion, the final stage output goes WAY down. I'm not sure whether to expect some distortion in this circuit under drive conditions, or whether it's indicative of a biasing or feedback problem somewhere.

I'm curious whether anyone else has built this transmitter and what your experience with it is. Not being well-acquainted with the IRF-510, I don't know whether to expect variations between units might account for the apparent low output of this one.

By the way, 9.373 Mhz xtals this transmitter calls for are not standard. I had a couple made for me by Quartslab in England. I'll probably want to sell the extra at my cost (about $23) if anyone is interested.

3807 2009-11-25 20:42:53 w7zoi Re: Question: VXO 20 meter QRP transmitter from EMRFD
Hi Kevin, and gang,

You may well be the first to have built this rig. I was quite enthused about the x 1.5 frequency multiplier scheme, but I'm generally alone with this.

I really got curious when I read your post, so I fired up the original box and it delivered 4.08 W at 11.99 volts, according to the typical ham instrumentation here. My scope is new, but has not gone through the kind of calibration that we used to enjoy in industry. When I first fired things up, I thought that the power was down, but I checked and found that I had the bandwidth limit on with my scope. I kicked it to full bandwidth (50 MHz) and I got the 4 W number. The measurement was done with a 5W BNC 50 Ohm terminator right at the scope.

I looked at the signal on the 51 Ohm resistor between the driver and PA and it was indeed distorted when the drive was a maximum. This should not be an issue, for it was harmonic distortion that will be stripped off by the output filter. The distortion could impact efficiency.

I have read that IRF510s from various manufactures will offer a variety of performance. The part I had in this rig was made by Harris. (I think that's what the "H" indicates
3811 2009-11-26 13:00:20 w7loz Re: Question: VXO 20 meter QRP transmitter from EMRFD
Hi Wes,

Thanks for checking the outputs on the original unit. I'll try a couple other FETs and see if that makes a difference.

I built this partially because I was intrigued by the multiplier scheme, and the fact that it allows the oscillator to free run. Pretty cool!

This tx will be a companion to the 40m binaural receiver and the 20m binaural converter built previously.

73 and Happy Thanksgiving