Roger,
I will follow your advice and work on eliminating the turn-on transient.
There are 56 Ohm gate resistors
That's quite high for such large FETs. I would use much lower ones. Note
that I'm assuming that these FETs are similar to APT5020 and 5022 ones.
I couldn't find a datasheet for the 5023.
> and a small amount of gate to drain
capacitance “negative feedback" (gate and drain wiring is twisted pair).
That gives you just a few pF, which is swamped by the FET's internal
drain-gate capacitance. But the inductance of such wires is significant,
and can resonate the existing FET capacitances, causing the
oscillations! You should strive for minimum inductance in the gate and
drain wiring, even in your relatively high impedance, very low frequency
amplifier.
A somewhat effective feedback would take a resistor of a few hundred
ohm, in series with a capacitor of 100nF or so, for each FET.
The screenshots that I attached to my post email did not appear on the
amps website. I have uploaded them to:
http://qsl.net/v/ve7vv//Temp/Waveform at antenna.png
http://qsl.net/v/ve7vv//Temp/Waveform at dummy load.png
I can see that your two FETs are very dissimilar. One comes on much more
softly than the other. I suspect that you have a single bias
potentiometer for both, and that the two FETs have significantly
different threshold voltages. You should either use matched FETs, or
have a separate bias setting for each, and set the two idling currents
for the most symmetric waveform.
Other than that, the waveform at the antenna shows much stronger
harmonic voltage, because the antenna offers essentially no load at the
harmonic frequencies. Of course you shouldn't be putting that signal
into the antenna, without a low pass filter! Although the antenna will
be a poorer radiator at harmonics than at the fundamental, it's still
good enough to cause QRM to other services.
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|