Hi Markus,
Could the scope "blurring " be caused
by a high-frequency rather than low-frequency oscillation?
It's possible but what I see is not what I would expect in that
case. My experience has usually been that with a high frequency
oscillation I see two sine waves: the fundamental and another with
shorter period which is the HF oscillation. Here I don't usually see
that. I see one sine wave keeping the same period but with amplitude
"blur". No blur at the zero crossings, maximum blur at the positive
and negative peaks.
That is what I see most often when it is misbehaving somewhat
mildly. But sometimes I do see two "clean" sine waves with slightly
different period. To me this suggests oscillation near the
fundamental frequency. I can usually (but not always) make it switch
from the single blurred since wave to two clean ones with different
period by increasing bias or RF drive.
Have you tried a
simple "snubber" network (e.g. 1 nF + 10 ohms) at the drain?
No, but I will try it next. I don't have any high power 10 ohm
resistors, but I suppose if there is significant HF energy the 10
ohm resistor burning up might be a clue. :-) I maybe could make a
higher power 10 ohm resistor by putting a lot of chip capacitors in
parallel, don't know what that would have for stray capacitance though.
Paul N1BUG
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