Hello Eric,

Yes, the output is a square wave when I use the U3S I take the output
direct from CLK0 at 2X frequency (set in the U3S menu) via a co-ax to
the input of the W1VD amp to Pin 3 of 74F74 with no 50 Ohm load
resistor. It works perfectly. It makes me think other exciters are
doing something different at the end of the TX cycle. I know not what
though...

The amp is powered all the time, the output solely depends on the
presence or not of an input signal.

Same with the TS-590S, output from the DRV socket at under 0dBm sine
wave at 2X frequency from the 590 into a squarer. Squarer drives the
W1VD amp the same, (no 50 ohm resistor) direct to Pin 3 of the 74F74.
no issues. Run the 590 at 136kHz into a W1VD doubler and a pre amp and
the FETS's life is always short. Death always seemingly occurs at the
end of a WSPR or OPERA cycle. Again the amp stays powered all the time
from either a huge linear supply or an HP SMPS 50V supply from a
server.

I have noted that running the over current limiter on the linear on
the cusp of triggering helps. The SMPS is ruthless in the doubler
scenario and FET life is *always* on the edge. The gate waveforms seem
OK, it's the drain waveform that shows the spikes as the oscillations
decay away. I would love to get to the bottom of this as I have too
many different set ups to remember :) I assumed it was some sort of
feedback from the antenna perhaps, but I am a novice and once I found
what saved me from spending loads at Farnell I gave up chasing the
cause.

Tuesday, February 26, 2019, 12:12:08 PM, you wrote:

> Or in the case of a U3S with a square wave output at CLK0 (I assume it's
> square)... a complete positive half cycle without any further "noise", etc.

> On 2/26/19 7:04 AM, Eric NO3M wrote:
>> Just a thought, perhaps the RF is ending exactly at a zero crossing, 
>> whereas a typical PC/soundcard + exciter may not be and that is 
>> causing this issue.





-- 
Best regards,
 Chris                            mailto:[email protected]