Re: R: Re: LF: TXing 2200m WSPR

[Andy Talbot <andy.g4jnt@gmail.com>]

Sounds more like feedback.    RF feeding back into the driver stages.   The LF component you see is, perhaps, a beat between generated RF and oscillatory RF due to feedback. Or feedback itself oscillating at LF.  Or something like that

Vaguely recall something similar happening here in the dim and distant past.   Perhaps a couple of times even    In fact, I think it explains why my 475kHz 400W Class-E PA which is built open-style has an aluminium screen above the input circuitry (Of course, I may have added that due to an earlier poor-screening experience - it's all too long ago to remember).

'jnt

On 29 December 2017 at 11:56, N1BUG <paul@n1bug.com> wrote:

Hi Chris,

At PS side of the choke, no RF behaving or misbehaving.

At output transformer side of the choke, mainly negative-going pulses which become distorted similarly to the output when misbehaving.

Drain waveform is like a sine wave with regular shaped positive half, negative half is double peak. When misbehaving, same basic shape but blurry just as the RF output is... looks as if amplitude constantly changes from cycle to cycle, as though modulated by a lower frequency.

Gate waveform almost a pure sine wave (remember this is a class AB/B/C amp), develops amplitude blur or modulation when misbehaving.

My "gut" tells me this amp is oscillating at a frequency well below the band (maybe at VLF) but only when feeding the antenna. Perhaps the fact the antenna is a very reactive load at VLF sets it off? I don't know why it would happen at different RF power levels with 13V / 19V PSU unless it is proportional to current somewhere...

I'm about ready to take this amp down to the train track and strap it on a rail... might be rather satisfying. ;-)

Paul N1BUG



On 12/29/2017 06:26 AM, Chris Wilson wrote:

Hello N1BUG,


What  does the scope show if you look at each side of the choke in the
PA?  I  did  some  tests ages ago using the stacked chip caps on the PA
side  of  the  choke  compared  to  conventional  leaded  caps and the
difference  was  marked. I would certainly try a proper dust iron core
for  the  choke, ferrite can't be right... I don't use a LPF after the
U3S but my amp is "Class D" and is happy with being driven with square
waves.

Friday, December 29, 2017, 11:13:03 AM, you wrote:

One final thing.

Thought maybe something in the antenna was breaking down (arcing or
corona), so made another check:

I now have the capability to run the PA on 13V or 19V. At 19V I can
make more power into the antenna before the signal goes dirty,
compared to 13V. RF power is RF power. If it was something failing
in the antenna system I think it should happen at the same RF power,
no matter what DC supply voltage I use on the PA...

Paul