I had to use the (carefully resonated and matched) antenna for this
test rather than a dummy load but it appears the maximum voltage
spike in the ringing does indeed vary almost linearly with applied
DC drain supply voltage. If I were to put 50V on this amp I strongly
suspect it would not go well for the FETs! Something is very wrong here.
I tried a few other things just to see if I could learn from it.
I swapped out cables between amp > LPF and LPF > dummy load, using
several different combinations of cables from short to long,
different types, etc. No observable change in ringing.
I tried reversing the LPF in and out connections: no change.
I tried amp directly to dummy load without LPF. Interesting. The
first voltage spike remains virtually unchanged but all the
succeeding ones were greatly reduced.
I looked at the DC voltage on the PSU side of the choke. It looks
perfect. No ripple, noise, variation or any other anomaly that I can
see on the scope.
Paul
On 04/09/2018 11:34 AM, N1BUG wrote:
> Yep, voltage breakdown of the FETs was my worry. However I may be
> making a false assumption. I assumed the voltage spikes in the
> ringing would vary linearly with applied drain voltage? Perhaps that
> isn't necessarily so?
>
> With 13V on the drains the first spike here is 'about' 64V.
>
> Efficiency seems fine. I wouldn't try to put an exact figure to it
> what with measurement accuracy in three places (voltage, current, RF
> power) but it 'appears' to be in the mid 90s.
>
> Paul
>
>
> On 04/09/2018 11:02 AM, [email protected] wrote:
>> Alan
>>
>> The main concern about the ringing is that the positive extreme stays below
>> the drain source
>> breakdown voltage ... with some 'headroom'. Otherwise ...
>>
>> Jay W1VD
>>
>>
>> ----- Original Message -----
>> From: "Alan Melia" <[email protected]>
>> To: <[email protected]>
>> Sent: Monday, April 09, 2018 10:21 AM
>> Subject: Re: LF: W1VD amp help - more waveforms
>>
>>
>>> Paul
>>> I think you need to step back from this a little. One point, design/cable
>>> routing at 137kHz is
>>> normally not that critical and it is unlikely it is having much effect.
>>> Many us have ''haywired''
>>> PAs at these frequencies without major troubles....even using croc-leads!!
>>>
>>> You are unhappy about the drain waveforms.......why?? the ringing is on the
>>> ''off'' device and so
>>> doesnt really matter in terms of efficiency. Consider what is happening.
>>> The FET drive goes off
>>> and the drain current starts to drop inductance in the drain feed causes
>>> the voltage to rise to
>>> try to keep the current flowing. Then on top of this the other transistor
>>> switches on and the
>>> transformer connection for the off side sees a rising induced voltage out
>>> of the transformer.
>>> There is bound to be overshoot but it does not absorb much power. It may be
>>> the generous ''dead
>>> zone'' (whilst a good idea) that is producing this picture.
>>>
>>> The critical part is the the flat zero volt line when the FET is on. The
>>> current pulse rings a
>>> bit, but the current drops in the second half of the pulse.....why?? This,
>>> I think, is really the
>>> loss of power. Does Jay's show this? The point I am making is don't get
>>> drawn up blind alleys
>>> because a trace just does not look ''pretty''.
>>>
>>> I think we have mentioned this before but the transient performance of the
>>> power supply may be a
>>> factor. There should not be great changes in the current draw because you
>>> are switching the PSU
>>> load from one transistor to the other but there can be higher speed
>>> transients that can affect
>>> power supply regulators in unexpected ways.
>>>
>>> As Andy said efficiency at 12v is not very meaningful because of the high
>>> rds of these FETs. It is
>>> usefull to run up a PA at low volts say for safety when the antenna needs
>>> to be tuned.
>>>
>>> Finally the scope probes are placing an. albeit small, capacitance across
>>> the drains (I am
>>> assuming they are correctly trimmed) The waveform can often be affected by
>>> where the ground is
>>> picked up from.
>>>
>>> Stick with it because you are close to being ''there''
>>>
>>> Alan
>>> G3NYK
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