Paul

Is your core 77 or 78 material?

Jay W1VD

To: <[email protected]>
Sent: Monday, April 09, 2018 2:19 PM
Subject: Re: LF: W1VD amp help - more waveforms

Jay,

Oops. I took a picture of the LPF this morning and uploaded it to
the server. Then I forgot to include the link in my earlier reply to
you. Here it is:

http://www.n1bug.com/vd-lpf.jpg

I used the same caps you did, .022 and .001 715P in parallel. I did
mount them a bit differently.

Here is an overlaid gate crossover:

http://www.n1bug.com/gates4-wdv13.jpg

That transformer very well may be my problem. I will have to find
*much* softer wire that will take soldering heat before I can do any
better. The trouble is knowing whether the insulation on wire can
take any heat before ordering the stuff.

For all current testing I am using the 3325B, 2Fo 5V square wave
with 2.5V DC offset fed directly to the input of the driver board.
The doubler is completely disconnected and not being used.

Paul


On 04/09/2018 01:41 PM, [email protected] wrote:

Paul

Thanks for the photos and information. I was more interested in the superimposed gate waveforms than the drains, though. A pix of the LPF as well would be appreciated ... think there was one last week.

Just for the record ... my doubler was never mounted inside the amplifier ... 
so you are covering

new ground with that. I would think having it external to the amplifier would be less problematic.

For testing, stick to driving it with the HP3325A for now ... which is what 
you're most likely
already doing.

Can't find much fault with the construction ... other than the somewhat loose primary winding on the transformer. In mine, the primary winding was nestled inbetween the turns of the secondary. This is

important to minimize leakage reactance ... which also contributes to excessive 
ringing.

Think I would continue to concentrate on getting the ringing down before going 
to high power.

One other thing ... have you double checked that you have the correct 
capacitance in the LPF?

Jay W1VD

----- Original Message ----- From: "N1BUG" <[email protected]>

To: <[email protected]>
Sent: Monday, April 09, 2018 11:34 AM
Subject: Re: LF: W1VD amp help - more waveforms

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