A question about parasitics

[n1ps]

I am about to do some upgrades to my E transmitter and it occured to me that I need further education in regard to those pesky parasitics that seem to plague from time to time.  I have run into them before, but I realized I need to understand more about them.  I am more interested in regard to Class E TXs, but certainly tube circuits would apply too.  This is why I posted it here and not on the E forum.

I have read (and heard) several accounts of finding parasitics.  What did they look like?  What should I look for?  What should I use to locate one assuming a TX does not take off and self distruct?  Would I see a VHF component with a spectrum analyzer?

This is one area I have a big gap (vacuum) of knowledge...well there are many...but we'll start here

TNX

Pete
~ps

[steve_qix]

Hi Pete!

At least for solid-state rigs like your class E rig, it is very easy to find out if you have a parasitic (probably you don't since you're using digital drive).  The way I do it is to connect a scope probe to the drain bus, and bring up the power supply voltage to about half.  I make sure the rig is properly tuned up, and then I put highly modulated - to the highest level the modulator will deliver - low frequency pulses into the rig and watch the scope pattern.  I generate these pulses by running the gain up a bit and tapping on the front of a condenser microphone.  It works very well with PWM or class H modulators, which will pass and modulate DC or near-DC frequencies.

Put the scope sweep down so it is fairly slow (maybe 100ms or even lower).   Look for any "grass" appearing above what would be the normal high peak.  If you don't see any, bring up the voltage somewhat and repeat the test, etc.  You will have to check every module in the transmitter (I seem to remember yours has 2 modules).

It is rare, but not impossible to have a parasitic with digital drive because the MOSFET gates are held either high or low all the time.  They are never floating.  Also,  I think you only have a maximum of 2 MOSFETs in parallel and usually there are not problems with this sort of arrangement.

A more likely cause of any possible parasitic with digital drive is coupling back to the driver inputs, but this is not at all common place.

[WA1GFZ]

Yes, you would see a VHF component on an SA. These are due to high drive Z as Steve said but also due to layout and leakage inductance in the wiring and transformer connections/design. You can see it on a scope if the scope is fast enough. I had a low frequency compoment modulating my drive signal once when I had too many drivers in parallel. My 160 meter rig was well before high performance drivers like the IXDD414 appeared on the scene. A low frequency modulation of the drive signal can cause a big boom when the FETs are not going into saturation. So I would start with zero drain voltage and look at the gate drive to make sure it is solid. Then bring up the drain voltage on a variac and monitor both gate and drain to make sure they are both clean.

[n1ps]

Steve TNX..that explains very well.  The "grass" description perfectly describes what I should look for.  I have heard you tapping on the mic before and wondered what exactly you were up to.

Frank...TNX also.  It makes sense that I can see it on a SA.  I happen to have an HP8558 and I will make sure to look for some "pips" up in the VHF band when doing final testing.

Now that I understand how to find them...what is the fix?  It appears the cause is mostly in the input areas around the gates (or grids in tubes).  Layout and presumably lead lengths can perhaps be a factor?  I have read about adding gate resistors (in linears especially) but have no idea why this fix works (or perhaps does not).

p

[WA1GFZ]

Gate resistor is good for isolating two FETs in parallel but it will slow things down a bit. Same thing as raising the driver output Z. My 80 meter final has 11 FETs in parallel without gate resistors. The gates are driven by a PC board trace 2 inches wide and resistor loaded at the far end. Drive end is a step down transformer. I've run it on CW by keying the drive and never had any stability problems. FET connections are in the order of 1/4 inch. Source leads bent down and soltered to the back side of the board making a big ground plane terminated to the heat sink with 1/8 inch spacers. The drains are all bolted to two coper plates making a very low Z connection to the output transformer primary copper tubes. The only time I have seen it unhappy is when the drive isn't high enough. I like push pull FET drivers transformer isolated to avoid cascading failures if a FET ever shorts.