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Gate Waveform

PostPosted: Mon Nov 02, 2009 7:36 am
by Gilly
Just finishing a new 80M Class E TX......

This is the waveform using a IXDD414 into one 11N90 FET....the CRO is showing both modules....

Image

This is the waveforms of one IXDD414 into two 11N90 FETs

Image

Any ideas whats causing the ripple at the top....I do have the same ripple on the DC rail into the IXDD414....
Are using bypass capacitors at the Vcc and En pins of the IXDD414's....


Thanks


Wayne

Re: Gate Waveform

PostPosted: Mon Nov 02, 2009 8:16 am
by kf1z
Wayne

Do you have that ripple on the DC buss, With NO signal applied to the ixdd414?


Is that waveform WITH DC applied to the drains? or without...?

I have seen that here on occasion... I can't remember what caused it...

May be a ground issue between the vfo and 414's.

Re: Gate Waveform

PostPosted: Mon Nov 02, 2009 5:46 pm
by Gilly
Hi Bruce

The TX is fully finished but just in its testing stages...

The 12VDC rail is well regulated and has no ripple .... and the FETs have no HV applied to the Drains....

The ripple only appears when the 2nd FET is connected...

The VFO can be switched to 160M and this is the waveform with the IXDD414 into 2 FETs....looks good..

Image

Have added a 1 Ohm resistor and some ferrite beads around the DC input to the IXDD414 without any change etc:

I seem to have come though better then the waveform in the photo...sorry...

Thanks


Wayne

Re: Gate Waveform

PostPosted: Mon Nov 02, 2009 7:46 pm
by kf1z
Wayne,

Email me a picture of your layout... showing the drivers and FETs etc....

Bruce

Re: Gate Waveform

PostPosted: Tue Nov 03, 2009 3:08 am
by Gilly
Hi Bruce


OK here is a photo....

Image

Went a bit over board with the ferrite beads etc: in the VCC to the IXDD414's...
The capacitors in that area are .47uF....ohh except for those ceramics....

Thanks


Wayne

Re: Gate Waveform

PostPosted: Tue Nov 03, 2009 6:43 am
by kf1z
Wayne,

I don't see much... :)

The only thing I wonder about is the vfo input to the ixdd414

Lead looks a little long , and maybe terminated with a 100 ohm resistor?, or 2 ?

Maybe you could send a pic taken from above, showing the whole thing...

Re: Gate Waveform

PostPosted: Wed Nov 04, 2009 7:17 am
by Gilly
Problem solved .....

I assumed the 12VDC supply was working properly...but it wasn't....
Poor regulation gave me those results.....

This is now the waveforms of one IXDD414 into 2 11n90's on 80M....with the fixed driver supply...The waveforms look quite good...

Image

My photos come out to large for some reason !!


Wayne

Re: Gate Waveform

PostPosted: Thu Dec 17, 2009 2:53 pm
by frank carcia
I bet you won't see the ripple if you single sweep the scope. I suspect low frequency AM modulation on the driver VCC. I have seen this before when I put smaller drivers in parallel.
The rise and fall times look clean so I don't think anything is taking off. Adding the second Fet puts a greater load on the driver. Film caps may not help as a bypass. I bet beads make it worse.

Re: Gate Waveform

PostPosted: Thu Dec 17, 2009 2:54 pm
by frank carcia
I bet you won't see the ripple if you single sweep the scope. I suspect low frequency AM modulation on the driver VCC. I have seen this before when I put smaller drivers in parallel.
The rise and fall times look clean so I don't think anything is taking off. Adding the second Fet puts a greater load on the driver. Film caps may not help as a bypass. I bet beads make it worse.