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SMPS LC filter... not!?

Started by George Herold September 13, 2016
Hi all, I'm having a big fail when trying to make an LC filter for a CUI
SPMS.  There's nothing to the circuit.  Here.

https://www.dropbox.com/s/xgesfnb3a6z0x1n/L-C-schem.JPG?dl=0

The problem is that no matter what I do to the Land or C's the output 
always has the same amount, (shape and size) of ripple.  
Here's a 'scope shot.  
https://www.dropbox.com/s/inv0w000gdm9le7/TEK0042.BMP?dl=0
Channel 2 is the input 
Chan. 1 is output.  


And here's the layout.  
https://www.dropbox.com/s/4q0n60q116j1hql/DSCN9494.JPG?dl=0

(I was having the same problem 
in an earlier layout, but thought it might be the lack of a good 
ground plane.. hence today's modification.)  

I'm doing something stupid, 'cause no matter what I do nothing changes. 

George H. 
On Tuesday, September 13, 2016 at 2:19:52 PM UTC-4, George Herold wrote:
> Hi all, I'm having a big fail when trying to make an LC filter for a CUI > SPMS. There's nothing to the circuit. Here. > > https://www.dropbox.com/s/xgesfnb3a6z0x1n/L-C-schem.JPG?dl=0 > > The problem is that no matter what I do to the Land or C's the output > always has the same amount, (shape and size) of ripple. > Here's a 'scope shot. > https://www.dropbox.com/s/inv0w000gdm9le7/TEK0042.BMP?dl=0 > Channel 2 is the input > Chan. 1 is output. > > > And here's the layout. > https://www.dropbox.com/s/4q0n60q116j1hql/DSCN9494.JPG?dl=0 > > (I was having the same problem > in an earlier layout, but thought it might be the lack of a good > ground plane.. hence today's modification.) > > I'm doing something stupid, 'cause no matter what I do nothing changes. > > George H.
Aw snap.. never mind, it's some dang ground thing. I can drag the ground of the 'scope probe around the pcb and get all sorts of "answers". George H.
On Tuesday, September 13, 2016 at 11:19:52 AM UTC-7, George Herold wrote:
> Hi all, I'm having a big fail when trying to make an LC filter for a CUI > SPMS. There's nothing to the circuit. Here. > > https://www.dropbox.com/s/xgesfnb3a6z0x1n/L-C-schem.JPG?dl=0 > > The problem is that no matter what I do to the Land or C's the output > always has the same amount, (shape and size) of ripple. > Here's a 'scope shot. > https://www.dropbox.com/s/inv0w000gdm9le7/TEK0042.BMP?dl=0 > Channel 2 is the input > Chan. 1 is output. > > > And here's the layout. > https://www.dropbox.com/s/4q0n60q116j1hql/DSCN9494.JPG?dl=0 > > (I was having the same problem > in an earlier layout, but thought it might be the lack of a good > ground plane.. hence today's modification.) > > I'm doing something stupid, 'cause no matter what I do nothing changes. > > George H.
First thought is to improve the scope probe grounding - you may be picking up signals in the ground lead. I found that the best way to look for small signals is to hardwire coax (eg RG174) directly with the screen soldered to the ground plane very close to the measurement point - I used to keep some short lengths of coax SMA terminated on one end just for that purpose. I can often see whether there is any signal in the probe ground just by gripping the probe cable tightly with my hand - if there is any change in the signal on screen there must be signal in the ground that is corrupting it. kevin
On Tuesday, September 13, 2016 at 2:39:16 PM UTC-4, kevin93 wrote:
> On Tuesday, September 13, 2016 at 11:19:52 AM UTC-7, George Herold wrote: > > Hi all, I'm having a big fail when trying to make an LC filter for a CUI > > SPMS. There's nothing to the circuit. Here. > > > > https://www.dropbox.com/s/xgesfnb3a6z0x1n/L-C-schem.JPG?dl=0 > > > > The problem is that no matter what I do to the Land or C's the output > > always has the same amount, (shape and size) of ripple. > > Here's a 'scope shot. > > https://www.dropbox.com/s/inv0w000gdm9le7/TEK0042.BMP?dl=0 > > Channel 2 is the input > > Chan. 1 is output. > > > > > > And here's the layout. > > https://www.dropbox.com/s/4q0n60q116j1hql/DSCN9494.JPG?dl=0 > > > > (I was having the same problem > > in an earlier layout, but thought it might be the lack of a good > > ground plane.. hence today's modification.) > > > > I'm doing something stupid, 'cause no matter what I do nothing changes. > > > > George H. > > First thought is to improve the scope probe grounding - you may be picking up signals in the ground lead.
Quite Right!
> > I found that the best way to look for small signals is to hardwire coax (eg RG174) directly with the screen soldered to the ground plane very close to the measurement point - I used to keep some short lengths of coax SMA terminated on one end just for that purpose.
Hmm yeah that works. But I've got to send all the power supplies out of the box, with one ground connection. (This whole ground idea is over rated :^) George H.
> > I can often see whether there is any signal in the probe ground just by gripping the probe cable tightly with my hand - if there is any change in the signal on screen there must be signal in the ground that is corrupting it. > > kevin
I remember that I used a large diameter ferrite toroid as a common mode filter. It needs to be large enough to get the probe through the middle. 
Was I cheating?

Graham