From: Jason White <whitewaterssoftwareinfo@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Fri, 17 Jan 2020 12:06:34 -0800
Subject: Re: [EE] Measurement noise, signal superimposed on H-Bridge
 operation.
Thread-Topic: [EE] Measurement noise, signal superimposed on H-Bridge
 operation.
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Update: adding a 100nF decoupling capacitor between the Vin and the
floating GND close to the power connector reduced EMI by an order of
magnitude (400mv vs 10V pk-pk). Some is still present.

1) unfortunately no.
2) the FETs are SOT26, I can't try that very easily
3) I decouple the drivers themselves quite well. The unregulated Vin
switched by the bridge had no decoupling prior to me just adding it
now.
4) I don't, but I am stuck with it in the short term.
5) I might try, but the circuit layout is such that there already is a
central ground point (the FET). The H-Bridge is to the left of the
point and the power supply and logic is all to the right of the point.
6) The ground node is "floating" a NMOS FET can disconnect ground to
protect the circuit if the supply voltage gets too high. The ground
plane goes through the FET to the power connector.

On Fri, Jan 17, 2020 at 2:44 PM Sean Breheny <shb7@cornell.edu> wrote:
>
> Lots of possibilities:
>
> 1) Can we see the whole circuit?
> 2) Try adding ferrite beads in the gate lead of the mosfets (during turn-=
on
> and turn-off there can be a very high gain which can amplify RF oscillati=
on)
> 3) How well do you bypass the power and ground close to the h-bridge?
> 4) Why do you want shoot-thru?
> 5) Consider separate ground and power "islands" for the h-bridge and then
> connect them to the rest of the circuit at only one point each (and have
> lots of bypass caps sprinkled about the fets, linking the power and gnd
> islands)
> 6) Why does your diagram show a block called h-bridge which is itself
> switched by another low-side fet?
>
>
> On Fri, Jan 17, 2020, 1:38 PM Jason White <whitewaterssoftwareinfo@gmail.=
com
> wrote:
>
> > As a follow up: I have attached a screen capture showing the waveform
> > of the noise.
> >
> > The signal presents itself as 1 cycle of a sine wave at 12.5MHz
> > followed by 12 cycles of a sine wave at 62.5MHz on a 100MHZ scope
> > using a 25MHz differential probe. Less than 400ns in duration from
> > start to finish.
> >
> > Just writing this out made me think: while I would like to eliminate
> > the radiation from my circuit - I may or may not be able to. Perhaps I
> > can filter it out of my measurements?
> >
> > --
> > Jason White
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
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> >
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