R:[SI-LIST] : Adding inductors to ground?

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From: Vigliarolo Roberto (Vigliarolo@asamrt.interbusiness.it)
Date: Mon Mar 06 2000 - 03:26:58 PST


If you need to insert more resistance (to damp the resonance) than you can
afford (too much dc drop) you can try paralleling the capacitor of your LC
filter with a higher value capacitor (typ 5x) that has in series the
resistance you need to properly damp the circuit with no dc drop at all.
One caveat: you need to use the "right" value of resistance otherways you
end up with a peacky response!
 
Regards,

Roberto Vigliarolo

 Ferrovie dello Stato SpA
 Tecnica e Ricerca
 Unificazione Tecnica
 Viale Lavagnini, 58
 50129 Firenze
 Italy
 
 Tel: +39-055-235-3848
 Fax: +39-055-235-3581
 Email: Vigliarolo@asamrt.interbusiness.it

----------------------------------------------------------------------------
----------------------------

> -Ray wrote
>
> I've seen this effect also with badly designed discrete LC
filters used
> in the same sort of applications. The cure is the judicious
insertion of
> a small bit of resistance to damp the response. You need
enough to
> tame the peaking but not enough to mess up DC conditions.

----------------------------------------------------------------------------
-----------------------

>
> sweir wrote:
>
> > The usual good intention of this is to keep noise from the planes
> out
> of
> > the PLL, not the other way around. The fear is that the VCO will
> get
> > thrown off by PS noise. It can be more dangerous to use an
> inductor,
>
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > unless the filter is properly damped, than to use either a
> resistor or
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > direct connection. In either case, high quality bypass from the
> PLL
> VCC to
> > the PLL ground is very important. This is also one of the places
> where
> > judicious moating can be very useful.
> >
>
>
> Good point. I was involved in fixing a design that someone messed up
> where they used one of the little integrated ferrite low pass
> filters
> to feed the PLL VCC pin on an ASIC.
>
> As it turns out, if you look at the attenuation curve in the
> vendor's
> data book, the low pass characteristic appears well behaved and as
> you might expect. The gotcha' is that the parts are characterized in
> a 50 ohm system. Well if you are feeding the filter from a power
> plane
> then the driving impedance is probably in the sub-ohm region and the
> load presented by the PLL is in the couple hundred ohm region.
> Grossly
> mismatching the little lpf part like that causes a real peaky
> response.
> The one I saw had a peak of around 20 dB. The end result was that
> there
> actually was more noise on the PLL VCC pin coming from the power
> plane
> than there was on the plane. The filter didn't attenuate the noise
> but
> it enhanced it.
>
> I've seen this effect also with badly designed discrete LC filters
> used
> in the same sort of applications. The cure is the judicious
> insertion of
> a small bit of resistance to damp the response. You need enough to
> tame the peaking but not enough to mess up DC conditions.
>
>
> -Ray
>
>
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