Re: [SI-LIST] : Another capacitor question

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From: Ray Anderson (raymonda@radium.eng.sun.com)
Date: Thu Jan 27 2000 - 11:46:48 PST


The effect that I was concerned with in the RF/Microwave realm
was the fact that minor changes in the capacitive parasitics
due to mounting orientation were enough to detune narrowband
circuits like filters and matching networks.

Typically, in production our designs needed little if any
tuning as the circuit was comprised mostly of printed
components (inductors, stubs, t-lines, etc.) and chip
capacitors. When the capacitors were mounted the 'wrong-way'
the delta in the stray capacitance to ground was enough
to distort the passband responses and to cause gain loss
due to detuned matching networks. This caused a lot of
extra time to be spent in production to fine tune the boards
with soldered on shims or rework to re-orient the capacitors.

Once our purchasing department found they could save a couple
of cents per part by getting unmarked parts instead of the
laser marked versions. You couldn't tell the orientation
of the caps during inspection as they were cubical.
Couldn't tell the value either :)

Never really had any EMI problems that I could attribute to
the capacitor orientation.

In the SI world where there are very few narrow-band structures
I wouldn't anticipate an extra pF to ground to really cause many
problems.

It has been suggested that there may be a bit of change in part
inductance due to a change in current density across the part's
cross section caused by mounting orientation. This could change
a part's resonant frequency a bit, but I bet it is minor compared
to the delta's resonance caused just by component tolerance and
temperature effects.

-Ray Anderson
Sun Microsystems

>
> I am curious what effects these are and how they manifest
> themselves. i.e. are they purely EMI related (field orientation) or are
> there other effects also?
>
> Doug Brooks
>
>

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