Re: gEDA-user: High speed decoupling (was: Simulation of ceramic capacitors, pairs and groups)

[Bill Sloman <bill.sloman@xxxxxxxxx>]

At 01:46 8-3-2006, you wrote:
> John Doty <jpd@xxxxxxxxxxxxx> wrote:
>
> > An inductor is a concentration of magnetic energy. A wirewound
> > resistor also concentrates magnetic energy to some extent. For these
> > it's sensible to associate an inductance.
> > John Doty              Noqsi Aerospace, Ltd.
> > jpd@xxxxxxxxxxxxx
>
>
> John,
>
> What about the capacitance of a wirewound?  Wirewounds are usu. wound as a
> loop folded in half on a former to reduce inductance (small loop area) with
> the beginning and end of the loop touching thru a dielectric (glass, enamel,
> carbon, plastic, etc).  You get a capacitance bypassing the inductor and the
> resistor!
>
> Ever see or measure that?

Philips once published impedance bridge measurements for their MRS25 
0.6W spiral-cut thin film axial lead resistors, and the lumped 
parallel capacitance came out at about 0.3pF.

B P Kibble and G H Rayner in "Coaxial AC Bridges", now available over 
the web from the British National Standards Laboratory at

http://www.npl.co.uk/electromagnetic/publications/guides/ac_bridges.html

give a figure of roughly one picofarad for the sorts of non-inductive 
wire-wound resistors sued in standards labs.

L-trimmed thin film surface mount resistors seem to have something 
like an order of magnitude lower parallel lumped capacitance, and 
Rayner and Kibble like the Vishay thin film laser trimmed precision 
resistors, but don't put a figure on their parallel capacitance, 
beyond characterising it as "low" and presumably thus lower than the 
1pF figure they give for regular resistors over the page in the next section.

--
Bill Sloman, Nijmegen