Bill, W6WRT wrote:
>ORIGINAL MESSAGE:
>
>On Wed, 12 Aug 2009 13:06:41 -0700, Jim Barber <audioguy@charter.net> wrote:
>
>>
>>Looking for opinions, of course. I find myself replacing carbon
>>composition units all the time, and was wondering if metal-oxide would
>>be "better" than carbon-film for these purposes.
>
>REPLY:
>
>Either metal-film or metal-oxide resistors are significantly more stable than
>carbon comp.
Carbon *composition* resistors are notorious for changing value over the
long term (even unopened NOS). For all practical purposes they are
obsolete.
>Their only possible disadvantage is that some values may exhibit
>more inductance than carbon because the film is sometimes trimmed in a spiral,
>creating a tiny inductor. It is very small but at UHF it may be significant.
Carbon film resistors are cut in a spiral the same way.
The inductance of a few turns on a small former is very small, and for
example I wouldn't have any worries about using 2W metal film resistors
in VHF parasitic suppressors. To keep things in perspective, any amp
that couldn't tolerate this small additional inductance would need some
work on the *causes* of its instability.
Metal film resistors would be the choice for parasitic suppressors
because they have better performance at high temperatures. Two or three
in parallel might be needed for comfortable operation on 10m, so that
reduces the combined inductance even further.
However, the original question was about an impedance bridge, where
these small values of inductance *are* important. This applies not only
to the individual components but equally to the layout and style of
construction. HF measuring equipment requires VHF-style construction,
and precision HF measuring equipment should be built like it's intended
to operate on UHF.
For all those reasons, probably the best components for long term
stability in an impedance bridge would be 0.1% metal film chip
resistors. Since there will be "little or no RF power dissipated", there
is no virtue whatever in using 1W resistors - use the smallest you can
handle.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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