Re: reactance of 0805 and smaller SMT resistors

Mark Randol (ryvw50@email.sps.mot.com)
Fri, 13 Jun 1997 15:52:05 -0700

Charles Hill wrote:
>
> Inductance is a property of a current loop, not a device. As such, the current return path is as important as the signal traces. An 0805 device mounted on the end of a coaxial cable has about 0.2nH of inductance; this would be a minimum figure. This inductance is increased on a PCB with the addition of vias and connections to the mounting pads. A smaller device will have less inductance but that is not as important as how the device is mounted, the trace lengths, and current return path.

Sometimes it's easy to forget this when talking about the parasitics of
devices. But having a device at the end of a piece of coax doesn't
necessarily mean that a measurement of 0.2 nH is valid for all
frequencies unless you account for the length and *parasitics* (losses,
leakages, etc) of the coax, right? What about the added inductance
of the path connecting the center conductor and "shield". It could be
different if you are using RG58, RG8, or 0.085 semi-rigid due to the
diameter
differences. Most of the component vendors I've seen that publish
parasitics or frequency reposnse data also publish information regarding
the fixturing and other components in the test setup. So generally
when someone asks "what are the parasitics of X component", all the
other factors you mention are hopefully understood and accounted for
in both measurement and use of the device.

For example there isn't any such thing as an unconditionally stable
amplifier if you throw away the "no external feedback" assumption. No
one
bothers to mention it every time, but sometimes people forget, so
your reminder has value.

BTW, there isn't anything magical about coax for this measurement.
You could use coplanar transmission line, 300 ohm twinlead, or
lamp cord depending on the frequency of measurement, availability
of calibration standards, the desired accuracy and precision, etc, etc.

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Mark Randol, RF Measurements Engineer   | Motorola SPS, Inc.
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