[AMPS] parasitic suppressors

[Ian White, G3SEK]

Peter Chadwick wrote:
>
>>?  100 ohms.
>
>That must be an awful 100 ohm resistor that put in a parallel with an
>inductor gave a parallel resistance of 166 ohms. Until we hear what the 100
>ohms resistor measured on its own, I have difficulties with either that
>measurement result or the idea of it being 100 ohms.
>

I remember thrashing through all this with Rich, the last time this
topic came around.

With the typical 100nH/100ohm values we're talking about, the *measured*
value of equivalent parallel resistance will inevitably be greater than
100 ohms at higher frequencies. All it needs is a few nanohenries of
lead inductance connecting the parallel network to the binding posts of
the impedance analyser.

For example, with an ideal 100nH/100ohm network, it only takes 29nH of
series lead inductance to bring the equivalent parallel resistance up
to 166 ohms at 30MHz [1]. 

29nH is only about HALF AN INCH of wire, each side of the network!

This clearly shows how important it is to take the rest of the circuit
environment into account, when evaluating the performance of a
suppressor network.


[1] That calculation took about 1 minute total, using the NETCALC
program which is downloadable from my web site. (New users can expect to
take a bit longer  :-)


73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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