There's no easy answer to that question. The two biggest factors to
consider when selecting the right value are:
a) What frequency range are you trying to filter out? If your problem is
keeping an AM broadcast station out, you'll need larger values as compared
to targeting your own VHF/UHF emissions.
b) What is the source Z of the signal(s) being passed? As the Z goes up,
you'll typically want less C lest you have roll-off problems in audio
circuits, "round off" of square-wave digital waveforms (generally not a good
idea to be filtering high speed digital circuits in this manner), etc. If
the Z is fairly low, and if you're only dealing with AF and on/off logic
signals (COR, PTT, etc.), this becomes less of an issue. But you should
still do the math before picking a value.
Feedthru caps are but one tool for fighting RF ingress. The cap needs some
L or R to "work against" if it is to serve as part of a low-pass LC or RC
circuit. Ferrite beads (of the approximate mix for the targeted frequency
ranges) and/or chokes may come into play as well. Take a look at the
filtering on a fully-fitered Micor unified chassis station to get an idea of
Mother Mo's take on the subject. IIRC, on the receiver interconnect there
are two feedthru caps (1000 pF?) with a choke between them on most, if not
all, circuits to/from the backplane. I don't remember if all bands/models
used the same values for caps and chokes, maybe somebody else does.
--- Jeff
> -----Original Message-----
> From: [email protected]
> [mailto:[EMAIL PROTECTED] On Behalf Of Doug Hutchison
> Sent: Sunday, April 22, 2007 3:21 PM
> To: [email protected]
> Subject: [Repeater-Builder] Seeking feed thru' cap value.
>
> What value of cap should be used as a minimum for TX RX controller
> inter-connection. 1000pf, 5000pf?
>
> Doug - GM7SVK
>
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> Yahoo! Groups Links
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