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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: signal leads that pick up less ambient noise?
Date: Wed, 26 Feb 2025 20:40:22 +1100
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On 26/02/2025 4:55 pm, Edward Rawde wrote:
> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vpm0qm$29gbe$1@dont-email.me...
>> On 26/02/2025 4:39 am, Christopher Howard wrote:
>>> Bill Sloman <bill.sloman@ieee.org> writes:
>>>
>>>> On 25/02/2025 4:46 am, Christopher Howard wrote:
>>>>>> Google for coaxial feed through capacitors.
>>>> Capacitors and resistors don't ring. Adding inductance can introduce
>>>> ringing. but enough resistance can make the resonant circuit
>>>> critically damped and the voltages and current will decay
>>>> monotonically.
>>>
>>> So, when you use a coaxial feed through capacitors on your Faraday cage,
>>> do you add a resistor right after the capacitor, to reduce/eliminate
>>> ringing?
>>
>> The whole point about coaxial connectors is that the distributed capacitance and inductance gives you a R50R transmission line.
>> The only way to get "ringing" out of that is to fail to terminate the transmission line with  it's characteristic impedance. In
>> practice it is hard to do it perfectly and you do tend to get low level reflections, but they die out fast,
>>
>>> Or are you just trying that all your inputs on the board have
>>> resistors before whatever op amps or other components that they feed
>>> into?
>>
>> The message is rather more complicated than that. The later editions of Ralph Morrison's book do go into that in more detail than
>> the earlier editions.
> 
> The sixth edition only mentions the "feed-through" capacitor in one paragraph on page 65.
> The fifth edition does not mention them at all as far as I can tell.

Feed-through capacitors seem only to be used in RF electronics, and 
Ralph Morrison's book initially concentrated on regular industrial 
electronics. Later editions did move on to higher frequency applications.

I never used a feed-though capacitor anywhere in  the work I did - if we 
need to put a fast signal through a conducting bulk-head we used coax 
feed-throughs. Feedthrough capacitors are relatively exotic devices.

Your example of their application seems to be a case where an RF 
specialist went in for a bit of over-kill.

-- 
Bill Sloman, Sydney