[sdiy] RC, LC we all c RFin HiZ

[Czech Martin]

yes, this is called a low pass filter ;->

But: if it is going to have more then 20dB/dec (your capacitor)
it will soon get expensive.
Better try to keep RF out of the system in the first place.

Unshielded cable will build a pretty loop antenna for the magnetic
component, for example.
So , if you live in a RF drenched area, you have to be carefull with
wiring. If your area is clean, unshielded cable can be ok.
Please no banana war again.

It is also a question of the frequency domain, which can still
cause problems. Most op amps have gain-bandwisth products from 1 -3 MHz,
in rare cases 10MHz. For this range of frequencies it is possible
to use a capacitor. But: series inductance will limit the usefullness.
You have to keep the leads very short, but also the pcb traces (which
can turn out very difficult, and is thus often "forgotten").

In some cases active components will work as rectifiers for
RF signal, even if their normal range of frequency is by far exceeded.
Makes a good radio reciever. If the fields are very strong, this
can also introduce DC-shifts, or low frequency shifts.
But then you should consider moving house. Perhaps a health risk
also.
So, the problem is not only audio interference noise, but can also
lead to more complicated system malfunction.


I remember a IC design a couple of years ago, all was good, until the
customer drove near by a AM transmitter station with several MW
output. The circuit was not designed to handle anything over - say -
100kHz, but the RF stuff was rectified internally and caused very strange
offsets which drove in turn the designer crazy.

So: shield it (metal case, coaxial cable),
if that does not keep it out, filter the rest.
Good transformers often have a shield foil today.
Or better two.
Mains cables can be a good antenna, too.
A feritte core arround them can work wonders. 
You have seen these cylinders on your computer video
cable? That's it.

m.c.

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of mark s
> Sent: Dienstag, 13. Juli 2004 06:44
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] RC, LC we all c RFin HiZ
> 
> 
> let's start with an ideal (wish):
> 
> is there an effective way to eliminate, at the input of the 
> circuit, all 
> frequencies above, say 50kHz, leaving the audio band 
> untouched ? placed at 
> the input of a circuit, it would prevent all the bad stuff 
> from getting in 
> through the input cable, at least.
> 
> ok now go ahead and school me :^)  but please share your 
> techniques for 
> reducing AM, FM and the rest of it.
> 
> i'll ante: a 10nF or even better 20nF cap across the input 
> jack has worked 
> wonders for me. but sometimes you need more....
> 
> mark
> 
>