What screening to protect mic from cellphone interference?

What screening is necessary if I want to protect a domestic-grade electret microphone from the RF coming from a cellphone which is in use?
I sometimes hear radio or TV news broadcasts which still get cellphone interference so I would guess it is not all that easy to provide a reasonale level of screening.
On the other hand, you can get a cheap electret microphone designed to be worn in the ear (as a phone-recording microphone) which is remarkably resistent to cellphone RF.
http://www.teknikmagasinet.se/prod/stor_bild_png/290080.png
How do I make a domestic electret microphone capsule with its co-ax microphone cable (cdonsisting of shield and one core) resistent to cellphone RF?
Moz
--



Note: xposted to 4 relevant groups (GKSA limit) = alt.cellular
alt.engineering.electrical sci.electronics.design uk.telecom.mobile
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Generally the way intereference gets in is by speaker or power or signal wires acting as antennas. If a wire doesn't need to carry a high speed signal, you can bypass it with an RF choke in series and/or capacitors to ground. And put the circuit in a shielded enclosure. Chokes, and perhaps coaxial microphone cable, should help quite a bit.
With GSM phones, the worst interference seems to be when the network is getting ready to ring them. In fact, I can often tell when my phone is about to ring because of what happens to my computer speakers, or car radio. Chances are the phone won't be heard in its own headphone because it probably has it's audio jack bypassed with chokes and its amplifier not only well shielded, but also muted at this point. And nobody is yet on the other end to hear if the microphone is being interfered with. Presumably a phone that does voice recognition could even know exactly when it is transmitting its stronger pulsed signals, and thus if necesssary not listen right then, to avoid confusing itself.
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wrote:

Err...I hope that it doesn't do something similar to our brains?(Just now reading Stephen King's cell).I have measured the electrostatic field of my Nokia 1100, and is 11 V/m at, maybe, 1/2", when calling (when idle, something like 0.44 V/m)

-- Tzortzakakis Dimitrios major in electrical engineering mechanized infantry reservist dimtzort AT otenet DOT gr
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It is exceptionally unlikely to be the microphone that's causing the problem; it will be the electronics that it's connected to that is demodulating the signal, and therefore that needs to be filtered and/or screened.
Unless you understand electronics fairly well, it's difficult or impossible to explain how to do this. If you /do/ understand electronics, then the best hint is to beware of the inductances of even short lengths of wire. Surface mounted components and tiny track lengths are your friends.
Dave
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