Electrical Engineering - What screening to protect mic from cellphone interference?

Author

What screening to protect mic from cellphone interference?

Mozzy

2007-02-20, 8:25 pm

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/..._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

cs_posting@hotmail.com

2007-02-21, 3:25 am

On Feb 20, 7:21 pm, Mozzy <d...@account.com> wrote:

> 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.

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.

Tzortzakakis Dimitrios

2007-02-21, 5:25 pm

? <cs_posting@hotmail.com> ?????? ??? ??????
news:1172031439.228710.31640@l53g2000cwa.googlegroups.com...
> On Feb 20, 7:21 pm, Mozzy <d...@account.com> wrote:
>
>
> 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.
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)

> 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.
>

--
Tzortzakakis Dimitrios
major in electrical engineering
mechanized infantry reservist
dimtzort AT otenet DOT gr

Dave Higton

2007-02-21, 5:25 pm

In message <Xns98DE3A21981701A4D@127.0.0.1>
Mozzy <demo@account.com> wrote:

> 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/..._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?

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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