EMI ISSUES DUE TO WALKIE TALKIE

[kumar]

One of our equipments malfuntions when a walkie talkie handset is brought near it. Can one of you please write about walkie talkie signal ( i.e. carrier frequency, type of modulation, depth etc.). Is this a typical radiated susceptibility problem?

We may have to add filter capacitor for bypassing this frequency. How to findout the self-resonance frequency of the capacitor? I use a Rf signal generator "SynthNV" and "RF explorer" spectrum analyzer.

[xrunner]

Might be able to block rf from coming in via external wiring by putting ferrite on wires, depends on the freq.

[AG6QR]

One of our equipments malfuntions when a walkie talkie handset is brought near it. Can one of you please write about walkie talkie signal ( i.e. carrier frequency, type of modulation, depth etc.). Is this a typical radiated susceptibility problem?

We may have to add filter capacitor for bypassing this frequency. How to findout the self-resonance frequency of the capacitor? I use a Rf signal generator "SynthNV" and "RF explorer" spectrum analyzer.

What kind of walkie-talkie?  CB? FRS? Ham? MURS? Commercial? Police? Kid's toy?  Each of these work in different bands, and some services have multiple bands assigned.

They might have almost any frequency or type of modulation.  Most likely it'll be somewhere between 27 MHz and 1000 MHz, though there are possible frequencies outside this range.   AM is more common below 30MHz, while FM is more common above.  Various digital schemes are also possible.

If the walkie-talkie has an FCC Id somewhere on its data plate, look up that ID on the fcc.gov site, and you may get more information.  Or you might want to look at its output with that spectrum analyzer.

[xrunner]

Well it's funny you asked this question yesterday because I had just fixed an RF problem with a ferrite device. I was transmitting in the 18 MHz ham band and I was hearing some squawking coming out of some computer speakers sitting above the transceiver. When I unplugged the audio line going to the PC it stopped. I simply wrapped the audio line once around a ferrite near the speaker and presto - no more interference. Of course if the RF is entering the equipment in another way this won't work.

Watch this video by W2AEW -

[kumar]

Thanks for the replies.

Is there any way to find the resonance frequency of capacitor? Or is there any standard meter ( like ESR meter ) which can tell the resonance frequency?

[Maxlor]

A capacitor doesn't have resonance by itself. It's when you combine it with a resistor or inductor or both that resonance and filter characteristics start to appear. So when people say they're adding a filter capacitor for example, they're usually adding it to a point in the circuit where there is some resistance already, building a lowpass RC filter. And to calculate that, you'd use the formula f = (2 * pi * R * C)^-1.

[C]

Expanding on AG6QR's post
Don't forget the mixer frequencies the walkie-talkie may use while receiving or transmitting. For some radios these are not shielded that well.

C

[AG6QR]

A capacitor doesn't have resonance by itself. It's when you combine it with a resistor or inductor or both that resonance and filter characteristics start to appear.

But real world capacitors have parasitic inductance, which causes self-resonance at a high enough frequency.  In particular, the parastic inductance of electrolytic capacitors with spiral wound electrodes is often non-negligible.  The inductance of any capacitor's leads will become non-negligible if you take the frequency high enough.  (As an aside, inductors have parasitic capacitance that causes self-resonance, too).

http://www.home.agilent.com/upload/cmc_upload/All/exp79.pdf?&cc=US&lc=eng

[w2aew]

A capacitor doesn't have resonance by itself. It's when you combine it with a resistor or inductor or both that resonance and filter characteristics start to appear.

But real world capacitors have parasitic inductance, which causes self-resonance at a high enough frequency.  In particular, the parastic inductance of electrolytic capacitors with spiral wound electrodes is often non-negligible.  The inductance of any capacitor's leads will become non-negligible if you take the frequency high enough.  (As an aside, inductors have parasitic capacitance that causes self-resonance, too).

http://www.home.agilent.com/upload/cmc_upload/All/exp79.pdf?&cc=US&lc=eng

Yes, I even did a couple of videos on capacitor self resonance....


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