[?] Audio amplifiers and GSM interference.

This might not help/be interesting, but every single phone I'd had except for my current one (Nokia 6230) had had some sort of audio effect on radios, hifis and my pc monitor. Perhaps there's less of a problem because of the attention paid to reducing potential brain damage?

Reply to
Poldie
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Hi,

Low-cost commercially available audio pre-amplifiers are usually very vulnerable to interference from adjacent cellular phones switching, in the UK, at around 217Hz.

When designing an audio amplifier with high immunity to such interference, I can see the obvious value of adding suitable RF filtering (ferrite chokes and shunt capacitors with low-Z in the UHF band) to the input circuitry but wonder if a pre-amplifier using differential inputs (FET or Bipolar ?) rather than single-ended would be better. Transformer coupling of source to the amplifier input is another possibility, of course. To meet a current requirement that I have to meet, amplifier noise should not be too much of a problem since the input signal levels are in the region of 20-50mV pk-pk, with a bandwidth from around 300Hz to 5KHz, and the output level required is around 2 volts pk.pk. into a nominal

600 ohm load.

Does anyone with practical experience of dealing with this problem have any comments that they'd care to make about how to minimise such interference?

TIA - Dave

David C.Chapman - Chartered Engineer. FIEE. ( snipped-for-privacy@minda.co.uk)

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Reply to
David Chapman

Turn the damn phone off !

That stuff gets *everywhere*.

What kind of audio are you working with that only has 5kHz bandwidth btw ? Telecoms ?

Graham

Reply to
Pooh Bear

Obviously, but that's not possible if there's no PSTN or alternative means of communication where it is being used.

Yup.

I only need to record the audio received by the GSM phone, and since the highest audio frequency that GSM can pass is little more than 3KHz the amplifier doesn't need to provide a flat response out to 20KHz!

- Dave

David C.Chapman - Chartered Engineer. FIEE. ( snipped-for-privacy@minda.co.uk)

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Reply to
David Chapman

You mean the brain daamge resulting from talking on them too long ? ;-)

Graham

Reply to
Pooh Bear

For any given "victim" device, the only determining factors are the strength of the interfering field at the victim, and the frequency. Victims are only broadly tuned, so the frequency only really depends on which band the phone is operating in (i.e. 900 or 1800 MHz etc.)

Dave

Reply to
Dave Higton

1) Put it in a screening case (i.e. conductive - preferably metal) and take care that seams can under no circumstances fail to make contact over a length exceeding about 40 mm. Pay particular attention to paint or any other surface finish that will prevent conduction, and to the fact that the edges aren't flat. Assume there will be no contact except at points that are pressed together. 2) Take inputs and outputs via screened cables. Make the screens off such that the RF interference remains on the outer skin of the case. Once you take the interference to the inner skin, it's all over. 3) Lay the PCB out with a complete ground plane.

4) Pay great attention to the spurious components, most notably the inductance of all tracks. This means keep everything as small as possible. Surface mount is a great help in this respect. The placement of suppression components is as important as their values.

5) You are unlikely to need ferrites, which is a good thing as they are relatively expensive, if you've done everything else right.

Dave

Reply to
Dave Higton

I have found differenital circuits are not an effective way to reduce UHF interference because you can never maintain adequate amplitude and phase balance of the UHF signal on the two leads...

I suggest you add series impeadance i.e beads and chokes and shunt impeadance like small c's

many times a series 1 kOhm resistor directly at the suseptable device i.e at the transistor base or at the IC input pins will work very well.

The series 1 kOhm and the internal shunt c of the device will form a low pass filter that has little or no effect on the audio but attenuates the UHF.. You need to attenuate the UHF to the point that it no longer creates non-linearity or rectifies. thats when it gets converted to audio and you can hear it....if it is just a low level UHF signal on audio, it won't hurt anything, only when it is large enough to cause non-linearity and rectification and is then converted to audio, does it create audible interference...

Mark

Reply to
Mark

As someone else said, use a metal box for the amplifier. You must prevent the RF from getting inside the metal box which means as the other poster suggested that the seams must be well connected along their length, and also you must pay attention to the wires or connectors going into or out of the box. The ideal situation would be to use 100pF feedthrough type capacitors on all of the signal lines going in or out of the box, but these are expensive and a similar effect might be achievable by using the groundplane of a PCB as one face of the shielding box, with the circuitry surface mounted on the side of the PCB which is inside the box, and then feeding the signals through vias from the other side of the PCB, with 4 surface mount caps mounted radially connected between the via and the ground plane.

If extreme protection is not required, then you could try just building the amplifier on a PCB with a good ground plane (none of this star grounding stuff except near high power stages) and putting a PI type filter with 47pF then 47 Ohms then 47pF again in series with each input wire. If you can tolerate higher value resistors then that would help. The caps should be surface mount with less than 2mm of trace in series with them if you want the best results.

Chris

Reply to
Chris Jones

Ferrites expensive?? not in my experience. if you are willing to buy a thousand about us$2 total. you can use any small value miniature inductors (including surface mount) in their place also (replacing the 47 ohm resistors).

--
JosephKK
Reply to
Joseph2k

[...]

Here's my understanding; maybe someone more knowledgeable will correct me:

Differential amplifiers reduce interference by cancelling the common mode signal. For that to work, the interference has to be a common mode signal. In general, this is achieved by balancing the impedance between the two legs of the signal and the interference source (which is notionally ground-referenced), so that the interference couples equally to both legs.

If your signal is already unbalanced (meaning that the impedance of the two legs to ground is not the same), then a differential amplifier will not help cancel interference. (Interference rejection in balanced transmission has nothing to do with whether the "cold" leg is carrying an opposite-polarity voltage, as should be obvious if you consider the case where the signal is

0V.)

Something that has always confused me; maybe someone can explain:

How can ferrites help, when input impedances are > 1k, as in much audio circuitry? I thought ferrite impedance was around 100 ohms or so, max. I wouldn't think it would be able to create enough voltage drop to make a difference. Is there something else going on?

Reply to
Walter Harley

Actually, victims often have internal resonances that can be very sharp. I've seen thermocouple gear whose quantifiable RF sensitivity peaked at over 20:1 in a few very narrow frequency regions.

John

Reply to
John Larkin

What I know:

Fet front-ends are much less likely to rectify RF than bipolar. Jfet and CMOS opamps can be tens or hundreds of times less sensitive than bipolars.

Multilayer boards, with a solid ground plane, help a lot.

Ferrite beads are magic.

The smaller the better.

Wiring resonates.

John

Reply to
John Larkin

I mean relatively expensive in that they are many times the price of resistors and capacitors; your figures bear that out as against resistors at maybe $0.01 and capacitors maybe $0.02.

Beware. Ferrites for interference suppression are (usually) deliberately lossy, so they dissipate the interference. Inductors are deliberately as little lossy as possible, so you can end up with resonances.

All these problems are soluble by careful design done by someone who knows what he's doing, of course.

Dave

Reply to
Dave Higton

At RF frequencies, trace impedances and IC input impedances are low. At 1 GHz, an opamp input impedance will be ballpark 50 ohms.

And a ferrite bead is a lossy inductor, so it kills resonances, and resonances are a major culprit here.

You can get surface-mount beads that are 600 ohms at 100 MHz, more at higher frequencies. A bead followed by a capacitor is an excellent lowpass filter.

John

Reply to
John Larkin

ONLY IF the trace lengths to the capacitor are short.

Dave

Reply to
Dave Higton

and it wont oscillate :)

maybe, maybe not. depends on how you do it. a "T" connection to the cap (with a nice long stalk) is terrible, the stalk inductance is detrimental, whereas a "V" subsumes the trace inductance into the filter :)

plus the cap inductance means you can only asymptotically approach some minimum L. hence 0508 caps, interdigitated leads etc.

cheers Terry

Reply to
Terry Given

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