Electronics > Beginners
Interference problem on a wireless mic system
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WaveyDipole:
Hi, I'm trying to help solve an interference problem someone is having with their 100v line PA system. It is in a small auditorium with a half a dozen ceiling mounted speakers in two rows and using two VHF wireless microphones. The length of the auditorium is well within the maximum range of the mics. The problem is there are several fixed spots in the auditorium where the microphone starts to ring and whistle and the mic radio signal drops significantly.  The spots in question seem to be almost halfway between any pair of speakers which seems more than co-incidence. They have tried various locations for the receivers etc and nothing seems to have made any difference.

One thing I looked at was the line output from the radio, which is unbalanced. The first image shows the audio signal but it appears swamped by the hash and the scope FC is getting confused. One might perhaps expect the amp to filter this out and the hash is too high frequency to be audible but I was wondering whether it might be being passed and causing some kind of intermod effect via feedback from the speakers?

I would also like to understand what I am seeing here as there seem to be several frequency components. The second image shows the output with mic switched off and the line quiet. In the following images I have tried to separate out the various components. I am guessing that the 38khz wave is SMPSU noise, but what might the 9MHz and 19MHz noise be?

The other curious thing was that when microphones were switched on and in proximity to the scope, the probes were picking up a signal approximately one third (about 60-70Mhz) of the carrier frequency. Would this be a harmonic?

Moreover, if this is down to the wall wart, what do I do to filter it?
ogden:
What microphones/receivers (models) used? What radio frequencies used?

Whistling is just acoustic feedback.

Someone shall consider introducing feedback eliminator.
Electro Detective:
Do the wireless mic receivers have some sort of small recessed adjustable 'squelch' type control on the back?

If yes, it pays to get a small screwdriver and play with that whilst in the "several fixed spots in the auditorium where the microphone starts to ring and whistle and the mic radio signal drops significantly"

you may get lucky and nuke your problem, or help minimise it

Watch out for loud runaway hash noise during extreme or unsure adjustments, best to have a compressor limiter in the chain on hard limit to be sure  :scared:

and check all the cabling too, many unbalanced cables today are crap, a single wire with a carbon shield layer inside the jacket that crumbles when flexed  :--

Good luck  :-+
Brumby:
First thing, as has been mentioned, is that the ringing and whistling you are getting is more than likely audio feedback.  At any one of those points you describe, I would guess that if you change which direction the microphone is pointing, you are going to find orientations where the problem is worse and others where it is not so.

Next, I would like to ask if you have the earth lead from your scope connected properly.  If it is not connected at all, then you are going to have everything connected to the probe acting as an antenna picking up who knows what.  Those screen shots look like the sort of crap you could get.  Before making such a connection, ensure there is no significant voltage difference between the scope earth and the earthing point on the device under test.  Use a Lo-Z voltage range on your meter, if you have one.

I am also curious as to the transmit frequencies used.  These should be marked on the microphones and the receiver.

One last point - harmonics are always multiples of a given frequency, not fractions.  That doesn't exclude there being some other explanation, though.
WaveyDipole:
Thanks for the replies. All good points and here are some answers to the questions raised:


--- Quote from: ogden on May 04, 2018, 10:42:23 pm ---What microphones/receivers (models) used? What radio frequencies used?
--- End quote ---

Trantec S3.2. 173MHz and 174MHz.


--- Quote from: ogden on May 04, 2018, 10:42:23 pm ---Whistling is just acoustic feedback.

Someone shall consider introducing feedback eliminator.

--- End quote ---

Well perhaps I was a bit inaccurate when I described it as whistling. Its more like a howling warbling echo effect that threatens to break into a whistle at any moment. The amp does have feedback filtering so could it possibly be the effect of the amp attempting to deal with it accoustic feedback?

What I don't get however, is why this affects the strength of the radio signal? The signal dips by a couple of LEDs (there are I think 5 or 6) when this problem is encountered.


--- Quote from: Electro Detective on May 05, 2018, 01:19:58 am ---Do the wireless mic receivers have some sort of small recessed adjustable 'squelch' type control on the back?
--- End quote ---

Yes, they do indeed but adjustment does not solve the problem.


--- Quote from: Electro Detective on May 05, 2018, 01:19:58 am ---and check all the cabling too, many unbalanced cables today are crap, a single wire with a carbon shield layer inside the jacket that crumbles when flexed  :--

--- End quote ---

The cabling looks like proper microphone cable, looks of good quality two copper core screened with rubberised (silicone I think) insulation. However, the mic has an unbalanced output employing a 6mm mono jack, but the amp is designed for balanced mic inputs which are wired inside the amp casing using screw connectors. I have not had a look at the the inside of the amp and the specifics of the wiring, so I'm not sure at this point how the connection is made. Unscrewing the jack cover at the receiver end I can see that the screen is disconnected and the two inner cores are being used to connect signal and earth. The length of the receiver to amp lead is about 2m.


--- Quote from: Brumby on May 05, 2018, 04:12:04 am ---First thing, as has been mentioned, is that the ringing and whistling you are getting is more than likely audio feedback.  At any one of those points you describe, I would guess that if you change which direction the microphone is pointing, you are going to find orientations where the problem is worse and others where it is not so.
--- End quote ---

The orientation of the microphone does indeed have some impact although sometimes upgright cancels it out, other times horizontal does. Pointing at or away from the speaker seems to have no bearing, although if you put the microphone right up to the loudspeaker (maybe less then 6 in away) then you get the sharp audio feedback whisle. In normal use, I would estimate that it would be more than 1.5m or more away.


--- Quote from: Brumby on May 05, 2018, 04:12:04 am ---Next, I would like to ask if you have the earth lead from your scope connected properly.  If it is not connected at all, then you are going to have everything connected to the probe acting as an antenna picking up who knows what.  Those screen shots look like the sort of crap you could get.  Before making such a connection, ensure there is no significant voltage difference between the scope earth and the earthing point on the device under test.  Use a Lo-Z voltage range on your meter, if you have one.
--- End quote ---

I confirm that I did have the earth connected via the croc clip. It is possible perhaps that earth lead picked up the 173/174MHz transmission from the Microphone, but I was surprised by the level of 38khz noise. The readings were taken with the amp switched off and disconnected from the receiver. Instead, plugged in a spare 6mm jack and connected the probe to the jack contacts.


--- Quote from: Brumby on May 05, 2018, 04:12:04 am ---One last point - harmonics are always multiples of a given frequency, not fractions.  That doesn't exclude there being some other explanation, though.

--- End quote ---

Yes, that’s a fair point although looking at it again, the frequency readings are not even a fraction of the transmission frequency. I wonder whether the FC is just not making proper sense of the mix of noise and showing frequencies incorrectly?. It should be easy enough to test this by having everything turned off except the mic. presumably then I should only see the frequency radiated by the mic transmitter.
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