Electronics > Beginners
Phone noise on an audio set, how to explain this to layman
Kjelt:
How can I tell in layman's terms why an 800MHz or higher portable phone with its 1W power can cause interference on an audio set (20-22kHz), you know the noise just before the phone rings.
I have real problems explaining this to a layman. Who can help ? Perhaps an idea for a Dave's eevblog episode.
coppercone2:
envelopes and mixing
draw a bunch of sine waves with a line on top like their smoothed by a frequency specific rectifier and cap to make pulses
maybe an equivalent circuit could be a HPF that takes the line noise in, followed by a rectifier and filter to make some kind of pulse wave that is then fed back into the original signal.
app note 96 does a good job explaining dc offset with rfi rectification. just modulate the offset
http://www.analog.com/media/en/training-seminars/tutorials/MT-096.pdf
bson:
Semiconductor junctions rectify RFI into DC, producing an offset voltage. When a transmitter is pulsed you end up with a pulsing offset voltage, and if the pulsing frequency is within the bandwidth of the circuit it will manifest itself as an induced squarish wave. If the circuit is an audio circuit and the pulsing is in the audible portion of the spectrum you will hear it as a square wave, which sounds raspy, especially at low pulsing frequencies.
Brumby:
The key phrase here is "in layman's terms".
I would presume that this means any arguments based on electromagnetic theory are going to be less than helpful - so I offer the following explanation:
First, I expect the person can understand how a radio signal might get into an audio circuit - but does not understand how something operating at 800MHz could be heard. That being the case.....
Imagine you are in a car driving over a smooth roadway and that your car has reasonably good soundproofing. You pretty much aren't going to hear the tyre noise - and if you did, it would be a constant low level noise that you would soon ignore. This is like a radio transmitter that just sends out a steady, unchanging signal. Whether tyre noise or a radio signal, unless you are actually looking for either one, you just aren't going to notice anything.
Now let's say you just drove over an expansion joint. You will get a little "thump" because of a change in the tyre and road conditions. You still aren't hearing the tyre noise - but you can hear a (different) noise when something changes quickly. A single little thump might not catch your attention either - but let's say you stray over a ripple strip. Now you have a lot of little (sometimes not so little) thumps - and you will certainly notice that.
It is the same sort of thing that happens with a radio signal. If you have a sudden change in the signal, then you could hear a click. This is not you hearing 800MHz - but hearing a change in that 800MHz. Now let's say there are a number of such changes that happen at a rate that we can hear. Just like the ripple strip, we hear a series of clicks - which we would describe as a buzz.
The 800MHz signal isn't going to get far in the audio chain, but it doesn't have to. The buzz that gets created becomes the audio signal that fits right in the zone - and will travel along it just like any other audio signal.
For entertaining examples of this, check out some videos by searching for "musical road".
Richard Crowley:
It is not clear (at least to me) WHY you have to explain it?
Isn't the fact that it happens prima facie evidence? And that the results are undesirable?
There were problems with RFI long before cell phones came along.
There is a story (not authenticated) that one famous Hollywood director banned the presence of cell phones on (or NEAR) the set. His assistant had a hammer and bag of nails available any time a cell phone of the cast or crew went off (either audibly or RFI into the audio) during production. The director would confiscate the offending device and actually nail it to the nearest doorpost (indoors) or fencepost (outdoors). There have been times when I dearly wished I could do that. >:D
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