How does RFI manifest in the audio range?

  • Thread starter Landru
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Summary
How does RFI, which is beyond the audible frequency range, manifest as noise within the audible range?
So I understand that grounded shielding of audio equipment is necessary to block both EMI and RFI from audio equipment, but what I can't find information about is how, for example, radio frequency interference "RFI" at 150kHz, can interfere with audio equipment in such a way that you will hear noise that is within the audible range of 20Hz to 20kHz. Can someone explain how the RFI which is beyond the range of human hearing, becomes a noise that is audible?
 

berkeman

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"Audio Rectification" due to semiconductor junctions where the RF gets rectified and the resulting envelope is in the audible range. Just think of simple AM receivers using a diode as an envelope detector:


And some times the induced problem is worse than just some hum or voices coming out of the equipment. We once had a PC blow up on a ship application where a crew member walked by the PC and keyed a pretty high-power walkie talkie. Turned out that RF interference coupled into a loop in the power supply that was part of the output voltage control circuitry. Boom!
 
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Another possible source, in sampling digital systems anyway, is aliasing.
 
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A third possibility is intermod. An RF signal can interact with another nearby RF signal and mix down to an audio frequency.
 

berkeman

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Summary: How does RFI, which is beyond the audible frequency range, manifest as noise within the audible range?

but what I can't find information about is how, for example, radio frequency interference "RFI" at 150kHz, can interfere with audio equipment in such a way that you will hear noise that is within the audible range of 20Hz to 20kHz.
BTW, 150kHz is a typical switching frequency for SMPS, but it's not really RF. RF scans for interference typically start at 30MHz, although the EN 61000-4-6 immunity test does run from 150kHz to 80MHz. And since one of my communication transceiver technologies uses signaling with Differential Manchester encoding at 39-78kHz (and another is centered on 1.25MHz), I run a lot of immunity tests on our products in that frequency range.

Any rectification, intermod, or CM-DM conversion is bad, and we try to design to reject those effects.
 

tech99

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Summary: How does RFI, which is beyond the audible frequency range, manifest as noise within the audible range?

So I understand that grounded shielding of audio equipment is necessary to block both EMI and RFI from audio equipment, but what I can't find information about is how, for example, radio frequency interference "RFI" at 150kHz, can interfere with audio equipment in such a way that you will hear noise that is within the audible range of 20Hz to 20kHz. Can someone explain how the RFI which is beyond the range of human hearing, becomes a noise that is audible?
It will become audible if the amplifier is non linear, so that rectification and detection take place.
It is possible that the amplifier is non linear at radio frequencies whilst being linear at audio frequencies. This is because solid state amplifiers rely on negative feedback circuits for linearity and these circuits might not be effective at high frequencies.
 

sophiecentaur

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RF scans for interference typically start at 30MHz
That's OK if your installation is not just done the road from a 200kW LF or MF Radio transmitter. It was a common occurrence for people to hear the MF radio broadcasts on their Band 1 405 line TVs. Frequency mixing can cause intermodulation products just about anywhere you care to imaging.
 

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