Question about audio and RFI

[Speakerguy]

How does RFI show up in audio-bandwidth systems? I have heard that the RF signal couples onto a trace or traces within the system and "gets rectified and demodulated" by the circuitry. Does anyone have a link or can someone give a longer explanation of this with diagrams, exampe circuit schematic, what circuits are prone to this, etc? Thank you for any help!

 

[audioguru]

Most properly designed audio systems have an RF filter at the input. The input transistors in an opamp or even an ordinary 2N3904 transistor has plenty of gain at the 1MHz AM broadcast band and even at the 100MHz FM broadcasst band. The transistors can rectify and demodulate an RF signal and create audio interference sounds.

 

[RadioRon]

How does RFI show up in audio-bandwidth systems? I have heard that the RF signal couples onto a trace or traces within the system and "gets rectified and demodulated" by the circuitry. Does anyone have a link or can someone give a longer explanation of this with diagrams, exampe circuit schematic, what circuits are prone to this, etc? Thank you for any help!

The most likely points of entry of RFI are on those conductors that make the largest antennas. In residential audio systems, this is usually the speaker wires but can also include other distribution wiring, like power supply lines to remote amps and so on. The RF is fed, by these antennas, into circuit boards and boxes where it distributes itself through coupling to many circuits. The most common effect, by far is that the semiconductors on those circuit boards act as rectifiers and demodulate any AM information in the RF signal. Once the AF is in the audio amplifier, it will be amplified and is often heard in the speakers. So the most easily heard RF interference is that which has audio bandwidth AM modulation on it, including AM broadcast stations, CB, and Amateur Radio. Amateurs and CB operators usually use single side band AM which can sound like Donald Duck on a stereo amp. GSM cell phones have strong AM modulation in that their transmitters are turning on and off at a fast rate, approximately 220 Hz I think and this is easily heard in audio systems too. Other cellphones also modulate their amplitude, but in more compex ways so it is much harder to identify them. FM radios like FRS portables or Police and Fire communications only have their amplitude modulated when they turn on or off so these types of radios usually don't cause noticeable problems in audio systems.

One reference that I have on hand is "Radio Frequency Interference, How to Find and Fix it" published by the ARRL. It is old and I"m sure there are others but all can easily be found in the ARRL bookstore on the web.
https://www.arrl.org/catalog/?category=Interference%2FDF&words=

All circuits are prone to this problem but some more than others. Since the mechanism is rectification, the most likely circuits to cause problems are rectifiers, any transistor that has a PN junction (all bipolar transistors and many others), bipolar ICs, and well, for that matter, almost any IC that has overvoltage protection diodes at inputs, which is most families I think. So, while almost all semiconductors can be the site of rectification my experience is that CMOS and MOS not quite as bad as bipolar. For rectification to occur, the RF voltage coupled to the component has to impress a voltage across one of the devices junctions. This voltage has to be large enough to rectify, and the frequency of the voltage has to be low enough that the capacitance across the junction doesn't simply cause both sides of the junction to have the same voltage. So, it is less likely that a 2 GHz cellphone signal will interfere with an audio amp than, say, a CB radio at 27 MHz because the transistors in the audio amp will rectify the 27 MHz signal much more readily than they will the 2 GHz signal. There are many other factors of course, including the power of the RF signal. AM radio stations nearby are transmitting at many thousands of watts so they will interfere much more readily than a 4 watt CB transmitter.

Interference of this sort is fairly common. The onus is usually on the owner of the audio system receiving interference to fix this sort of thing. The person generating the RF is usually doing so legally and so if you suffer RF rectification, the problem is with your equipment acting as a radio receiver when it is not supposed to, and it is up to you to fix that and not to try and shut down the radio transmitter that you might believe is the "cause" of the problem.

 

[Speakerguy]

Thank you both very much. I think at this point I need to just go through the math to prove to myself that simple rectification demodulates the signal. I used to be real good at this stuff back in college, so we'll see how much math I can remember. Thanks again!