What elements create oscillations? how about picking up radio stations?

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jcharles00

Well-known member
Joined
Dec 28, 2005
Messages
51
Location
Indiana
In troubleshooting my Green pre, I have run into some oscillations, and thought the question might be a little deeper than just a "how do i get rid of it" question. I've read on here that opamps on their own can sometimes oscillate or pick up radio stations. Same with large capacitors. (as I read in the notes of a THAT demo kit) Are these oscillating components "bad" and need to be replaced, or is it generally the result of poor shielding / something else wrong with the circuit?


In my case, I'd like to eventually stop the oscillations, but I would like to 1. figure out where it's coming from and 2. understand why it's happening. and I guess a distant #3 would be learning to identify different types of oscillations by the way they sound.. (is there a site with mp3 examples of audio problems?)
 
NewYorkDave said:
If you're picking up radio stations then detection (rectification) is your problem, not necessarily oscillation.

forgive my ignorance, but the only thing on the subject i could find here was about cell phone interference.. which made it sound like the input of opamps are frequently culprit and the phenomena happens because a trace/lead is the correct length to pick up a RF wave. is that correct?

how would you correct for this? change lead lengths on culprit components?

i googled RF detector and all i get is "find out if your house is bugged" links. lol

thanks for the info!
 
Read about FM demodulation and AM demodulation.  You are creating a diode detector or a tank detector in some way if you are picking up FM.

You are most likely picking up AM though.  High gain parts will pick up signals if they aren't terminated properly(both input and output).
 
Look at how a crystal AM radio receiver works and you'll get the idea. All it takes is a a high gain circuit, diode and capacitor and you'll receive a range of radio stations, although it may be many at once since tuning and selectivity requires extra components.

Oscillation happens when your amplifier is unstable and the gain/phase margin is not adequate. This can happen with inadequate compensation and stray coupling between the output signal and the non-inverting input. Layout and wiring are usually the culprits for this to occur if the circuit design is not.
 

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