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Russ,
I have some HF oscillation on AVCC module. I got 3.437VDC on both outputs which is OK. However I also have some HF on those pins. On right channel output I have 72mV at 261kHz and on left channel output 76mV at 261kHz. Is that normal operation?
George
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No, its not normal. We have only seen this occur once before, and that was only on one channel. In that case the AVCC module was damaged.
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How are you measuring? Do you have it connected to the load? The AVCC module needs to be connected to the load to be stable. Also you need to be careful with some capacitive probes. They can setup oscillation themselves. Edited by user Tuesday, October 25, 2011 11:18:19 AM(UTC)
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Russ,
What load? Isn't B III the load? I have measured between GND and OUT pins on AVCC module plugged into B III board with input at 5.247VDC.
George
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That is what I was asking. :) Yes the BIII would be the load. Is it possible the AVCC module ever saw more than 5.5V?
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No, I never had more than 5.25VDC there. The DC output is right so looks like the AVCC module is working fine except for oscillation.
George
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Interesting. This is the first I have ever seen this occur. One thing to try would be to short R6 and R8(make each 0 ohms) and see if that clears things up. It likely will not, but its something I have wanted to try in a case like this. :) Edited by user Tuesday, October 25, 2011 12:45:34 PM(UTC)
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OK, I will try this. Could you please post schematics of this module. Thank you.
George
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JKC wrote:OK, I will try this. Could you please post schematics of this module. Thank you.
George No I won't post the schematics. There are people looking to copy it. But I am happy to help.
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Double check all of the caps, and the solder joints in general. Rarely we find one with a poor joint.
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Shorting R6 and R8 is not working. Can I put some caps on the outputs?
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Can, but should not be necessary. Still it might be a short term fix.
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JKC wrote:Shorting R6 and R8 is not working. Can I put some caps on the outputs? OK, that is what I was expecting. I just wanted to double check. You might try putting a larger cap across C1. If C1 is defective or loose that should help. A decently low ESR 47uf should work fine. Edited by user Tuesday, October 25, 2011 12:53:26 PM(UTC)
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There is no change after putting 100uF low ESR cap across C1. I wonder If measuring leads are contributing to this oscillation. I am using Agilent U3401A multimeter with built in frequency counter. The measuring leads are 3ft long. Unfortunately, I have no scope to see this oscillation.
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I will try caps on the output. How big cap can be put there?
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I think the problem lies as you surmised with the method of measurement. You can try a cap of say 22-47uf at each output, but that may actually only mask the issue.
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Problem is fixed by adding 4.7uF tantalum caps on the output. 1uF cap worked on one side but not on another, however it lowered oscillation to about 20mV. Now with 4.7uF it is clean on both sides.
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Sure I expected you would see that, but like I said I suspect that just mitigates the effect of your probes. Still it won't hurt a bit to keep them in there.
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