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| 62.5kHz ringing Noise on signal |
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| StillTrying:
"So can we conclude that ground plane was the problem?" It depends on how the 'distortion' changed with the extra cap. If the frequency and timing of the 'distortion' changed quite a bit the problem is likely right there. My crystal ball says only the amplitude of the pulses lowered a bit so it's probably somewhere else. |
| StillTrying:
Did the OP find where the 62.5kHz pulses are coming from. r? |
| syntax333:
Contrary to what I thought, carving the PCB board to isolate OPA657 did not help. After testing circuit with working version on breadboard I found out the problem was the switching buck converters on the Transmitter side. However, it was probably due to common ground for PSU (Counterfeit LM2596 has switching frequency of 62.5 KHz :palm:), Analog and Digital signals. I didn't know that I should separate ground of analog circuitry from grounds of Digital and PSU. Probably counterfeit PSU switching caused noise on the ground plane thus on the analog signal. After I changed buck converters to linear 78XX regulators on both transmitter and receiver the problem was solved. In addition, I also changed 555 circuit which could cause also noise on ground plane due to its switching, to dedicated negative voltage supply IC (ICL7660). Thank you all for your valuable suggestions which helped me to find the problem. |
| T3sl4co1l:
Oh, those stupid regulator modules? Counterfeit or not, doesn't really matter, so much as their layout is terrible, and filtering nonexistent. You can fix that a bit by connecting to the module with three wires, which in turn connect to your board in a single point (well, not actually a single point obviously, but as near to a single point as is practicable). Two ceramic caps tie the two power leads together with the ground (eliminating high frequency differential noise), and the module having no additional connections eliminates common mode noise. (Well, perhaps. Ideally, the module sits inside a metal box, with the single-point connection being made at a single hole in said box. Or something topologically equivalent to that; it can be practical to realize with traces and pours on a PCB for example, for some degrees of "topology".) If that's not enough filtering, then chokes can be used in series with the power lines, then another pair of ceramic caps, to get a pair of pi filters. And so on. All the while, make sure ground fully surrounds the filter, on top and bottom, well stitched with vias. Tim |
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