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Originally Posted by sciguy125
Alright, the supply is clean. Well, if I put my scope at 5ns, it doesn't see any noise.
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Phil,
I am more interested in what you see when the scope is at 50-150 ms(full screen). Connect the probe to the output of your regulator. Set the scope to "Line" on the trigger to lock the scope to the power line. Set vertical sensitivity to less than 1 volt and AC couple. Do you see any signal that is not moving on the scope? This is power supply/power line related stuff. It can occur at 60Hz or 120Hz, which is 16ms or 8 ms rep rate. It is not uncommon for three terminal regulators to output 60Hz noise when the input has a lot of ripple. Adding caps on both the input and output will not cure this problem. If you have a low voltage at the wall, then you step down transformer is not capable of giving greater than the 2 volt minimum for correct operation of the regulator. (A 7805 needs more than 7 volts input at all times to produce a clean 5 volt supply. Anytime the input falls below the 7 volt minimum, the output 5 will also fall. If you see any noise at all, it should be in microvolt range. Anything greater than .25 volt peak to peak is likely going to trigger the brownout detector and cause the oscillator to have a lot of FM modulation. When this occurs anything is possible.
One thing we didn't discuss before is grounding. Are you absolutely sure that the power supply and board grounds are firmly attached? Are you using a scratch built power supply? If so are all the power supply filter caps tied together close to the negative terminal on the power supply rectifier. Considerable current exists in this path and if you stretch that out, a large voltage drop is developed at power line frequencies across your grounds. When building power supplies, one needs to think of a single point where all power supplies grounds tie together. Then a single wire from that point out to your board. This way, all the filter current passes through the component leads and not through the circuit being powered. Let me know what you find. I will check CD each day if you need any help.