Alright, here’s a run down of my testing so far.
Interesting results!
I find it interesting that the lower uF low ESR caps perform better.
I guess that means the inductive reactance (from the ESL) is more than I expected and is dominating the overall impedance at that frequency. Using a smaller capacitor reduces the ESL and the total impedance, even though the capacitive reactance is higher.
Vpp is around 3 mV which I would think would be good enough
I would think so, too, especially given what I measured below.
I got some more caps in to try out and discovered that I need to clean up the 12 V power as well.
That sounds plausible, as it was fairly noisy in your previous screenshot. Meanwhile, 2 or 3 mV peak-to-peak on the 5 V rail should be more than good enough and is getting down into the margin of error of our measurement methods.
Kudos on your progress so far!
Also, here is what the linear power supply output looks like for comparison.
Looking at your screenshot, I was thinking, “That’s got to be induced noise. There shouldn’t be switching noise like that in a linear supply, and the peak-to-peak is higher than I would expect from a linear supply.” So I took a closer look at the PSU and realized that
the long board PSU is not a linear supply at all. And on the short board, only the 12 V rail is using a linear regulator. All this time I’ve just been repeating what I’ve read elsewhere and making my own ass-umptions without taking a close look. How embarrassing!
It stands to reason that there would need to be switching to generate the negative voltage rails. On the short board, the 5 V regulator is used in switching mode, and the oscillations there are used to derive -6 V via a transformer. I don’t have the schematic for the long board but can see that both 5 V and 12 V regulator chips are switching regulators, and it looks as if the -12 V is similarly derived.
I took some time to measure and characterize the ripple and noise for both the short and long boards:
- 1x probe with pig tail ground lead, placed directly on the PSU card edge
- AC coupling
- Floppy drive plugged in for load. (I was lazy for these quick measurements and just used what I had at hand. The floppy drive doesn’t provide a big load, but it was enough to get -6 V from the short board. Perhaps I’ll re-do the measurements with a better load later.)
I used the FFT feature of my oscilloscope to get a sense of the frequency distribution of the noise. For each measurement, I started with a 25 MHz range to see where the largest peaks were, then narrowed the range to zoom in on them. I’ve included one screenshot with the 25 MHz range just to show that there’s not much to see beyond 5 MHz.
Note that the 5 V and 12 V rails on both boards have ripple of less than 20 mV peak-to-peak, while the negative rails have considerably more.
Long board
The long board regulators oscillate at about 50 kHz.
5 V
12 V
-12 V
Short board
The short board’s 5 V regulator oscillates at about 20 kHz.
5 V
12 V
The 12 V rail uses a typical 7812 linear regulator and thus has very low noise (notice the scale). The 20 kHz ripple is probably induced from the 5 V rail next to it.
-6 V
.