Hi TomC,
finally another step! Should anybody still checking, here the results of denoising the + 5.5V converter (became quiet her, what happened? I also have lots of other things to do but every third day or so I'm able to spend a few houres continuing the SDS-Free-Of-Noise-Project)
So, a few things to say about this switcher:
It behaves a bit different than the +3.3V (the 5.5 controller is not from Texas Instruments). Killed two chips untill I found out that it is very critical concerning the input. Therefor one 33uF Tantal is placed directly on the chip's input. The reason is that the 15uH torroid I used for the filter out the main switching frequency towards the 8.4V input - don't know if it is actually necessary but want to have the thing as clean as possible - 'softens' or correctly said, increases the supply impedance for the controller too much causing it to work more or less erratic. The TI controller of the +3.3V switcher didn't show any problem with a 15uH coil at it's input but I think I will give it also some more uFs for to be on the sure side,
Another difference between these chips is that the Alpha & Omega chip exhibits much more low frequency noise,
and there where ocasional sharp transients on the output of about +/- up to 2 mV. I'm still not sure but got rid of them after some houres of testing different layouts and also replacing the chip several times.
It's very time consuming to trace things like that down and perhaps another chip might be used eventually but do not think that's really necessary as the +5.5V in turn mostly supplies other converters and regulators - we'll see.
You can see the low F noise on the pics showing the output voltage.
I do not have pics of the performance of the original circuit/PCB layout as my 1st SDS's adapter PCB is depopulated and my 2nd SDS I would have to take the adapter PCB out as otherwise there is also the noise generated by the supplied circuitry and therefor no definite measurement could be made. But as far as I remember the noise of the original is far more - in the 50 to 100mV region if not more.
On the pics you can also see some switching noise remainder on the input but that I'm sure will be gone after the circuit has been shielded entirely. I just did not have the time to do that here especially as I know it's effect quite well from denoising the +3.3V converter.
So, hope somebody will read and that it will not take too long to have the next result ready.
Best wishes