Hi TomC,
I just read through your Theory Of Operation paper and schematic for the OWON SMPS and was quite impressed. Increased my understanding of whats going on inside these things significantly.
And, by the way, found the data sheet for the MOSFET and will attach it.
First about the residual noise and your noise figures.
In general I have to say that when testing/calibrating scopes you never use probes. The scope must perform the same whatever you connect to the input. So, stating any results by feeding the scope via a probe puts two things in one pot and nobody can tell which component is responsiuble for what and how much.
The only step in a calibration procedure of a scope where you need something like a probe is the input capacitane adjustment and this - at least in Tek labs - is done with a so called NORMALIZER a simulated 'perfect' probe with a known and calibrated capacitance matching the specified input capacitanc of the scope. And you adjust the input capacitance of the scope then for a flat square wave like you would do later with a probe that in turn will have to be adjusted to match the scope's input capacitance.
As a reply to you noise figures I will describe the noise problem of the OWON now as I experienced it when testing the machine and try to interpret it a bit of course according to my current knowledge (I might be wrong because of having overseen something - but don't think so):
Test setup example: Connect the output of whatever device to be maintenanced or tested to CH I input and a probe to CH II input.
Then shorten the tip of the probe with the groundstrap of the probe. How much noise do you see on screen provided the shorted tip end of the probe is as far away from the scope as possible?
Answer: nothing or almost nothing (no noise). Then take the ground strap off from the tip and touch the scopes input connector which is supposed to be solid ground. And what do you see on screen? Answer: Lots of spikes!!!
And now start tipping around inside of the device to be maintenanced/tested.
What will you see?
I can not tell at what level or what phase or spectrum but it will be noise everywhere except if you are lucky and find a point of Zero for all frequencies (not very likely).
I also depends on whether and how the device is grounded by itself, through the mains cord or whatever.
Conclusion is, that the OWON pollutes every conducting thing you connect to it's ground as good as it can. Using long wires and in addition grounding the thing to your central heating radiator the OWON has a very hard job. Is the thing a floating battery operated device and the connection to the OWON very short and done with HF wire it will pollute the thing entirely but you will not be able the see it on screen because the tip of your probe might be in perfect phase with the thing connected to OWON's ground. But after connecting something else with big mass or even grounded to the thing your measurements will show different again.
Its not the hot side only of the input that is pulluted! It's the COLD SIDE AS WELL. And that means actually you can not predict the displayed noise at all.
So I have to say that any noise figure you give can only be true for the individual setup you had.
To the attempts of OWON reducing the noise by circuit mods I can only say that the result can be only marginal. They should know better. Of course as you pointed out in your paper 'slowing down' the slopes would help but there are hard limits as you wrote (Power consumption / component sizes).
How much reduction did you get with the belly strap and hand wound toroid? and what material did you use, chopper foil?
And another thing I would like to ask you: please can you check what is written on the backlight switcher controller and the screen supply switcher controller. I can not fine the sheets in the net and the screen supply is a bit complex as it prvides the power up sequence for the various display voltages. Found the spec sheet of the display with power up sequence detailed but no app circuit given. I will retrace it but you know missing a little and killing the diplay wouldn't be so great. Datasheets are always a good backup also if some resistor breaks when unsoldering or jumps away.
And my postings - all a bit rough and preliminary. The final design of the smal converters I will do after I have everything together.
1. 3.3V buck converter: In the box is what was on the I/F PCB just added Inductors + tantalums as filter
2. 5.6V buck converter: removed all components and replaced it with a linear 7805 + diode for 0.6V increase. maybe will keep it but will decrease battery time should I ever need it. To be decided later.
3. 1.9V Buck converter: to be found on the main PCB. removed all parts using hot air but replaced the controller then with the one that did the 5.6V before. The reason was that the original was supplied by the +5,6V converter and by taking it off the main PCB could feed it directly with the 8.4V also taking away current from the 5.6 now linear regulator.
And for the former 1.9V controller 8.4V would be above or just max specs. As the former 5.6v controller has an enable input I could use this to keep up with the correct power up sequence and also power on/off dependencies. (1.9V only present if 5.6V present) It works fine.
So that's it for now. have removed the backlight switcher but not boxed yet. And will continue working on the display switcher.
Next photos will be the input shielding I did and minor mechanical mods for to make a bit more space between button PCB and main PCB (.6mm) but have to dissasemble the thing again before.
Keeping up the good sprit,
Sarasir