PICO Tech TA041

[mikec]

Hi,
Recently acquired a pico-tech TA041  (25MHz) differential probe for testing power supplies etc. So far I like the product and really don't have any complaints about the quality. In fact this appears to be the very same unit as the Lecroy and BK Precision PR-60.
I have found one peculiarity while testing a power supply. The test setup is as follows:
Rigol DS1052E and TA041 (switched to the x10) connected to CH1 and powered by internal batteries. I also have a standard probe connected to ch2 (x100).
CH1 and CH2 are DC Coupled.

I used the TA041 to test the primary side of the power supply (SMPS battery charger) and I was able to confirm every checked out ok here (although there was a lot of ringing on the transformer and I have attached a picture as maybe this is something the differential probe is causing).

The problem I observe is when I probe the secondary side just after the diodes and across the filter caps from the output of the transformer. I firstly probed with the standard probe and I could see a stable 15VDC signal across the filter capacitors. I then just out of curiosity probe the exact same points with the TA041 and i get a completely different result on the scope, see picture. I also probe with my fluke multimeter and it measures on DC around 15VDC.

So I am happy to trust the measurement I am getting from the standard probe and multimeter, but can anyone explain how or why the TA041 differential probe is providing this erroneous result?

Note is is no coincidence that the measured frequency is 50Hz as that is the main line frequency that I am plugged into, as for why it is measuring 67.2V I am not sure. I am really curious as to what the differential probe is picking up here.

Thanks

[mikec]

After doing some more testing I wanted to add more info to this thread, hopefully it can twig some ideas as to why I get the results I see 
I have actually narrowed this down and realize that this thread probably belongs in the test and repair forum (if any admins agree and can move it feel free)..

I have attached a rough schematic/block diagram of the battery charger circuit I am testing.

The first test shows a standard x10 probe connected to TP2 and ground clip connected to GND (0V) and shows the expected +15VDC.
The second plot shows a measurement again of TP2 but this time using the TA041 differential probe (red probe connected to TP2 and Black connected to GND (0V). Again this shows the expected +15VDC no problem.

I repeat the exact same tests for TP3. Third plot attached for the standard probe which shows the expected 15VDC and fourth plot for the differential probe.

I understand that I most likely have some significant common-mode noise present on the secondary side of the charger.. this can come from a number of sources for example the Y-class capacitors C5 and C4, transformer etc.

I have measured this CM noise with a Fluke 179 RMS meter by placing the red probe on any point of the circuit  and the black probe on the EARTH  at the power outlet circuit (not DC ground). I get roughly around 90VACrms.

So I basically understand how there is 50Hz AC noise present what I can't understand is why the signal becomes rectified at TP3? I understand there are diodes in the circuit but I don't see an AC signal at TP2 least of all a rectified signal. Also as can been seen on plot 5 using the differential probe connected to OUT+ (red) and Out-(black) I can see the full un-rectified signal with DC offset, the RMS value is lower though.

Finally I understand that by connecting the standard probe clip to the circuit I am grounding out the CM Noise so therefore I don't see the AC signal for the TP3CH2 plot.. So therefore which probe is actually giving the correct plot? (note this power supply is a 2 core L+N insulated class product i.e. has no connection to EARTH on the mains socket)

Appreciate any help.. I can provide plenty of other data if it helps.