I actually don't see any published data on the Rohm switcher for stability analysis, so nothing to really compare that to. It does seem to be measuring correctly with how the converter behaves though.
The active PFC converter is similar to what can be found in this ST document:http://www.st.com/resource/zh/applicati ... 004041.pdf
Figure 8 shows a similar circuit as I've got. I'm using a cousin of this part with some enhancements, however similar in nature. This doc shows a lot more in depth information here vs the vendor I am using, so this would be better to reference for the general idea.
The nature of these converters makes them pretty noisy. Over the sine wave cycle of mains, the frequency of the converter moves around. This is great for minimising EMI/EMC, but terrible when we're trying to see the loop response since I have many frequencies now in the output spectrum. That along with the strong 120Hz (or 100Hz for 50Hz mains) signal make this a bit more tricky.
Measurement is made by breaking the loop on the secondary side as I have shown. I was able to use a 22 ohm resistor and drew out the points where I inject from the transformer and where the scope is measuring.
I should have captured some time domain plots using the picoscope, but a notch filter as I built plus putting a digital filter at ~10kHz on both channels made this very easy to measure. I'll have to repeat this at the correct signal amplitude since that was again at the high 1V drive. I have a hunch that would allow me to measure in the mV stimulus range and get correct crossover data.
It will be a little time until I can get back to measuring that, but I'll check hopefully in the next week.