I fitted a grounded plug to my FY-6600, but still need to get Y caps for it.
In my view:
Y-cap are safety. Because mains to any touchable item *must* be safe, even when it fails, they must be super safe, tested to 10KV as well as (I presume) lots of mechanical stresses. That is why the capacitor spanning the transformer must be a Y-cap, as it is potentially violating the galvanic isolation.
But you are planning to put a cap between an already safe item (the BNC is already touchable safe) and earth (also already touchable)
So while the habit for these things is to talk about y-caps, other caps should also be OK. I did put a Y cap, because I had one, but on my other 2 supplies that I modified, I put X-caps (because I had only 1 Y-cap)
Also, as I wrote earlier, the mains-to-0V Y-cap should be significantly higher then the transformer parasitic capacitance
And the cap from 0V-to-earth should be significantly higher than that to divide up the floating mid-voltage to null potential as much as possible.
But it should not be too high. At least, not if you really want to use the signal gen as floating.
Because it does have "probe loading" consequences. If/When you connect the signal to a DUT that is not 0V potential, it will first have to charge the cap to whatever potential the DUT is. And you do not want that to be too much of a jolt.
So the original Y-Cap is about 1nF (I measured)
The extra Y-cap I used is 4.7nF (that was the only one I had)
I would try to not make it higher then 10nF
Of course, I am new to this particular topic, so do take my view with some critical thinking.
It would be very helpful to have spectra from a calibrated commercial SA to compare to the RSP2 and scope FFT plots. I'm trying to sort out which of these things I should believe and when. So references are very helpful.
I was also playing with SDR based SA. But I did not get any result that I would think usable. Nice to locate FM radio stations.
But as electronics SA: when you know what peaks to expect, you can find them. But only being able to find what you already know is there is of limited use. And I have to many other peaks that I suspect are artifacts, clouding up the picture.
Of course, my SDR is one of those toy DVB-RTL sticks that I had lying around. That RSP2 does look like it is giving better screens, but are they true?
If not, it may be better to not spend $150 for a RSP2, and put that toward a real SA instead?