I'm a *tiny* bit dubious of the Jim Williams circuit - if the HT supply you are characterising suffers a flashover, with nothing to limit the transient current out of the coupling cap except its ESR and the cable impedance, the transient current through one of the MUR-110s may be rather higher than you'd like, so that 50R input its direct coupled to had better be fairly robust. Of course, in his day, the testgear he used typically was that robust. Modern low voltage highly integrated gear typically isn't, unless the designer put a *LOT* of effort into input protection.
Does anyone have any hints and tips for measuring millivolt ripple on a 1500VDC psu?
How about using a 1uF PP film cap int (HV rated) into a resistor load to ground (with bidirectional zener across it for some scope protection) - measure noise across R.
Should get down below 100Hz with that.
Or is that a dumb idea?
Replace the resistor load with a 2:1 divider, with the clamping at the tap for the scope, compensated for the loading of the scope probe, or maybe with the tap buffered by a video OPAMP driving a 50R cable terminated at the scope and I think you'd be onto something. The top resistor in the divider, with only a few pF of compensation across it limits the max clamping current. Also, there was a topic that got bumped today:
https://www.eevblog.com/forum/beginners/multimeter-input-protection-what-are-these-bjts-doing/msg719790/#msg719790 that's worth a look as the transistor clamping described can have much lower leakage and junction capacitance than a pair of Zeners. Alternative very low capacitance and leakage clamping if the scope input can stand >100V would be a NE-2 neon bulb.
Its behaviour could be characterised with a cheap chinese coin cell stack in a plastic pipe
* to get a low noise approx 1KV DC bias voltage, and driving the bottom end of that from a signal generator.
* A 1KV CR2016 stack is only 334 cells and just over half a meter tall. Split it in two and it can easily be fitted in two 30cm tubes bonded side to side, with a conducting crossbar to link them and retain the cells at one end and two contact springs retained by pins at the other. Insertion is easy if you stack cells 16 at a time and wrap the edges with just under a single turn of 25mm sellotape. then insert them with a slotted plastic plug that can be removed (and replaced again) after complete assembly preventing contact with the conductive crossbar, so you don't have to work live on a 1KV DC source capable of over 200mA short circuit current.