Yes, I've got the capacitors and poor-man OP, collected the precision resistors, some P and N JFETs, and now I want to place a big order on Arrow to assemble a collection of OP suitable for testing (Kindly please contribute to my fresh topic in this forum regarding the OP), also Kelvin cables for my little multimeters and I want to come with a schematic this month after some experiments.
Also looking for one of these small CPLD/FPGA boards to implement the PWM generator, I've looked at your experiments with ATtiny and friends, this stuff really has jitter and bundle of other problems, I'll take my chances first with the original Datron schematic with JFET switches, with a bit of improvements from Kleinstein and you and many others.
In the end I'll learn something at least or even produce some usefull results.
It's clear from your results that the MCUs with embedded 16/8 PWM are not the answer as far as I can see, if the FPGA/CPLD implementation with external TCXO it's not absolutely jitter free, I'll do a discrete implementation, there is not very much that I can do for the analogue part with my resources, but I totally don't accept any BS on the digital part, it has to be perfectly synchronized and jitter free.
So yeah, if the couch that I must carry tomorrow (along with other house chores) will leave me some energy, I'll test the summer/filter/buffer thing with humble TL081, as the couplings are mostly AC and try to see how much is the noise/drift using a fixed set of 2 values: zero and the output of the LM399AH reference. Also a good time to test the dual 18V power supply.
If I can't maintain these two values and this stage drifts or has noise already, then it's no need to bother with PWM dividers and worry about charge injection, the whole base has to be redone with better components until it's statically stable.
Cheers,
DC1MC