Hello John,

Thanks for your comments.

LDO: The op amp top middle is the control integrator. It responds to the sampled amplitude and controls the negative resistance. So yes, there is in effect a dc path all the way round the system.

The quadrature squared edge is used to generate the sample pulse which has to be at the peak of the reference phase.

Yes, the sum of squares technique is one I considered way back in time and it was later used in an app note by Analog Devices or Burr Brown. It is not guaranteed to be ripple-free because of tolerances. Sample-hold when used on one peak is the same on every cycle.

The ultimate resolution of the Radford instruments turned out to be 1ppm or 0.0001 per cent at 1kHz. I am hoping to get 0.001 percent using off-the-shelf components first time round, perhaps better in the next generation.

DMS: By adding appropriate proportions of +/-I and+/- Q you can generate any desired vector. So yes, it does null exactly. I have not added autonull yet, this is a treat in store for you, it used four OTAs to detect, feed back and reinject I and Q. In those days we used CA3080.

Thanks for your kind welcome, nice to be here,

mike