Capacitance Meter or LCR

high

precisely

Yes, this resistance is a critical component (if you want some bandwidth) and this is a pb I haven't thought about yet. Thanks for having given the begining of a solution.

Nothing very exciting I suppose. The post were about some detail points. Most are there, I guess :

Hmm, not so sure. What you need is a sensitive vector measurement, and a null tuning mean. This will give you "coarse" nulling. Then you boos the detector gain to precisely measure the remaining unnulled component at the nulling node(s) and some math will give you the extra digits through ako precision virtual nulling. If you have a sensitive enough and linear enough detector, the figure can be astonishing and, I think, will compensate for not having standards to switch. I don't have the GR1616 manual but have found the 1615 one. The detector is (obviously) only for nulling, and all the sensitivity comes from the dynamic range given by the switched standards and the ratio transformer. Furthermore the vector measurement capability allow for noise averaging which the magnitude null detector can't provide.

Another thing to consider is that the ratio transformer bridges are rather bandwidth limited.

I'll email you about this, 'cause I can't disclose it.

Do you really mean that they've implemented the detector at the RF level and not after an IF amplifier?

Hmm, that could explain a lot of things and that's why I've prefered the other way (which is possible with todays components and was probably much more difficult when the hp4275 was designed). Another reason is that I certainly don't have the same ressources capabilities and I have to cheat a bit :-)

It's a critical issue, and can single-handedly destroy accuracy.

Thanks, I'll check it out.

The reason a ratio transformer is needed has nothing at all to do with sensitivity, and everything to do with accuracy, linearity and stability.

OK, use my harvard.edu email address.

Yes.

There certainly was no problem implementing a vector IF strip when the 4275 was designed, quite the contrary: Implementing their accurate RF circuit at 10MHz was definitely a challenge. I'm not sure why they made their choices, but it's clear they knew what they were doing!

component...

That's not what I meant. I was speaking of possibly approaching those bridges performances (at least the easiest one) with the ABB by using the fact that I have a highly linear and stable vectorial detector. If I fell inclined and have some time when I have my two IF/detector boards debugged, I might try to push the limits and see where we go.

much

cheat

That's not the detection that's causing problem, but rather phase stability of the IF amplifiers if you want really good accuracy. I've run into such problems and was very very happy to have really modern opamps to help solving this issue. I agree that doing the detection at the RF stage might have been tricky, but having followed the other route I can foresee the difficulties at the moment it was designed and I guess I understand their choice.