Hi Warren,
Interesting re. changing gain of the op amps. If the Rx II MDS is limited by the noise floor of the op amps, then it is not clear to me that changing the gain will affect this in a beneficial way. The spec sheet for the op amp suggests to me that the circuit is well designed and the noise is limited by the op amp itself, not by the gain resistors.
However, if you are interested in experimenting, I have a thought. Those 10 ohm resistors before and after the switching mixer seem to me to be non-essential and possibly decreasing the sensitivity. I was thinking of bypassing them myself as an experiment. I was thinking of this from the perspective of eliminating some thermal noise and simple resistive loss. However, eliminating them would also increase the gain of the op amps since the input impedance, together with the feedback impedance, determines the gain. The mixer itself presents, I think, a source impedance of 12 ohms, so eliminating the 20 ohms of resistors should increase the voltage gain by X3 or so. I would be very interested to hear what, if any, change in MDS measurement eliminating those resistors would make.
(A caveat, I am writing this after two glasses of wine.)
73,
Roger
