Thanks guys, for the responses already - I should have asked here in the
first place. Here's a consolidated answer:
Reg, I could use a low level scanner if it was for a general or one-time
measurement, but this is to be a compact, self-contained AC divider, for
isolated low level signal generation for experimenting with lock-in
analyzer setups. The transformer modules to be used came from a Fluke
5200A attenuator board. I initially planned on only the low and middle
band transformers, for 10 Hz to 100 kHz with good flatness, which covers
the range of all my LIAs. But then I figured I'll include the high band
(up to 1 MHz) too, using the last, little transformer. Since it's AC,
I'm not worried about thermal issues.
David, thanks for the link. I have seen and have the Keithley low level
measurements bible somewhere, but I think this is the newest edition
I've seen. I have copied the pdf into several places now.
Todd, great find! It looks like the 262 used good old copper-copper
rubbing (to cut the oxides) contacts for low level and low thermal
characteristics. Also note the lowest, final dividing resistor network
is all-copper resistance and connections. I've seen this before in old
Keithley gear. My model 148 nV meters use similar in the feedback
divider, and also heavy lug Cu-Cu switches in the front end ranging. I
see the reference to the Cramolin oils in the 262 manual - these were
indeed the forerunners of what became Deoxit. It's a long, complicated,
and kind of interesting story - I studied it a few years ago.
So anyway, I'm feeling better about going with my mechanical switches
with Au and Ag contacts and Deoxit combo. If worse comes to worse, I
guess I could scrape off the plating and go Cu all the way, but I don't
think that will be needed.
Ed
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