Todd,
you already have sent me the appropriate SelfCAL circuitry, but I did
not recognize (understand) it as such!
I also received a hint from another volt-nuts, John, about the complete
documents for the DATRON 1271 on the KO4BB site, which is very similar,
and also contains a SelfCAL module, being a bit different than the one
inside the 1281.
The schematics for the 1271, anhow reveal how the transformer inside
this module provides different voltage ratios. The transformer there is
labelled +9.93V, +/-7.23V (that's the main references value), 1.83V,
1.00V and 0.19V.
These ratios were generated with a precision switch mode / chopper
circuit, and as the ratios on a transformer are very stable, you only
have to measure them once during initial calibration.
Obviously that is good for ppm ratio accuracy in the SelfCAL procedure.
So thanks again
Frank
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