You are absolutely correct: this is "a cheat at best." So, being an American of the old-school, I take that as a compliment.
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I found your comment "The 2MHz and 8MHz specification of the 3458A is quite mediocre . . ." most interesting: if I understand correctly, the 3458A is not so good above 2MHz, so why should I get all fussy about nailing the spec, particularly on the lower voltages? Hmmm.
The weakest link in my chain is the signal generator, in particular the granularity of the the amplitude adjustment, so the voltages are just 'close'. That is where the suggested instruments shine: the RMS voltage can be spot on. Maybe I'll go shopping on eBay . . .
Well, I had in mind, that you'd cheat, or fool yourself by this bootstrap idea, as it's a self-referred, or circular measurement.
Sorry, there was no pun intended, especially not related under any aspect to the current American president
Concerning the ACV specification, please consider, which other DMM offers that high bandwidth, up to 10MHz.
The accuracy is still at 4% up to 8MHz, and I remember many old HP TVMs , like I think the hp400E, which also measured up into that frequency region, with a similar accuracy.
So these RF specs of the 3458A are quite OK, but it really shines up to 1kHz, where it competes with the highest grade calibration instruments.
Anyhow, the ACV performance is often criticised by PMEL metrologists, for some reason...this said AC pre-amplifier could have been designed much better.
The oscillator does not need fine amplitude adjustment, any random value is accepted in these cal steps.
It only has to be stable, a few tenths of percent maybe, as a transfer is already made between different output frequencies.
I also assume, that the specification is better than these 30 year old data imply, when they had to use these in-sensitive 2mV output single thermocouple. If you use a much better MJTC, like the ones offered by NIST (SRM6001), you could proof that better accuracy to a lower degree of uncertainty.
Frank