[Radio Wrangler]

We've got into thermal transfer standards. There's a fairly common HP thermal RMS voltmeter, pretty much the same as the Solartron one the 3400A and a wider bandwidth 3403A

These amplified units will, of course clip large peaks so the crest factor of the signal is limited (literally!)

THere are non-amplified ones for RF/ microwave use like the 432A power meter and the 478A family of bolometric thermal heads. These add a neat trick by using thermistors as the load resistor for the RF. THe thermistor is part of a bridge which senses its temperature and adjusts the DC drive to the bridge to get the thermistor exactly on the target value. Add RF and the extra heating means the bridge re-balances itself with a lower amount of DC power into the thermistor. The reduction in DC power equals the RF power.

DC substitution and sensing all in the same component.

This unit was taken further as the NIST type IV RF transfer standard.

DAvid