True RMS converter

The AC signal would be applied to a small heating element that was matched with a thermistor, which could be used in a DC measuring circuit.

Thermal-based power meters are the norm for millimeter wave (MMW) RF work.

Analog electronic circuits may use: Unlike thermal converters they are subject to bandwidth limitations which makes them unsuitable for most RF work.

The slew rate limitation of the operational amplifier used to create the absolute value (especially at low input signal levels) tends to make the second method the poorest at high frequencies, while the FET method can work close to VHF.

Specialist techniques are required to produce sufficiently accurate integrated circuits for complex analog calculations, and very often meters equipped with such circuits offer true RMS conversion as an optional extra with a significant price increase.