Trying to measure an AC amplitude, one would ideally like a V**2 square-law component (yes, I know about thermal converters). But, what about other U-shaped I-V transfer characteristics? Is there a good way to make an AC level detector that's accurate to one percent or better, over a wide bandwidth (10 to 10**7 Hz)?
Following such a detector with an integrator would make AC level-crossing detection easy: just use an op amp integrator with the (+) terminal at some set-voltage point.
One example would be an absolute-value circuit (but those mainly use op amps, and high frequency accuracy suffers). Another is two comparators in a window-detector configuration, but THOSE have unwanted high gain and switching transients. A third is a diode bridge, but the forward voltage is temperature-dependent and the output is differential, not ground-referenced (mixers using transformer coupling don't suit the lowest frequencies).
So, what OTHER detection schemes should I consider?