there is another quite simple solution involving a multiplier/divider IC (as far as I remember AD435) and a simple buffer circuit. Please give notice if you are interested.
Reviewing this classical document, I was aware that Vahab actually didnt't mention if the application is kHz or MHz domain. For low frequency (e. g. audio) range, a log/antilog circuit, either as an IC or built from individual parts, is offering superior dynamic, for high frequency, only gilbert-cell multiplier circuits or thermal detectors are usable.