[G0HZU_JMR]

The AD8307 is a log detector and it is very sensitive to odd order harmonics and this can cause confusing results. A picture speaks a thousand words so here is a chart showing the measurement uncertainty 'window' for the AD8307 if there is a third harmonic present. This is based on carefully measured data using two vector signal generators made by Agilent. These allow the phase to be accurately adjusted in 0.1degree increments.

You can see that the uncertainty is fairly negligible if the harmonic is suppressed more than 30dB and so this would be fine for making absolute power measurements.

However, if like me you want to use the AD8307 to make relative measurements such as a low insertion loss measurement then it really is important to make sure the signal source has very low odd order harmonics. 50dB suppression would be nice in this case and I usually try for 60dB.

Otherwise, an attempt to measure a lowpass filter with an insertion loss of 0.3dB could give very strange results because the effect of the harmonic is present during the through calibration but the filter will then remove this error offset when it is placed inline. This means it is possible to falsely measure 'gain' from a passive device using the AD8307 log amp.