Measurement and Control of RF Power (Part III):Response of Log amp to Different Signal Types
Log amps are generally specified for a sine wave input. Like diode detectors, log amps
are not true rms responding. Differing signal waveforms in a log amp shift the effective
value of the log amp’s intercept upwards or downwards. Graphically, this looks like a
vertical shift in the log amp’s transfer function (see Fig. 19.), the logarithmic slope being
unaffected. The figure shows the transfer function of the AD8307 when alternately fed by
an un-modulated sine wave and by a forward-link CDMA channel of the same rms power
(9 channels active.) The output voltage differs by the equivalent of 3.55dB (88.7 mV)
over the complete dynamic range of the device.