Detecting Fast RF Bursts Using Log Amps:Monolithic logarithmic amplifi ers (log amps) can handle signals withdynamic ranges up to 100 dB. They are capable of responding to RFbursts that are as short in duration as a few tens of nanoseconds.However, when demodulating logarithmic amplifi ers are used todetect fast RF bursts, strange tails sometimes appear at the outputwhen the applied burst shuts off. An example of this was seen ina log amp tutorial article that appeared online in Analog Dialogue33-3 (1999), and in print in Volume 33. This article explains acommon cause of these tails and offers suggestions on how toeliminate them.Understanding Demodulating Logarithmic Amplifi ersThe fi rst thing to understand about log amps is that, while theyprovide information about power, they actually respond to voltage.In communications technology, the term log amp generally refers toa device that outputs a voltage that is proportional to the logarithmof the envelope of the input signal, scaled to base-10. A power ratioof 100:1 corresponds to 20 decibels (dB)—or a voltage ratio of10:1 into a given impedance.