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