While a step attenuator is very useful during testing of RF modules, one sometimes lacks an attenuator which could be voltage controlled. For example, such a device might be of interest when debugging or adjusting AGC circuits : we then need to monitor the AGC action when the RF signal is first presented then allow the system to recover (up to 10 seconds) before sending the next signal. The pulse input found in many RF generators (at least mine) produces fast rise and fall times but is not well suited to long duration and repetition periods related to AGC testing.
A few years ago I designed a controlled attenuator while trying to improve the pulse response of my IF amplifier AGC. It is a simple device, built in a hurry, using components at hand. Its attenuation-voltage curve is far from linear but time response is satisfying as far as communication receivers are concerned.
The schematic diagram shows that the circuit uses a standard Pi structure where RF switching diodes (BA482) are used in place of resistors. The control voltage swings between 0 and 4 V thus giving a 0 to 50 dB attenuation.
The insertion loss is better than 1 dB over the useful frequency range and the maximum attenuation is 50 dB at 1 MHz and 42 dB at 30 MHz.
