Noise figure recovery measurement [microwave FET amplifiers]

In most high performance radar systems the performance of the low-noise FET amplifier is critical to overall system dynamic range. Low noise FETs inherently have small gate geometries which means their power handling capability is reduced. This limitation can cause the performance of low-noise front-end amplifiers to be severely degraded when high level spurious signals are incident on the input to the system. In order to analyse the system response to a high level signal it is necessary to measure both the gain and noise figure recovery times of the front-end low-noise amplifier. Interpulse gain recovery times are relatively straightforward to measure, even for very rapid recoveries (i.e. <50 ns), since a significant low level signal can be used to monitor the gain. Interpulse noise figure recovery times, however, are more difficult to measure and for this reason it is generally assumed that, for good low noise amplifiers, the noise figure and gain recovery times are comparable. The author presents a qualitative discussion of the sources of noise in microwave FETs and a simple measurement which has been devised to verify the above assumption