Talk:Surface acoustic wave sensor

The Surface Acoustic Wave compressor is used in modern radar receivers. This allows the transmitter to use a longer transmit pulse width with a lower peak power, and therefore semi-conductor amplifiers as opposed to single high power high voltage devices such as Magnetrons, Klystrons and TWTs, because the overall power per pulse is approximately the same, thus maintaining the Probability of Detection per pulse at a high level.

The transmitted pulse is long (typically 50-60 micro-seconds, as opposed to 1 micro-second), but is frequency modulated downwards throughout its duration, also called "down-chirp". The disadvantage of a long transmit pulse is a large non-receive dead space close to the radar which can only be overcome by interleaving short transmit pulses into the transmit cycle for close range coverage. The other disadvantage is poor target range resolution, which can be overcome using a SAW pulse compressor in the receiver.

The long receive pulse is compressed by a SAW device. The transducers either end are tuned so that the initial higher frequencies enter and exit the SAW device further apart, and thus travel over a physically longer path, while the following lower frequencies enter and exit the substrate much closer. The resulting different path lengths result in different delays, and this compresses the received pulse from 60 micro-seconds to about 1 micro-second, thus restoring the target range resolution.

Using semiconductor based amplifiers removes the need for EHT power supplies, and also reduces the reliance on a single or small number of final amplifier stages, allowing a "soft fail" mode where the radar output reduces incrementally on amplifier failures and can still operate. If the amplifiers are "hot swap-able" then the radar can be repaired without taking it off line, which is important for both military and civilian ATC applications.