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Impulse Response as a Measurement of the Quality of Chirp Radar Pulses

Traditional measurement of the quality of a linear FM chirp has used Frequency Linearity or Phase Linearity. The linearity of frequency or phase of the chirp tells only part of the story. If a reflection or other time-related mechanism results in a delayed copy of the intended pulse (a secondary pulse) added to the main pulse, the linearity measurements may not easily discover the problem. Also, if a chirp pulse has periodic incidental modulation (particularly amplitude modulation), such an error may also very difficult to determine.

Impulse Response has commonly been included in the measurements made by vector network analyzers. As such, it is performed on components or assemblies as a forward transfer measurement. The Network Analyzer supplies the swept RF stimulus in addition to measuring the resultant output signal.

A measurement of the quality of a complete radar transmitter can be made using the FM chirp from the radar's own exciter as the swept RF source. In this manner, the quality of the generated chirp as well as all modules that the chirp passes through will be tested together as a complete assembled radar. This Impulse Response measurement is also called Time Sidelobe measurement.

The most obvious measurement is of a reflected and delayed copy of a chirp included with the main pulse. Such a secondary pulse can be simultaneously measured for both time delay and relative amplitude by plotting the impulse response of the main chirp pulse. This is functionally equivalent to a TDR using the chirp itself as the excitation signal.

Another defect, which this process can easily discover and measure, is incidental periodic modulation. Incidental modulation adds sidebands to the sweeping RF carrier. These sidebands effectively create lower amplitude copies of the chirp that appear as pulses both preceding and delayed behind the main chirp.

All of these defects would appear as false responses in a radar receiver.