In a microwave test bench, why is the microwave signal amplitude modulated at 1 kHz?

In a microwave test bench, why is the microwave signal amplitude modulated at 1 kHz? 

(1) To increase the sensitivity of measurement (2) To transmit the signal to a far-off place (3) To study amplitude modulation (4) Because crystal detector fails at microwave frequencies
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Answer :

In a microwave test bench, why is the microwave signal amplitude modulated at 1 kHz? 

Because crystal detector fails at microwave frequencies

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