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In microwave measurements we attempt to measure power and power ratios as measuring current and voltage is often not possible. Microwave power varies over a wide range, down to a microwatt or less, so instrument noise needs to be suppressed. Improving the signal-to-noise ratio is done by modulating the test signal, often by a square-wave modulator keyed by 1 kHz, so at the detector or receiver side we can distinguish the useful signal from noise.
One popular design is to use a PIN-diode modulator. A PIN device is a P/N junction diode with a high-resistivity layer in it. Such design slows diode response, so it would not behave as a detector. Instead, it presents different resistance at microwave frequencies (typically above ~20 MHz) according to DC current we send through it. The PIN diode is installed in a transmission line , to present an "open" circuit which passes the signal, or, a "short" circuit which reflects the signal back. This allows to pass or attenuate the signal; done by 1 kHz square-wave current, we have an amplitude modulator
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