The 6Hz signal source acts as the output from a PIR sensor (component) when waving your hand in front at a rate of 6Hz. The signal amplitude is incredibly small and requires a gain of ~10000 to be useful. What's helpful is that PIR sensors have really low noise, meaning you don't need complicated filtering, especially if you supply the sensor with a very stable voltage.
Because PIR sensors only detect temperature changes/moving objects, adding a mechanical chopper that blocks and uncovers the sensor at a rate of 6Hz allows for the detection of stationary hot objects. When the detector is covered by the chopper it has time to cool down before being exposed to IR again.
