When checking an active (magnetoresistive) wheel speed sensor, what type of signal pattern should be observed on an oscilloscope?

1. A variable direct current (DC) voltage flatline that rises to 12V when stopped
2. A digital square wave toggling between two distinct current or voltage states as the sensor tone wheel rotates
3. An analog sine-wave waveform whose amplitude increases as wheel speed rises
4. A high-frequency radio signal shifting between 90 MHz and 100 MHz