In the first session of this laboratory the tachometer signal is deformed when the rectifier is attached.  Here is the circuit that was attached to the tachometer output.  In this diagram, the signal:

• v_in(t) is the tachometer voltage.
• v_out(t) is the voltage at the output of the diode rectifier.

        Here is a photo of the oscilloscope screen in that situation:
 
 

Now, there are a couple of things to observe about this signal.

•  The input voltage - which should be a pretty good sine wave - is distorted.
• Interestingly, that distortion only occurs when current is flowing through the diode in the circuit shown above.
• That distortion is caused by the current that is flowing.  That's because the tachometer has some internal resistance.  When current flows through that resistance, there is a voltage across that resistance, and the voltage at the terminals drops.  It is exactly the same phenomenon as occurs when you go to your car on a dark winter morning, turn on the lights, and hit the starter.  The lights dim because there is an internal resistance in the battery, and the terminal voltage drops when you draw current.
• When current is flowing, the output voltage does not match the voltage ag the terminals of the tachometer.
• The mis-match is approximately a half volt.
• That half volt is voltage that appears across the diode.  When current flows through the diode, there is a threshold voltage of about a half volt in the diode.  And, that voltage is pretty much constant as long as current is flowing through the diode.