Sunday, April 17, 2016

Measuring output impedance of the crystal checker

Just for fun, I wanted to see about cleaning up the output of the crystal checker.  I had fun putting different side capacitors on the output just to see what would happen on the output.  But I finally decided that I needed to measure the output impedance.  I had seen this video, but I was hung up on sizing the AC coupling capacitor first.  I asked on reddit, and an nice user pointed out that you want the cap to be low impedance at the frequency you care about.  (At which point I felt very silly for asking something so obvious.)

My fundamental frequency is at 3.57 MHz, so I calculated I need at least a 45nF capacitor.  I tried using a 470nF (what I had handy), and got an open voltage of 10.16v. Adding a 4.7k resistor off the other side of the cap as load, I got a voltage of 9.76v. So that would seem to say that my output impedance is 192.6 Ohms.

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