I believe that I am having trouble getting accurate measurements of SFDR. I assume that picoscope 188.8.131.52 calculates SFDR with harmonics, as opposed to without harmonics. In default settings, with a "1 kHz," 0.707V sine wave looped back from the generator to channel A, it measures an average SFDR of 65.06 dBc. I can clearly see that the highest harmonic is ~3 KHz and it is closer to (almost exactly) 100 dB down from the fundamental. Within C weighting, this region is flat and I would expect a(n) SFDR measurement of 100 dBc. Please correct me if I am mistaken on this.
OK, so, I switch the generator to a square wave and the software measures the SFDR average as being 9.158 dBc. Now, on the screen, the peak at ~3 kHz is "exactly" 8 dB down from the fundamental. So, I would expect a measurement of 8 dB.
Why the discrepancy? Is there an error in my assumptions regarding "C" weighting?
PicoScope® 6 - PC Oscilloscope software version: 184.108.40.206
Copyright © 1995-2014, Pico Technology Ltd
Model: PicoScope 4262
Serial Number: ARxxxxx
USB Version: 2.0
Calibration Date: Tuesday, January 7, 2014
Hardware Version: 1
Driver Version: 220.127.116.11
Firmware Version: 18.104.22.168 / 22.214.171.124