Why is my resonance phase lag discontinous?

I am programming a phase-locked loop to track the resonance of a tuning fork so that the tuning fork is always on resonance. This will be used in condensed matter physics through atomic force microscopy.

Now, I am not certain about how to carry this out, so I was wondering if someone here could help me.

I have programmed an FPGA to output an arbitrary driving voltage, and this drives a tuning fork. This tuning fork then generates a signal that is eventually converted to a voltage and read by my device. Now, I also have a lock-in amplifier to detect the phase lag between my DAC (technically, the NCO that outputs via DAC) and ADC signals. I have made a graph of the phase difference after taking 4000 steps around the resonance frequency. Also, there is a graph of the LIA's detected magnitude*2.

I do not know what any of the transfer functions are, I apologize.

Phase difference (phase lag) graph on left. Resonance centered at 32.7635 kHz However, my phase difference graph is likely incorrect. I found the following that displays continuous "pretty" graph that may be what I am supposed to have.

Phase lag in blue

Does anyone know why phase graph has a max/min of 1/-1? If so, does anyone know how I may fix it?

Thank you

• Your phase in the Labview plot, is in normalized radians, right? That is +1 = 180 degrees and -1 = 180 degrees?
– Ben
Feb 9 at 18:53
• I would assume so. However, I did not explicitly specify for the detected phase to be normalized, nor did I normalize the data. Feb 9 at 18:54
• You could unwrap the phase, and the discontinuity would go away
– Ben
Feb 9 at 19:06
• I tried using the LabVIEW "Unwrap Phase PtByPt" subVI, but it produces the same output zone.ni.com/reference/en-XX/help/371361R-01/ptbypt/… Feb 9 at 19:18
• the VI expects an input with a ±pi range. You have a ± 1 range. You should miltiply your phase by $\pi$
– Ben
Feb 9 at 19:19