# XY graph modulating/demodulated signals in FM

I am using Labview to simulate a communication channel using FM modulation/demodulation (with MT Modulate FM VI and MT Demodulate FM VI to be more precise).

I made a XY-plot (see below) showing the amplitude of the modulating signal and the amplitude of the demodulated signal but I am experiencing some difficulties to understand what it means.

The modulating signal here is a simple tone at 10kHz, amplitude of 1V with a 1V offset. Modulation index set to 1. Increasing the modulating signal frequency increases the width of the ellipse on the plot.

How can I interpret this graph and what causes this ellipsoid form? As commented below, this ellipsoid suggests a phase shift between the modulating and the demodulated signal, what causes this phase shift?

• What is your EE question? May 8, 2016 at 10:22
• The ellipse (rather than a straight line) suggests there is a slight delay (phase shift) between input and output. You can infer what the phase shift is : if it were 90 degrees you would have a perfect circle (see "Lissajou's Figures" for more information.
– user16324
May 8, 2016 at 10:31
• @Andyaka How can I interpret this graph and what causes this ellipsoid form? Sorry if this post should not be on this stack exchange site (I wasn't sure where to post it). May 8, 2016 at 10:31