# Phase Vs. Time about VSA

Question asked by Agilentfans on Dec 25, 2008
Latest reply on Jan 2, 2009 by dgun
Dear all:

In Application Note 150-15, it mentioned some concepts in page38 as follows
"amplitude modulation, AM, is the variation of the carrier magnitude, A(t), versus time and is extracted from I(t) and Q(t) by taking the square root of the sum of the squares of I(t) and Q(t).
AM = A(t) = sqrt[I2(t) + Q2(t)], Phase modulation, PM, is the phase variation, Ã˜(t), versus time and is equal to the arctangent of [Q(t) / I(t)]. Frequency modulation, FM, is the derivative of the phase shift verses time, dÃ˜/dt. That is, FM is the derivative of PM:
PM = Ã˜(t) = arctan[Q(t)/I(t)]
FM = derivative of the PM = (dÃ˜/dt)"

My question is about the view of phase Vs. time, why this Ã˜(t) is not limited to zero degree to 360 degrees? some times, we can get phase Vs. time result is from -2000 degrees to -3000 degrees within one pulse, if this means that  the center frequency we set in the VSA has frequency offset related to the real signl center frequency or other reason?

The attached file is extracted from A/D symposium, Thanks for anyone's warm reply in advance!