VCO's FOM

A Figure Of Merit is a quantity used to characterize the performance of a VCO relative to other VCOs' of the same type.
The power, phase noise, frequency of oscillation, offset from carrier trade offs all are taken in the FOM value:




.
w0 is the oscillation frequency
dw is the frequency
dw is the offset from the carrier
L(dw) is the phase noise at the specified offset
Pdiss is the DC power consumed by the VCO core.


FOMT also factors in the tuning range.
FOMT is given by:


.

where FTR is the frequency tuning range of the VCO.
See a spread sheet of the FOM calculation

Phase Noise Measurement

Phase Noise is a basic property in VCO design. Measuring Phase Noise requires a carefull treatment.
One way of is to measure phase noise directly on a spectrum analyzer. This measurement can be done as long as the analyzer has better phase noise than the measured source.
VCO phase noise can be measure when the VCO is locked in the PLL loop or in open loop.
The first method is easier to measure but then you see phase noise of both VCO and the loop.
The second method gives directly the VCO noise, but you have to hold Vtune with a very clean source.

EVM Calculation

The error vector magnitude represents theEuclidian distance between the ideal symbolcoordinate and the actual recorded symbol.
EVM is a way to measure and quantify the performance of a comunication system. A comlex signal sent by an ideal transmitter or received by a receiver would have all constellation points precisely at the ideal locations, however various imperfections in the implementation (such as carrier leakage, low image rejection ratio, phase noise etc.) cause the actual constellation points to deviate from the ideal locations. Informally, EVM is a measure of how far the points are from the ideal locations.

EVM calculation:
EVM ≈ 100%*Sphi(f)*π/180 ........Error vector magnitude due to phase noise.

Jitter calculation

Jitter is the time domain represantation of the frequency fluctuations.
Jitter due to phase noise:

Jitter = Sphi(f)/(foscx360\deg)

where Sphi(f) is the Spectral density of phase modulation, also kown as RMS phase error [degrees]:

Jitter can be defined to variations of the signal amplitude, frequncy, or phase. It can be measured in terms of RMS, peak to peak etc.