Vector Signal Generator and Analyzer Terminologies,Glossary

This page of T&M World covers Vector Signal Generator and Analyzer terminologies. This VSG and VSA glossary include power vs time, phase vs time, IQ vs time, EVM, spectral flatness, gain response, CCDF etc.

Power vs Time

The power versus time plot is instantaneous power of a complex OFDM signal plotted on sample by sample basis. Amplitude of signal is derived in unit of dBm and plotted. It is similar to time domain plot displayed on oscilloscope.

It is expressed as follows:
Power in dBm = 10 * log { ( I^2 + Q^2 )/1mW }
X-axis : Time in seconds or samples
Y-axis : Power in dBm

Phase vs Time

It is similar to power vs time explained above. Here phase value for each IQ pair is computed and displayed w.r.t. time.

Phase value of IQ sample is calculated as follows:
Phase in degrees = tan^-1 (Q/I) * (180/π)
X-axis: time in seconds or samples
Y-axis: phase of IQ sample

IQ vs Time

IQ vs time plots refers to I amplitude versus time and Q amplitude versus time separately. This measurement displays raw I and raw Q output of down converter.

This measurement is helpful for analyzing any imbalance between I and Q components.


EVM-Error Vector Magnitude

The full form of EVM is Error Vector Magnitude. It is a measure of modem's performance with various kinds of impairments (baseband and RF). When modulated signal goes through impairments, the constellation changes. This constellation change or impaired IQ data need to be corrected at the receiver using front end algorithms in order to have minimum bit errors in the decoded data. The equation mentions EVM between measured IQ value and referenced ideal IQ value. Here |P1| is the absolute magnitude of ideal IQ vector.


As we know CCDF is the short form of complementary cumulative distribution function. It helps in analyzing peak to average values of baseband IQ data with respect to time. This will help in proper design of Power Amplifier taking into consideration nonlinear region and other saturation requirements.

It plots percentage of time IQ power is above the average value on Y axis and time samples on X axis.
X-axis:power in dB
Y-axis:units of percent

Spectral flatness or gain flatness

It is a measure of amplitude or magnitude response of time domain samples. It measures difference between amplitudes of two consecutive samples (in time domain) or frequency points (in frequency domain).

Test and Measurement Terminologies

Spectrum Analyzer Terminologies➤
VSG VSA Terminologies➤
Network Analyzer Terminologies➤