Tektronix SPECMON3 Manual page 314

GP Digital Modulation
Measurements for OQPSK and SOQPSK modulation types
Measurement
EVM
Offset EVM
Phase Error
Mag Error
MER (RMS)
IQ Origin Offset
Frequency Error
Gain Imbalance
Quadrature Error
Rho
xxx
302
Description
The normalized RMS value of the error vector between the measured signal and the ideal
reference signal over the analysis length. The EVM is generally measured on symbol or
chip instants and is reported in units of percent and dB. EVM is usually measured after
best-fit estimates of the frequency error and a fixed phase offset have been removed.
These estimates are made over the analysis length. Displays RMS and Peak values with
location of Peak value.
Offset EVM is like EVM except for a difference in the time alignment of the I and Q
samples. For EVM, I and Q samples are collected at the same time, for every symbol
decision point (twice the symbol rate for offset modulations). For Offset EVM, the I and Q
symbol decision points are time-aligned before collecting the I and Q samples. In this
case, one I and one Q sample is collected for each symbol (half as many samples as the
same number of symbols for (non-offset) EVM.
The RMS phase difference between the measured signal and the ideal reference signal.
Displays RMS and Peak values with location of Peak value.
The RMS magnitude difference between the measured signal and the reference signal
magnitude. Displays RMS and Peak values with location of Peak value.
The MER is defined as the ratio of I/Q signal power to I/Q noise power; the result is
indicated in dB.
The magnitude of the DC offset of the signal measured at the symbol times. It indicates
the magnitude of the carrier feed-through signal.
The frequency difference between the measured carrier frequency of the signal and the
user-selected center frequency of the instrument.
The gain difference between the I and Q channels in the signal generation path.
Constellations with gain imbalance show a pattern with a width that is different form
height.
The orthogonal error between the I and Q channels. The error shows the phase
difference between I and Q channels away from the ideal 90 degrees expected from the
perfect I/Q modulation. Not valid for BPSK modulation type.
The normalized correlated power of the measured signal and the ideal reference signal.
Like EVM, Rho is a measure of modulation quality. The value of Rho is less than 1 in all
practical cases and is equal to 1 for a perfect signal measured in a perfect receiver.
Signal Quality Display
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