Teledyne LeCroy QPHY-UWB - Manual (Page 22)

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QPHY-UWB-OM-E Rev C
Real and Imaginary Waveforms (I and Q components)
These two waveforms are the result of the discrete Fourier transform (DFT) processing, and display the Real and
Imaginary components of all the tones contained in each symbol. The points in these traces are the same as the
points in the constellation. These traces have horizontal axis units of time, but 112 points from each symbol are
from the same symbol (the same time); i.e., the time scale is # symbols * symbol duration. This is sufficient to
deduce which packet, or approximately which symbol within a PSDU, caused a visible disturbance; but this time
scale does not match the acquisition’s time scale, which of course includes inter-packet gaps, inter-symbol gaps,
and preamble and header symbols that do not contribute to the constellation.
Error Vector Magnitude (EVM) Measurement Parameter
Error Vector Magnitude is the distance between the point where each tone, as magnitude and phase, actually fell
and where it should have fallen. Ideally each tone should fall exactly on the constellation point but, in reality, each
tone falls in the proximity of the ideal constellation points, creating a “cloud” around the ideal points of the
constellation.
One Error Vector Magnitude (EVM) value is calculated for each packet (all the symbols contribute) by the root
mean square calculation. From section 8.7 of the MultiBand OFDM Physical Layer Specification, EVM is
calculated for each packet as follows, which is the square root of the mean squared distance from expected
constellation points of all the data and all the pilot tones in each of the symbols after the packet header to the end
of the packet:
The EVM value is then calculated by averaging the values from each packet:
EVM is displayed with statistics on. The “value” is a number calculated for the most recently processed packet.
The value, min, and max are EVM values from one packet. Num is the number of packets contributing to the
mean. The EVM result you should look at is the mean value, which is the result of averaging the values from
each packet, as required.