Teledyne LeCroy QPHY-UWB - Manual (Page 23)

QPHY-UWB Software Option
QPHY-UWB-OM-E Rev C
23
The sdev value, because it is shown in the same units as all the values, requires a word of explanation. But first,
please note that this measurement has not much value in considering sdev as it’s not required by the test. Linear
units must be averaged to find the mean and the sdev. Then both are converted to dB. Sdev reflects the
distribution of EVM values from different packets. It is expected to be much smaller than EVM, as shown. If sdev
is larger, all else being unchanged, min and max will be farther apart.
Frequency Error Measurement Parameter
This parameter measures the drift in the timing error that is due to a mismatch between the signal timing and what
it should be, based on our timebase. The timing error is derived from Common Phase Error measurement. The
drift between the first PSDU symbol and the end of the packet divided by the time gives a frequency mismatch
between the signal and the ideal frequency. This is the frequency error, and it is represented in parts per million.
EVM/Constellation Controls
Include in Constellation and EVM
The default settings for this menu are as shown in the following figure:
In this mode, the Constellation display includes data from both Data and Pilot Tones for all the PSDU symbols of
every packet included in the waveform under analysis. These default settings can be quickly set by checking the
Use Defaults checkbox. Uncheck the Use Defaults checkbox to use the controls on this menu.
The WiMedia PHY Compliance and Interoperability Test Specification specifies in section 3.5 that “The EVM Rx
shall compute and correct for common phase error (CPE) after a CPE filter has been applied as described in
Section 3.5.1”. The intent of the filter is to cause relatively rapid and/or large fluctuations in the carrier frequency
(that is, phase noise) to degrade EVM. The filter -3 dB bandwidth is approximately 266 kHz. When Filter CPE is
checked, the required filter is applied. When Filter CPE is not checked, we completely correct for CPE on a
symbol-by-symbol basis; that means phase noise will not degrade EVM up to a much higher frequency. The
difference observed reflects how much EVM is degraded by fluctuation of the carrier frequency, that is, phase
noise. C&I Testing requires Filter CPE to be checked.