Rohde & Schwarz CMU-K20, Operating Manual

Page: 121 / 646

CMU-K20/K21/K22/K23/K24 Non Signalling: Modulation Measurements
1115.6088.12 4.43 E-11
Measurement Menu (Modulation – 8PSK)
If the 8PSK modulation scheme is selected (see Application softkey in section Test Settings on page
4.39 ff.), the graphical measurement menu Modulation displays quantities characterizing the 8PSK
modulation accuracy.
•
The measurement control softkey Overview 8PSK (which changes to EVM 8PSK, Magn. Error
8PSK, or Phase Error 8PSK if the corresponding application is selected) indicates the measurement
status (RUN | HLT | OFF) and opens the configuration menu Modulation Configuration (press twice).
The hotkeys associated to the measurement control softkey define the scope of the Power
measurement.
•
The other softkeys to the right of the test diagram are combined with various hotkeys. The
softkey/hotkey combinations provide test settings and switch over between different measurements.
The entry of values is described in section
Measurement Menu (Power) on page 4.9 .
The measurement menu Modulation can be accessed from any other measurement menu of function
group GSMxxxx-MS Non Signalling using the Modulation hotkey. It can be opened also from the Menu
Select main menu (with the associated key at the front of the instrument).
Quantities characterizing the 8PSK modulation accuracy are determined as follows:
The actual modulation vector of the received signal from the mobile station is measured over the complete burst
and stored. From a comparison of this measured modulation vector with the (computed) ideal signal vector, three
non-redundant quantities are calculated (see
Fig. 4-17
):
Phase error Difference in phase between the measured and the ideal signal vector.
Magnitude error Difference between the magnitudes of the measured and the ideal signal vector.
Error vector magnitude Magnitude of the vector connecting the measured and the ideal signal vector. In contrast
to the previous quantities, the error vector magnitude cannot be negative.
These three quantities are calculated as a function of time and displayed over the whole useful part of the burst
(symbol 6 to symbol 162), each of them in a separate graphical measurement menu. In addition, the peak and RMS
values of all three quantities are calculated (over the whole display range or over the first ten symbols only) and
displayed.
Finally, the Modulation measurement provides the following scalar quantities:
95
th
percentile Limit value below which 95% of the values of a measurement curve are located. The 95
th
percentile of a measured quantity has the same unit as the quantity itself. In the 8PSK
modulation measurement, the CMU determines 95
th
percentiles of the Error Vector
Magnitude, the Magnitude Error, and the Phase Error.
Origin offset Origin offset in the I/Q constellation diagram reflecting a DC offset in the baseband
signal (see
Fig. 4-18
on page
4.44
and
Equation 4-1
). The origin offset corresponds
to an RF carrier feedthrough.
I/Q imbalance Amplitude difference between the in-phase (I) to the quadrature (Q) components of the
measured signal, normalized and logarithmized (see
Fig. 4-18
on page
4.44
and
Equation 4-2
). The I/Q imbalance corresponds to an unwanted signal in the opposite
sideband.
Frequency error Difference of the measured frequency from the expected frequency.
For the tolerance check all three phase error curves can be fitted into a tolerance template and checked.
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