Rohde & Schwarz CMU-K20, Operating Manual

Page: 116 / 646

Non Signalling: Modulation Measurements CMU-K20/K21/K22/K23/K24
1115.6088.12 4.38 E-11
Modulation Measurements
The menu group Modulation comprises the functions for measurement of the modulation parameters of
the RF signal transmitted by the mobile phone. The measurement results are displayed in the graphical
measurement menu
Modulation, the popup menu Modulation Configuration is used for configuration of
the measurements.
The characteristics of the modulation measurement, the measured quantities and the measurement
menus depend largely on the modulation scheme
(GMSK or 8PSK modulation). For the sake of clarity,
the two modulation schemes are explained separately throughout the remainder of this section.
Measurement Menu (Modulation – GMSK)
If the GMSK modulation scheme is selected (see Application softkey in section Test Settings on page
4.39 ff.), the graphical measurement menu Modulation displays the results of the extended phase and
frequency error analysis.
•
The measurement control softkey Ext. Phase Err. GMSK controls the measurement, indicates its
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 Modulation
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
Test Settings on page 4.3 .
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).
Frequency and phase errors are determined as follows:
The actual phase of the signal received from the mobile station is recorded during the entire burst and stored. The
transferred data is demodulated and the training sequence searched for. The middle of the training sequence is
used for time synchronization (transition between bit 13/14).
The complete data content of the burst is then mathematically modulated using an ideal modulator. The resulting
ideal phase is compared with the measured phase. From the difference between the two quantities (the phase
difference trajectory), a regression line is calculated using the Mean Square Error method. The
phase error is the
difference between the phase difference trajectory and the regression line; it is calculated and plotted over the
whole useful part of the burst (147 symbols). The average
frequency error in the burst is equal to the derivative of
the regression line with respect to time.
The Origin Offset and the I/Q Imbalance characterize the accuracy of the I/Q modulation. They are defined and
measured in analogy to the 8PSK modulation scheme; see (see
Fig. 4-18
on page
4.44
and
Equation 4-1
).
For the tolerance check the phase error trajectory is fitted into the tolerance template and checked for tolerance
violations. According to GSM specifications, a maximum peak phase error of
±
20
0
, a maximum RMS phase error of
±
5
0
, and a frequency error of 0.05 ppm referred to the carrier frequency is allowed.
The CMU evaluates the phase error with a resolution of 4 measured values per modulating symbol. This
corresponds to a sampling rate of approx. 1 MHz.
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