Rohde & Schwarz CMU-K20, Operating Manual

Page: 47 / 646

CMU-K20/-K21/-K22/-K23/-K24 Non Signalling Mode
1115.6088.12 2.13 E-10
Additional Information... Alternative Settings
and Measurements
... on Step 7
"
Phase and frequency errors
GSM equipment can use different modulation schemes; the
basic scheme is GMSK modulation, which is a constant-
envelope, binary, differential phase-shift keying scheme. It is
important that the modulation scheme is adhered to as strictly
as possible. GSM specifies a peak phase error of max. 20°, a
RMS-weighted phase error of max. 5° and a frequency error
of max. 0.05 ppm of the transmit frequency.
The limits may be modified in the Limits tab of the Modulation
Configuration Menu which is opened by pressing the se-
lected Ext. Phase Err. GMSK softkey once again. The Modu-
lation Configuration menu is analogous to the Power Con-
figuration menu explained on the previous pages. According
to the requirements of the measurements the two configura-
tion menus differ in two points:
•
Phase errors are relevant within the useful part of the
burst. Therefore, a fixed upper and lower limit for the
phase error is specified. It is not necessary to discriminate
between different areas of the burst (see item
#
below).
•
The absolute value of the phase error is a measure of the
quality of modulation, whereas the sign is of secondary in-
terest. This is why the display modes
Minimum and Maxi-
mum can not be selected separately, the CMU displays
the extreme values instead (display mode Mini-
mum/Maximum ).
'
chapter 4, p. 4.38 ff.
The measurement principle for
phase and frequency errors is
explained at the beginning of
section
Measurement Menu
(Modulation – GMSK).
For configuration settings see
section
Measurement Configura-
tions (Modulation Configuration).
As a second modulation scheme,
the CMU supports 8PSK-
modulated traffic channels (in the
so-called EDGE channels).
#
Measurement curve
The diagram in the Modulation menu shows the useful part of
a normal burst with a length of 148 bits, The time scale of the
diagram, ranging from 0 bits to 146¾ bits, is thus shorter than
in the
Power/t Norm. GMSK diagram. The ordinate is sym-
metric around 0 , ranging from –20 dB to +20 dB.
'
chapter 4, p. 4.41 ff.
$
Statistical quantities
The table below the phase error diagram gives an overview
of the phase error averaged over the current burst
(Phase Er-
ror RMS), the extreme value of the current phase error
(Phase Error Peak), the current Origin Offset and I/Q Imbal-
ance, the current frequency error, and the statistical distribu-
tion of these three quantities
. The values in the three col-
umns are calculated as follows:
•
The Current column contains the frequency error, RMS-
averaged phase error and peak (Max./Min.) phase error
for the current burst.
•
The Average column contains the three quantities aver-
aged over the last statistics cycle.
•
The Maximum column contains the extreme values of the
three quantities within all bursts measured.
'
chapter 3.
In this chapter a comprehensive
description of measurement con-
trol and on the definition of statis-
tical quantities is given.
'
chapter 4, p. 4.41 ff.
As a last measurement group in
Non Signalling mode, the Spec-
trum measurement assesses the
off-channel power due to the
modulation and due to switching.
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