English - Rohde & Schwarz

Enhanced accuracy in BER measurements
by precise level calibration of CMD
NRVD
ROHDE & SCHWARZ
FSE
NRV-Z4
FIG 1 Test setup for calibration of BER level at
RFOUT2 output of Radiocommunication Tester
CMD
The specifi cation for static BER sensitivity
in GSM base-station receivers calls
for tight tolerances that approach the
limits of what is physically feasible. The
tolerable level uncertainty of the tester
for BER measurements must therefore
be reduced accordingly.
The level of Radiocommunication Tester
CMD54/57/59 at the RFOUT2 output
can now be calibrated much more precisely
with the new option CMD-U92
and a simple test setup (FIG 1), consisting
of Power Meter NRVD, Power
Sensor NRV-Z4 and Spectrum Analyzer
FSE fi tted with option FSE-B22.
Calibration is performed using the sensitive
Spectrum Analyzer FSE under the
conditions stipulated by GSM 11.21
with a modulated signal at +50 dB
higher level in the unused timeslots
(FIG 2). The user can also carry out
calibration. The absolute measurement
uncertainty of FSE is eliminated by
means of a reference measurement
performed with the accurate Power
Meter NRVD in the presence of a nonpulsed
CMD output signal. In the subsequent
measurement with a pulsed
CMD output signal, the small linearity
error of FSE (< 0.2 dB with option
FSE-B22) alone contributes to the total
calibration uncertainty of CMD-U92
(< 0.5 dB).
Calibration of CMD is performed
between –85 dBm and –91 dBm at a
+20 dB higher level to obtain suffi cient
signal/noise ratio for measurement
with FSE. The exactly calibrated BER
level of CMD can then be reduced
to the level required for BER measurements
(–105 dBm to –111 dBm)
by means of a precision 20 dB attenuator.
Roland Minihold
–40
–50
–60
–70
–80
–90
–100
–110
–120
RF2
OUT
Ref Lvl
–32 dBm
CMD with CMD-U92
ROHDE & SCHWARZ
Marker 1 [T1]
–35.50 dBm
5.000000 ms
–130
Center 935.2 MHz 500 µs/
REFERENCES
– Calibrating CMD output level for static BER
measurement. Rohde & Schwarz Application
Note 1MA24_0E
– CMD-U92 – Special calibration for CMD57,
output level –85 dB to –91 dBm at
50 dBc ADJ.TS. Technical Specifi cation,
Stock No. 1129.0906.02
Reader service card 166/12
FIG 2 Measurement of used timeslot with Spectrum
Analyzer FSE in time domain
RBW 300 kHz RF Att 0 dB
VBW 3 MHz
SWT 5 ms Unit dBm
SUMMARY [T1]
RMS AVG –85.86 dBm
T1
Test Articles hint
T2
1
A
SGL
TRG
1RM
EXT
News from Rohde & Schwarz Number 166 (2000/I) 33