1GA16_1L.DOC
ROHDE & SCHWARZ
33
Except for ZOOM FFT, measurements can be carried out as single measurements (SINGLE, started with
F11) or in the form of a sweep (SWEEP, F12). After pressing the SNGL FRQ softkey (F10), the frequency
for a single measurement is entered under FREQ RNG (F7) the frequency range for the sweep. % or dB
can be selected under UNIT (F9).
For single measurements the measured values are also numerically displayed. During sweeps the result
display is switched off as standard to increase the sweep speed. To switch on the result display a single
measurement should be performed before the sweep is started.
Prior to each RESTART of a single measurement, the frequency of the useful signal can be changed at
the post-processing level (SNGL FRQ, F10).
DIN IEC 268, part 5, recommends frequency sweeps up to 5 kHz; measurements at higher frequencies
are not useful as the 22-kHz analyzer measures the 3rd order distortion for instance only for signals up to
approx. 7 kHz.
5.5.1
Special Features of THD+N Measurement (with post-FFT)
The relatively high distortion factor of sound converters permits measurements to be made in the FAST
mode. In this mode of the THD+N measurement useful-signal suppression to obtain precision is left out.
This halves the measurement time which is particularly important for swept measurements.
Since the useful signal is displayed in its original size, post-FFT shows harmonics and useful signal in
their correct proportion.
For measurements without post-FFT display, the FFT size for
calculating
the THD+N value is reduced
from 8 to 1k to further increase the measurement speed.
5.5.2 Special Features of Zoom FFT
The FFT measurement function is offered exclusively as zoom-FFT as normal FFT is displayed (and
swept) as post-FFT of the THD+N measurement.
FFT size and zoom factor can be set via the 1-out-of-N selection softkey, the zoom factor as a power-of-
two integer from 2^1 (2) to 2^7 (128).
FFT display is normally in Volts while the distortion (also with post-FFT) is displayed in % or dB. In order
to obtain the same scale for FFT and post-FFT, the FFT is processed after the first measurement:
•
set the graphics cursor to maximum (also the useful signal),
•
the voltage value found there is displayed and stored as a reference value for the display scale,
•
the value is used as reference depending on the selected unit:
- %V/Vr, if the selected UNIT is %,
- dBr, if the selected UNIT is dB.
Summary of Contents for UPD
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