Rohde & Schwarz R&S UPP, User Manual

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Instrument Functions
R&S
®
UPP
390 User Manual 1411.1055.32 ─ 10
Here a compromise must be made between measurement speed and measurement
accuracy:
●
A small (initial) FFT length and a large rejection bandwidth ("Rejection Bandwidth")
mean a short measurement time; the filtered-out sideband of the fundamental is,
however, relatively large. Non-harmonic signal components close to the fundamen-
tal are not measured.
●
A large (initial) FFT length means a long sampling time; the filtered-out sideband of
the fundamental is, however, relatively small so that signal components (non-har-
monics and noise) close to the fundamental are also measured.
If the selected FFT value is not large enough to determine the fundamental of the mea-
surement signal, the length of the FFT is increased gradually by a factor of 2.
The maximum FFT value depends on the bandwidth or sampling rate:
●
at a 22 kHz bandwidth (analog) the FFT value can be increased to a maximum of
64 k,
●
at sampling rates of less than 50 kHz (digital) or at a 40 kHz bandwidth (analog)
the FFT value can be increased to a maximum of 128 k,
●
at sampling rates higher than 50 kHz (digital) or at a 80 kHz bandwidth (analog) the
FFT value is increased to a maximum of 256 k,
The maximum integration time in the two-channel analog analyzer (fast mode) is there-
fore around 1.4 s.
Sideband suppression is designed for the shortest possible measurement time. For
very low frequency signals (less than 12
Hz), sideband suppression can be improved
– at the cost of a longer measurement time – by increasing the minimum FFT size to
128
K (with 48 kHz sampling rate and 22 kHz bandwidth) or 256 K (with 96 kHz
sampling rate and 40 kHz bandwidth).
The energy of the noise and (if applicable) the harmonics (i.e. without fundamental)
can be displayed as RMS value with "Meas Mode" ="Level THD+N" or set in relation-
ship to the total RMS with "Meas Mode" = "THD+N". The total RMS value is band-limi-
ted to "FrqLim Upp" and is identified in the following figure with
RMS value .
An ultra-pure sine-wave signal is required as the measurement signal. The selected
frequency of this sine-wave signal should be such that the significant components of
the distortion spectrum are still below the upper measurement limit.
The "Sine" signal function of the internal generator is recommended as the signal
source. The sine-wave level and frequency can be varied or swept.
Distortion Measurements