User manual

30
Analysis
9.2 Frequency Analysis (FFT)
In general, the FFT in an oscilloscope works differently than
in a spectrum analyzer and is affected not only by the time
base setting, but also by the available number of used acqui-
red data points when calculating the FFT. The R&S®HMO1002
allows you to include up to 128k point in the FFT.
The FFT menu in the ANALYZE section allows a quick Fou-
rier transformation which displays the frequency spectrum
of the measured signal. The changed display allows you
to determine the most frequent frequencies in the signal
and the corresponding amplitude. Once the FFT key was
pressed, the screen will be divided into two graticules.
The upper left of the display shows information about the
settings in the time range, the area between the upper and
the lower window shows details about zoom and posi-
tion, and the section below the large FFT display window
indicates the settings (Span and Center) in the frequency
range. The lower FFT display window will be outlined in
white when the FFT is activated. This means that the large
knob in the time range section is used to select the span.
ThespanisspeciedintheunitHz(Hertz)andidenties
the width of the shown frequency range. The span posi-
tion can be determined by selecting the center value. You
may use the horizontal encoder X Position for this purpose.
The shown frequency range ranges from (Center - Span/2)
to (Center + Span/2).
The soft menu key MODE allows you to choose from the
following display types:
❙ REFRESH:
This mode calculates and displays the FFT without
additional evaluation or editing of the captured data. The
new input data is captured, displayed and overwrites
previously stored and displayed values.
❙ ENVELOPE:
IntheEnvelopemode,themaximumdeectionsofall
spectra will be stored separately in addition to the current
The FFT is not suitable for the analysis of very slow signals (Hz-
range); this type of analysis requires a classic oscilloscope mode.
The minimum increment depends on the time base. The greater the
time base, the smaller the span. Another important element for the
FFT is the setting “Max. Sampling Rate” in the ACQUIRE menu.
spectrum and will be updated with each new spectrum.
These maximum values will be displayed with the input
data and create an envelope curve. The spectrum is
located within the envelope limits. This forms an area or a
sleeve including all occurrences of FFT signal values. With
each signal parameter change the envelope curve will be
reset.
❙ AVERAGE:
This mode calculates the mean value from several
spectra. It is applicable for noise reduction. The soft menu
key #AVERAGES allows you to select the number of
spectra used to calculate the mean value by setting the
universal knob in the power of 2 from 2 to 512.
The menu entry POINTS allows you to select the maximum
number of capture points to be included in the calculation by
using the universal knob in the CURSOR/MENU section. The
possible settings are 2048, 4096, 8192, 16384, 32768,
65536, 131072 points. The soft menu WINDOWS allows you
to improve the FFT display in case of irregularities at the
margins of the measurement interval. Irregularities are cal-
culated as a leap by a computing algorithm and interfere with
the measurement result. In the event of a bell-shaped win-
dow function, the margins with lower values are multiplied
and the impact is damped. The soft menu item WINDOW
allows you to choose from the following window functions:
HANNING: The Hanning window function is
bell-shaped. In contrast to the Hamming win-
dow function, it is equal to zero at the margin of
the measurement interval. Therefore the noise level is re-
duced in the spectrum and the width of the spectral lines
is increased. This function is useful for a precise amplitude
measurement of a period signal, for instance.
HAMMING: The Hamming window function
is bell-shaped. In contrast to the Hanning and
Blackman window function, it is not equal to
zero at the margin of the measurement interval. Therefore
the height of the noise level in the spectrum is greater
than with the Hanning and Blackman window function but
less than with the square wave window function. Howe-
ver, the spectral lines not as wide as in other bell-shaped
functions. This function is useful for a precise amplitude
measurement of a period signal, for instance.
BLACKMAN: The Blackman window function
is bell-shaped and its waveform features the
steepest fall-off among the available functions.
Is is zero at both ends of the measurement interval. The
Blackman window function allows you to measure the am-
plitudeswithhighaccuracy.However,itismoredifcult
to determine the frequency due to the wide spectral lines.
This function is useful for a precise amplitude measure-
ment of a period signal, for instance.
RECTANGLE: The rectangle function multiplies
all points by 1. This results in a high frequency
accuracy with narrow spectral lines and incre-
Fig. 9.2: FFT