38
Subject to change without notice
A n a l y s i s
The FFT menu allows a quick Fourier 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.
You can activate the
frequency analysis by pressing the FFT
key
9
in the ANALYzE section of the control panel. Once the
key was pressed, it will be illuminated in white and the screen
will be divided into two graticules. The upper section displays
the voltage time curve whereas the lower section lists the re
-
sults of the Fourier analysis. The FFT is calculated including a
maximum of 65,536 acquired data points. Additional points at a
consistent span result in a smaller frequency increment of the
FFT. The number of points for the output data is half the size
of that of the input data.
Fig. 9.5: FFT illustration
The upper left of the display shows information about the set
-
tings in the time range, the area between the upper and the
lower window shows details about zoom and position, 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 specified in the
unit Hz (Hertz) and identifies the width of the shown
frequency
range. The span position 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 ( Span/2).
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 set-
ting “Max. Sampling Frequency” in the ACQUIRE
menu of the HMO instrument.
The soft menu key MODUS allows you to choose from the fol
-
lowing 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 deflections of all spectra
will be stored separately in addition to the current spectrum
and will be updated with each new spectrum. These maximum
An equation consists of an operator (mathematical function) and
up to two operands. You can use the
universal knob to choose
one of the following operators:
– Addition
– Subtraction
– Multiplication
– Division
– Maximum
– Minimum
– Square
– Root
– Amount
– Pos. Wave
– Neg. Wave
– Reciprocal
– Inverted
– Common logarithm
– Natural logarithm
– Derivation
– Integral
– IIR Low Pass Filter
– IIR High Pass Filter
For each corresponding equation, the input channels CH1, CH2,
CH3, CH4 and an adjustable constant are allowed as OPERAND
(
sources). For the formula MA2, MA1 is added as source, for
MA3 the added
source is MA2, for MA4 it is MA3 and finally for
MA5 the added
source is MA4. From these five equations, you
can create, save and retrieve a total of five different sets. New
equations can be added by using the universal knob to select
the menu item NEW in the formula set editor. Pressing the soft
menu key ADD allows you to edit the new equation.
Fig. 9.4 illustrates how in formula
MA1
channel 1 is added with
100 µA. Press the key
EDIT CONSTANT
in the menu for enter
-
ing constants and use the universal knob to choose from the
following constants:
– Pi
– 2x Pi
– 0,5 x Pi
– User 1 . . . 10
(up to 10 customized constants are available)
For instance, if you select
USER1
as constant, you can press
the soft menu key
VALUE
and use the universal knob to select a
numeric value. You can apply the same method to set a DECIMAL
POINT and enter an additional S prefix (soft menu key PREFIX).
You may choose a UNIT from the same SI prefixes as those that
are available in the soft menu EDIT. Press
SAVE
to store these
settings as
USER 1
and return to the menu to edit the equation.
You can store up to 10 of these customized constants. When
saving a formula set, you may also add a comment (soft menu
key
COMMENT)
. Press the key
SAVE
to save this formula set with
the determined name and comment to the selected location.
Stored formula sets may be reloaded at any time. Press the
MATH key to activate the Mathematics menu and then press
the MENU key below the SCALE VOLTS/DIV key. This menu
displays the menu item
LOAD
. This will start the file manager
which will display the internal memory or the connected USB
stick as possible storage location. Select the respective formula
set file and press the key
LOAD
to load the file.
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 acquired data
points when calculating the FFT. The HMO series allows you
to include up to 65k point in the FFT resulting in a very high
resolution for this price bracket.
The FFT is not suitable for the analysis of very slow
signals (Hz-range); this type of analysis requires a
classic oscilloscope mode.
