User manual
39
Analysis
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:
IntheEnvelopemode,themaximumdeectionsofall
spectra will be stored separately in addition to the current
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 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
calculated as a leap by a computing algorithm and interfere
with the measurement result. In the event of a bell-shaped
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.
window 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-
plitudeswithhighaccuracy.However,itismoredifcult
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-
ased noise. This function can be used for pulse
response tests with start and end values of zero.
The menu item Y-SCALING allows you to scale the FFT in
the amplitude logarithmically (dBm / dBV) or linear (V
eff
).
The unit dBm (Decibel-Milliwatt) refers to 1mW. The unit
dBV (Decibel-Volt) refers to 1V
eff
. The displayed values
refer to a 50Ω terminating resistor. You can use an external
terminating resistor parallel to the high impedance input.
You can deactivate the FFT function by pressing the soft
menu key FFT OFF or pressing the FFT key on the control
panel again.
9.3 Quick View
Pressing the QUICKVIEW key
10
in the ANALYZE section
activates several basic automatic measurements. The
QUICK VIEW function allows a quick overview of the typi-
cal signal size. Measurement results are displayed at the
bottom of the screen and with a cursor on the signal. The
followingvemeasurementvaluesaredisplayeddirectlyin
the signal:
Fig. 9.8: Advanced FFT menu