46
Subject to change without notice
C o n t r o l s a n d R e a d o u t
STOP
Attention!
The logic channels and the related settings can
only be activated and displayed if the optional probe
HO2010 is connected.
37
FFT
(pushbutton)
– digital mode only –
Pressing the FFT pushbutton switches over to FFT, if digital Yt
mode is present.
Note:
In this state FFT mode can only be left by switching over
to analog mode. If digital mode is not to be left, please press
the FFT pushbutton again to display the FFT menu which offers
an Off function. Pressing the FFT pushbutton once again cause
the FFT menu to be displayed.
Note:
The signal can be input at CH1 or CH2. After pressing the
VERT/XY pushbutton
32
the channel can be selected. If DUAL
was present before, when switching to FFT, the channel that
previously served as trigger source is selected.
37.1 Window
There are different “windows“ available that cause different
signal data calculation and their display on the frequency axis.
After pressing the function key, the required “window“ function
(Hamming, Hanning, Blackman, Rectangle) can be determined
by the INTENS knob
2
. Please refer to the table below.
Note:
The FFT frequency resolution is the quotient of sam-
pling rate and the number of FFT points (4 kPts). At a constant
number of FFT points, the resolution is proportionally better
as the sampling rate is lowered. The Nyquist frequency is the
highest frequency a real time digital oscilloscope can measure
without aliasing. This frequency is half the sampling frequency
(sampling rate). In case of higher frequencies the number of
samples is too low. If the sampling frequency (rate) is possibly
too low, the readout displays ALS?.
37.2 Mode
The desired signal capture mode can be determined by the
INTENS knob
2
after pressing the function key. The following
modes are available:
37.2.1
Refresh
The signal capture is performed in real time refresh mode. The
result is displayed on the frequency axis (Yf).
37.2.2
Envelope
As in Yt mode frequency maxima and deviation are displayed
on the frequency axis (Yf).
37.2.3
Average
Shows the result of signal capturing processes after FFT cal-
culation and averaging.
37.2.4
Number
Determines the weighting accuracy of the FFT calculations in
the following averageing process. Numbers between 2 and 512
can be chosen by aid of the INTENS (2) knob which are displayed
by the readout (e.g. avg#256). The accuracy increases with
higher numbers but requires more time. Averaging enables
to reduce amplitude changes (noise) and frequency changes
(jitter) in the display.
37.3 Scale
The TIME/DIV-SCALE-VAR knob enables up to 20 fold X expan-
sion of the total spectrum display. This means that in case of a
span of e.g. 500 MHz, the span is reduced to 250 MHz if 2 fold
expansion is active; or 25 MHz span as the result of a 20 fold
expansion. The current center frequency setting is not affected
by scale factor changes.
On condition scale (X expansion), only a fraction of the spectrum
is displayed. The fractions lost can be made visible by turning the
HORIZONTAL knob, changing the center frequency setting.
37.4 dBV V
(rms)
The amplitude unit can be selected by pressing the function key
and will be displayed with intensifi ed brightness.
V
rms
refers to 0 Volt (linear scaling of the Y axis) which is indica-
ted by the reference symbol at the left border of the graticule.
0 dBV equals 1 Volt (logarithmic scaling of the Y axis). If the
reference symbol is 5.5 cm (div.; typically) above the noise
level and is the current scaling 20dB, the noise level is 110dB
(typically) below 1 Volt.
37.5 Off
The function key “Off“ switches over from FFT to Yt mode and
the FFT menu off.
38
AUXILIARY INPUT
(BNC socket)
In analog mode this BNC connector serves as an input for
external trigger or Z axis (intensity modulation) input. In digital
mode it can only be used as a external trigger input.
C O M B I S C O P E
USB
Stick
COMP.
TESTER
PROBE
ADJ
POWER
MENU
MENU
OFF
OFF
43
42
41
40
44
Window
Criteria
Optimal for measurement of
Hanning /
Hamming
Good / ideal frequency resolution but worse ampli-
tude resolution as at rectangle.
Hamming offers a slightly better frequency resolu-
tion than Hanning.
Sine wave, periodical signals as well as narrow band static
noise; transients or burst.
Blackman
Good amplitude, perfect frequency resolution.
Mono frequent signals for detecting harmonics of higher
order.
Rectangle
Best frequency resolution and lowest amplitude
accuracy.
Transients and bursts, settling time analysis; sine wave sig-
nals with equal amplitudes and stable frequency; broadband
static noise with relatively slowly varying spectrum.
