Hameg CombiScope HM2008 Manual Download

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Subject to change without notice

Off“ is active, the whole memory will always be displayed. Tur-
ning the control CCW will decrease, turning it CW will increase
the time base speed. Depending on the signal capture/display
the time base can be set from 50 s/cm to 5 ns/cm in a 1-2-5
sequence (e.g. ”A:50ns“) and will be calibrated. There is no
variable function as in analog mode.

28.2.2

Search Zoom only

(Zoom time base speed selection)
One of the functions ”Search“ or ”Zoom only“ may be selected
in the ”Zoom“ menu after pressing HOR VAR pushbutton

”Zoom Off“ is equivalent to time base A in analog mode. With
”Zoom only“ a portion of the display in Zoom ”Off“ can be ex-
panded over the whole screen. This is possible because there
is a very large memory for signal capture/display. The ”Zoom
Off“ display will present the whole memory contents. With
”Search“ both the ”Zoom Off“ and the expanded ”Zoom only“
displays will be visible. With ”Zoom only“ only the expanded
display will show up.
The Zoom time base speed will be indicated in the readout ”Z:...“
and is calibrated. Turning the knob CCW will decrease, turning
it CW will increase the time base speed. This can be selected
from 20 ms/cm to 2 ns/cm in a 1-2-5 sequence. The maximum
expansion depends on the time base A setting.

28.3 XY digital mode

As XY mode is untriggered, all trigger related controls (LEVEL
A/B

, MODE

, FILTER

and SOURCE

are deactivated.

The same applies regarding the ZOOM function (HOR VAR

and

X POS DELAY

), the X magniﬁ er (MAG x10) and all functions

not meaningful in XY mode.

STOP

Attention!

The TIME/DIV-SCALE-VAR knob

is active, be-

cause sampling is required in digital XY mode and
consequently the sampling rate must be set. Thus
the readout displays the sampling rate but no time
deﬂ ection coefﬁ cient.

It is recommendable to choose a suitable sampling rate in Yt
mode and then switch over to XY mode. A suitable sampling rate
is when both signals are displayed with one complete signal
period. With increasing number of displayed signal periods a
degradation of the XY signal display takes place.

28.4 FFT (digital mode)

28.4.1

Preliminary note:

To avoid erroneous spectrum displays, it must be checked be-
fore selecting FFT, that the Yt (time base) signal display is suited
for calculation in FFT. This means that the time base setting
(sampling rate) must enable the display of minimum one signal
period; with complex signals this is regarding the signal with the
lowest frequency. On the other hand the sampling rate must not
be too low (to many signal periods) to avoid so called aliasing.
The signal display height should be between 5 mm (0.5 div.) and
8 cm (8 div.). Signal display heights

8 cm, cause the danger

that the dynamic range is exceeded, so that signals deformed
by limiting effects to square wave form, become digitised and at
least show spectrum displays spectra that do not exist in reality.
If the sampling rate is too low the readout displays “ALS“; if the
signal is too high “overrange ±“ will be displayed.
Such problems can be avoided, by pressing the AUTOSET push-
button

before switching over to FFT or during FFT mode.

28.4.2

Scaling:

For better understanding in some cases the term sampling
frequency is used instead of sampling rate although the readout
displays the sampling rate. A sampling rate display of 40 MSa

(40 Megasamples per second) is the equivalent of a sampling
frequency of 40 MHz.

The current sampling rate setting in Yt (time base) mode
automatically sets the center frequency and span in Yf (FFT)
mode after switchover from Yt (time base) to Yf (FFT) mode. In
FFT mode the TIME/DIV.–SCALE–VAR knob

can be used for

changing the span (SCALE function, expanding the frequency
axis) and not the sampling frequency setting.

The readout displays the sampling rate, center frequency and
span setting together with other parameters in FFT mode. From
the sampling frequency (rate) setting the theoretically highest
displayable frequency (f

max

) results. In accordance with the

Nyquist-Shannon sampling theorem, the highest frequency to
displayed must be 2 * f

max

. At a sampling rate of 2 GSa/s – cor-

responding to a sampling frequency of 1 GHz – the maximum
displayable frequency is below 1000 MHz (2 GHz / 2). In practice
the frequency response of the oscilloscope must be taken into
account (e.g. 200 MHz –3dB); this means that the voltage height
of a 200 MHz is displayed with 0.707 V although the real value is
1 V. This error becomes larger with even higher frequencies.

The center frequency corresponds with the vertical graticule line
in the screen center with direct relation to the frequency span
that displays the frequency range from the left to right graticule
border lines. The display “Center:10.00MHz“ and “Span:20.0MHz“
indicates that the left graticule border line coincides with 0 Hz, the
vertical center line with 10 MHz and nearly 20 MHz are displayed
at the right border line. Under these conditions the sampling
frequency is 40 MHz (Readout: 40 MSa).

MAG x10

(pushbutton)

In analog mode only: pressing this pushbutton will turn on the
x 10 magniﬁer. No menu will be shown. If ”x10“ is illuminated
on the MAG pushbutton the magniﬁer is activated. The adjusted
time/cm will be shown in the top left readout. Depending on
the time base mode turning on the magniﬁer will have these
effects:

29.1 ”Time base A only“

The display will be expanded around the screen centre by a
factor of 10, the time/cm indication adjusted accordingly.

29.2 ”Search“ (A and B times bases alternated)

The time base A speed will not be affected. The time base B

C o n t r o l s a n d R e a d o u t

FFT-
Marker

VAR

x10

TIME / DIV

SCALE

VAR

X-POS