22
Subject to change without notice
requires a multitude of signal repetitions with invariant shape
in order to reconstruct it once on the screen. Therefore also
in this mode a very high ”effective“ sampling rate is achieved.
However, the samples are not taken step for step along the sig-
nal but randomly distributed over the signal period. An analog
computer is used to estimate the arrival of the next trigger, and
the time base is already started when it arrives. This has two
enormous advantages:
1
st
The rising portion of the signal can be shown without the
need for a delay line which would severely limit the achie-
vable bandwidth.
2
nd
Due to the randomness of the samples alias signals will be
broken up.
The foregoing explains why it is HAMEG policy to offer Combi-
Scopes rather than pure DSOs which combine the best of both
worlds although the cost of such an instrument is markedly
higher than that of a pure DSO, take alone the complicated high
frequency crt. It is the low cost of manufacturing which causes
the drive towards DSOs.
DSO Operation
The 100 MHz scope has 2 8bit a/d converters of the fl ash type
which is the best there is. The maximum sample rate of each
is 500 MS/s which is the rate available in dual channel mode
for the capture of single events. The maximum sampling rate
in all other operating modes is 1 GS/s.
Higher effective (!) sampling rates are possible as explained
above in equivalent and random sampling modes. As very many
signals repetitions are needed to reconstruct the signal once
any changes in signal shape such as noise will show up.
The reconstructed signal may be displayed either by showing
only the sampled points or with interpolation between them by
drawing straight lines.
The signals stored in DSO mode can be read via an interface and
documented. See the chapter ”Data Transfer“ for details.
DSO operating modes
In DSO mode the following operating modes are available:
1. Menu: ACQUIRE: Repetitive triggered signal acquisition and
display in usual Yt representation.
REFRESH: readout shows ”rfr“ (real time sampling) or
Random sampling:
readout ”RS:xGSa“.
The operating mode may be further subdivided:
Envelope:
readout
”env“
Average:
readout
”avg:x“
(x may be a number 2.. 512)
2. ROLL mode, untriggered continuous signal acquisition,
display will „roll“ over the screen from left to right in usual
Yt mode:
Roll:
readout
”rol“
3. Single sweep, triggered (menu: Trigger MODE) signal capture
in usual Yt mode:
Single:
readout
”sgl“
4. Untriggered continuous signal capture, display in XY mode
(Menu: trigger MODE):
XY: readout ”XY“
5. XY display of signals which were previously captured in Yt
mode and protected against overwriting by STOP:
XY: readout ”XY“
Signal capture is triggered in SINGLE, REFRESH, ENVELOPE,
and AVERAGE modes and untriggered in ROLL and XY modes.
The normal (Refresh) mode is similar to the operating mode of
an analog scope. Triggering will cause signal acquisition and
display from left to right. After the next acquisition the display
will be replaced by the new information. If automatic triggering
was selected there will be a reference trace in the absence of a
signal the position of which is dependent on the vertical position
control setting. Signals with a repetition rate lower than the rep
rate of the automatic triggering can not properly trigger so the
resulting display will be untriggered.
In contrast to an analog scope the last display will remain on the
screen if the signal disappears in normal trigger mode.
In SINGLE mode the signal will be acquired only once. Acquisi-
tion can start if STOP (RUN key) is not illuminated (if necessary
press RUN until STOP extinguishes). The next trigger received
will cause the single acquisition. After this STOP will light up
and the trigger mode will be automatically switched to normal
dc coupled if auto was selected.
The trigger symbol on the screen allows to directly see resp.
determine the voltage level desired for triggering in the normal
mode, the voltage follows from the position and the VOLTS/CM
selected. The ground reference will be indicated by a ground
symbol in the screen centre.
After selecting SINGLE the trigger level symbol may be positio-
ned using the LEVEL control. If e.g. the symbol is 2 cm above
the ground reference symbol the trigger level will be 2 cm x
Volts/cm (x probe factor if any).
Example:
2 cm x 1 V/cm x 10 (probe) = + 20 V.
Memory resolution
Vertical resolution:
The 8 bit a/d converters have a resolution of 256 possible ver-
tical positions. The screen display has a resolution of 25 points
per cm. This is advantageous for display, documentation and
post-processing.
There may be some difference between the display on screen
and documentation, e.g. on a printer, this results from various
tolerances in the analog circuitry involved. The trace positions
are defi ned by:
Median horizontal line:
10000000b
80h
128d
Top
line:
11100100b
E4h
228d
Bottom line:
00011100b
1Ch
28d
In contrast to an analog display with its theoretically infi nite
resolution this is limited to 25 points per cm in DSO mode. If
there is any noise superimposed on the signal this may cause
frequent change of the lowest bit and thus jumping of the trace
in vertical direction.
Horizontal resolution:
A maximum of 4 simultaneous signal displays may be shown
on the screen. Each signal display will consist of 2048 points
(bytes). 2000 points will be distributed over 10 cm. The resolution
is thus 200 points per cm. Please note that this a 4 to 8 times
improvement over customary VGA (50 points per div) or LCD (25
points per div.) DSO displays.
C o m b i S c o p e
