22
Subject to change without notice
invariant shape for e.g. millions of periods, it is unsuitable for
the capture of single events. Equivalent time sampling cannot
display the rising portion of a signal without a delay line.
3. Random sampling:
Random sampling is also very old (1952) and not invented for
digital oscilloscopes. It is similar to equivalent time sampling in
that it 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 achie-
ved. However, the samples are not taken step for step along the
signal 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 digital oscilloscopes which combine
the best of both worlds although the cost of such an instrument
is markedly higher than that of a pure digital oscilloscopes,
consider the complicated high frequency crt alone. It is the low
cost of manufacturing which causes the drive towards digital
oscilloscopes.
Digital operation
The 150 MHz scope has two 8 bit a/d converters of the flash
type, the best there is. The maximum sample rate of each is
500 MSa/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 GSa/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 digital mode can
be read via an interface and documented. See the chapter ”Data
Transfer“ for details.
Digital operating modes
In digital 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 you to directly de-
termine 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 po-
sitioned 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 digital 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.
C o m b i S c o p e
