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Subject to change without notice
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.
Memory depth
1 GS/s means that one million samples of the signal will be
taken and stored. With normal triggering and time base settings
of
>
20 ms/cm there will be 500,000 samples.
The screen display is calculated from the whole memory con-
tents. Within the menu Settings
>
Display several display modes
may be selected:
Dots:
the sampling points only are displayed.
Vectors: interpolation (sin x/x) or dot join is used to gene rate
a continuous trace.
Optimal: In this mode all samples are used to calculate
the display. This way the display of aliases is
less
likely.
The scope acquires with the highest sampling rate possible thus
preventing to a large extent the production of alias signals. It is
always possible to zoom through the memory in order to look
at details, and, thanks to the deep memory, signal details may
be shown which remain invisible with shorter memory digital
oscilloscopes.
Example:
This scope will sample with 1 GSa/s in single channel mode
down to a time base setting of 100 us/cm. This equals 100,000
points per cm. In MEMORY ZOOM signals of 150 MHz can still be
seen. Down to 100 us/cm hence aliases are not to be expected
due to the bandwidth limit of 150 MHz and the critical frequency
being
>
500 MHz.
Digital oscilloscopes with a shorter memory like e.g. 10 K
will only present 1000 points per cm which is equivalent to
a sampling rate of 10 MHz, thus signals > 5 MHz will cause
aliases , far below the scope bandwidth. A deep memory is one
of the most important criteria of a digital oscilloscope.
Horizontal resolution with X magnifi er
In principle, with a 10 x magnifi ed sweep, the resolution should
be reduced to 20 points per cm. However, the resolution re-
mains at 200 points per cm as the information necessary will
be calculated from the memory. The magnifi ed portion may be
selected with the X POS control. The fastest time base will be
5 ns/cm allowing 2 cm per period display of 100 MHz.
Maximum signal frequency in digital mode
The highest signal repetition frequency which still can be dis-
played well cannot be given exactly. This is dependent on the
signal shape as well as on its amplitude displayed.
While it is fairly easy to recognize a square wave, it requires
at least 10 samples per period to distinguish a sine wave from
a triangle. In other words, in practice, signals may still be re-
cognized if their repetition frequency is
<
1/10 of the sampling
frequency. For a well defi ned display, however, many more than
10 points per cm are necessary.
Display of aliases
As explained the maximum sampling rate must be reduced for
slow time base settings. This may cause aliases. If e.g. a sine
wave is sampled only with one sample per period and if it should
be synchronous with the sampling frequency a horizontal line
will be shown as each time the same signal point is sampled. An
alias may also take the form of a signal of much lower frequency
(beat frequency between signal and sampling frequencies),
apparently untriggered changing displays, or may look like AM
modulated signals. If an alias is suspected, change the signal
frequency or the time base or both. If aliases remain undetected
grossly erroneous results will be obtained which includes also
grossly (maybe orders of magnitude) false displays of signal
parameters like rise time etc.! Always watch for a stepped
display or printout: this indicates an insuffi cient sampling rate
and consequently a false display. With an insuffi cient sampling
rate e.g. fast, short pulses may be completely ignored.
The best method to detect any false digital oscilloscope display
is to switch to analog mode. In analog mode false displays are
absolutely impossible! An analog scope can at worst round the
edges of very fast signals.
Vertical amplifi er operating modes
In principle, in digital mode there are the same modes available
as in analog mode, i.e.:
– CH1
only
– CH2
only
– CH1 and CH2 in dual trace mode Yt or XY
– Sum
– Difference
The main differences of digital mode are:
– In dual channel mode both channels and signals are a/d
converted simultaneously.
No alternate or chopped channel switching.
– No
fl ickering display even with low frequency signals as
the signals are stored and continuously displayed from the
memory with a suffi ciently high rep rate.
– Trace intensity is always the same. This can be an advantage
and a disadvantage.
All so called Z axis (trace intensity) information is lost. In ana-
log mode the intensity depends on the signal rep rate and the
speed, thus mixed or unstable signals can be differentiated by
their respective trace intensity. Fast slopes of low frequency
signals are invisible in analog mode, in digital mode they will
be shown as bright as the other signal portions.
C o m b i S c o b e
