BK Precision 5105B, Instruction Manual, Page: 23 / 48

23
Subject to change without notice
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 will be taken of the
signal 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
contents. 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 as high a sampling rate as is 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
DSOs.
Example:
This scope will sample with 1 GS/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.
DSOs 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 criterion of a DSO.
Horizontal resolution with X magnifier
In principle, with a 10 x magnified sweep, the resolution should
be reduced to 20 points per cm. However, the resolution
remains at 200 points per cm as the information necessary
will be calculated from the memory. The magnified portion
may be selected with the X-POS control. The fastest time base
will be 5 ns/cm allowing a 2 cm per period display of 100 MHz.
Maximum signal frequency in DSO mode
The highest signal repetition frequency which still can be
displayed well can not be exactly given. This is dependent as
well on the signal shape 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
recognized if their repetition frequency is
< 1/10 of the sampling
frequency. For a well defined 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 hori-
zontal 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 print-out: this indicates an insufficient
sampling rate and consequently a false display. With an
insufficient sampling rate e.g. fast, short pulses may be
completely ignored.
The best method to detect any false DSO 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 amplifier operating modes
In principle, in DSO 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 DSO mode are:
– In dual channel mode both channels resp. signals are a/d
converted simultaneously.
No alternate or chopped channel switching.
– No flickering display even with low frequency signals as
the signals are stored and continuously displayed from the
memory with a sufficiently high rep rate.
– Trace intensity is always the same. This is 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 resp.
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 DSO mode they will be
shown as bright as the other signal portions.
Data transfer
Please note: Interface modules may only be exchanged after
the instrument was turned off. During operation the opening
of the interface must be covered.
There is an opening on the rear panel into which various
interface modules can be inserted. Instruments are delivered
with RS-232 installed.
The interface allows to either remotely control the scope or
read its settings. In DSO mode also the digitized and stored
signals may be retrieved. Interface cables must be shielded
and must not reach a length of 3 m or more.
D a t a t r a n s f e r