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Subject to change without notice
CombiScope
®
The oscilloscope HM1008-2 combines two oscilloscopes in
one: An analog oscilloscope and a digital oscilloscope. With a
touch of the Analog/Digital pushbutton you can switch between
analog and digital mode (oscilloscope operation). To avoid long
explanations, the terms analog and digital mode are used in
the following text.
HAMEG oscilloscopes are either analog or they are Combi-
Scopes, i.e. they contain a complete analog scope and the
additional hardware and software to sample, digitize, store,
process and display the signals. The HM1008-2 is a 150 MHz
1 GSa/s CombiScope
®
.
With a HAMEG CombiScope
®
the user is always sure: he needs
only to switch from digital to analog in order to see the true
signal. This is especially important when a signal is to be docu-
mented in digital mode. The user of a pure digital oscilloscope
needs to know the signal better than the scope!
The advantages of digital operation are:
– Capture and storage of single events
– No
fl icker with very low frequency signals
– Fast signals with a low rep rate or low duty cycle can be
displayed at high intensity
– Due to the storage of all signals they may be easily docu-
mented and processed.
– High quality crt’s and custom electronic parts are used.
The disadvantages of digital operation are:
– An analog scope displays the signal itself in real time. In
a digital oscilloscope the signal is not displayed but only a
low frequency reconstruction of the signal. The limitations
and problems of sampling operation as well as those of
analog/digital conversion hold. The display can not be in
real time as, after capturing a signal, the digital oscilloscope
must take a short time to perform calculations the result of
which will then be displayed later.
– Therefore the capture rate of ordinary digital oscilloscopes
is orders of magnitude lower than that of any analog scope.
Hence a digital oscilloscope is least suited to catch rare
events.
– There is no information in the trace, the trace is always of
equal intensity. Thus valuable information (so called Z axis)
is lost. Also the fast slopes of a pulse which are invisible on
an analog scope will be of the same intensity as the slower
parts of the signal, this is a gross misrepresentation. The
reason is that digital oscilloscopes ordinarily do not show
only the sampled points but they interpolate by drawing a
continuous trace.
– The vertical resolution is mostly only 8 bits. In an analog
scope there is no loss of fine detail by digitizing. Even if the
trace is not very crisp details can be seen in it.
– Due to the sampling and the lack of a low pass filter in the
input frequencies above half the sampling frequency will
cause so called aliases, i.e. low frequency ghost signals.
Sampling is practically the same as frequency conversion
or multiplication, it creates sum and difference frequencies,
beat frequencies which may be orders of magnitude lower
than the signal frequency and gives grossly erroneous re-
sults. In practice, therefore, only frequencies 1/10 or less
of the sampling frequency can be reliably displayed. The
meaning of the Nyquist theorem is mostly misunderstood:
if the sampling frequency is only twice the signal frequency
there will only be two points displayed on the screen: any
number of signal shapes may be drawn which fit through
these two points. The Nyquist theorem assumes that the
signal is a sine wave. It is easily understood that, in order
to depict an unknown signal shape one needs at least 1 or
2 points per centimeter; in other words: the useful signal
frequency is only 1/10 to 1/20 at best.
– An analog scope has a frequency response which follows
closely the Gaussian curve, this means in practice that also
frequencies far beyond the –3 dB frequency will be shown,
reduced in amplitude, but they will be shown. This not only
preserves fine detail of a signal but it allows also to see, e.g.,
very high frequency wild oscillations in a circuit. This is not
the case after sampling because all frequencies beyond half
the sampling frequency will be „folded“ back into the lower
frequency band.
– Due to limited memory depth the maximum sampling rate
must be reduced in a digital oscilloscope when the time
base is set to slow sweep speeds, it may be reduced from
GSa/s to kSa/s! Most users are not aware of this drawback,
they think that if they bought a digital oscilloscope with 100
MHz bandwidth and 1 GSa/s they are safe when measuring
kHz range signals. But such low frequency signals may be
distorted and possibly aliases displayed.
Please note:
This list of disadvantages is by far incomplete!
It only scratches the surface.
There are 3 methods of sampling:
1
st
Real time sampling
Here the Nyquist theorem must be observed, but, as mentio-
ned, in practice the signal frequency is far less than 1/10 the
sampling frequency. Consequently, with a 1 GSa/s rate signals
with up to 100 MHz can be adequately reconstructed. Obviously,
this is the only mode for single event capturing.
2
nd
Equivalent time sampling
This is the normal operating mode for all sampling scopes.
(Sampling scopes are very old, they are still the fastest scopes
with bandwidths
>
50 GHz because they have no input amplifier.
Sampling scopes are far superior to digital oscilloscopes be-
cause their Y resolution is identical to that of an analog scope).
In this mode consecutive periods of the signal are sampled,
each period contributes but one sample. The signal period is
thus scanned and very many periods are necessary in order to
achieve one full screen display. This way a very high „effective“
sampling rate is achieved, this method exchanges bandwidth
for time. In a sampling scope a very accurate display is created
which is, as far as the shape is concerned, almost as good as
that of an analog scope. In a digital oscilloscope, however, the
sample points are 8 bit a/d converted, losing resolution. The
bandwidth achieved is given alone by the hf properties of the
input and the minimum realizable duration of the sampling pul-
se, so 14 GHz at a sensitivity of 2 mV/cm and 50 Ω was standard
in the 1960’s. In a digital oscilloscope, however, which should
be used like an analog scope, a high impedance (1 MΩ) wide
range (e.g. 1 mV/cm to 20 V/cm) attenuator must be included
and also an input amplifier. This is why a digital oscilloscope
cannot reach the bandwidths of sampling scopes. Equivalent
time sampling suffers fully from the problems of aliasing. As it
requires the (not necessarily periodic) repetition of the signal in
C o m b i S c o p e
