Hardware Reference
In-Depth Information
It is also worth asking yourself a few questions about the PC that you intend
to use with the instrument:
•
What ports are available?
•
Is the parallel port dedicated to a printer or is it available for use by the DSO?
•
If the parallel port is unavailable, does the PC have a spare USB port or will
you need to purchase a hub? Or, can the PC be fitted with a second parallel
port by means of a suitable PCI or EISA adapter card?
In many cases you may find that you are faced with a compromise between
resolution and speed. However, a DSO with a 12-bit resolution and sampling rate
of 5G samples per second will be more than adequate for most general-purpose
applications!
Modern DSOs, with their PC connectivity, can also be fully integrated into
automatic test equipment (ATE) systems. In addition, the DSO is often used as
the front-end of a highly cost-effective data acquisition system.
Bandwidth alone is not enough to ensure that a DSO can accurately capture a
high-frequency signal. The goal of manufacturers is to achieve a flat frequency
response. This response is sometimes referred to as a Maximally Flat Envel-
ope Delay (MFED). A frequency response of this type delivers excellent pulse
fidelity with minimum overshoot, undershoot, and ringing.
It is important to remember that, if the input signal is not a pure sine wave,
it will contain a number of higher-frequency harmonics. For example, a square
wave will contain odd harmonics that have levels that become progressively
reduced as their frequency increases. Thus, to display a 1 MHz square wave
accurately you need to take into account the fact that there will be signal
components present at 3, 5, 7, 9, 11 MHz, and so on.
As mentioned earlier, it is wise to purchase a DSO with a bandwidth that is five
times higher than the maximum-frequency signal you wish to measure. Note,
however, that with some instruments the specified bandwidth is not available
on all voltage ranges, so it is worth checking the manufacturer's specification
carefully.
Most DSO have two different sampling rates (modes) depending on the sig-
nal being measured: real time and equivalent time sampling (ETS) - often
called repetitive sampling. However, since this mode works by building up the
waveform from successive acquisitions, ETS only works if the signal you are
measuring is stable and repetitive.
Basic operation of a DSO
The basic operation of a DSO is extremely straightforward and all of the
instrument's controls are accessed through software using a standard Windows
interface (see Figure 11.2). It is worth comparing this interface with the controls
and adjustments provided on a conventional oscilloscope (see Photo 11.2).
Unlike a conventional oscilloscope, most DSOs will provide you with more
than one type of view of the data that you are collecting. Typically these will
include:
•
an
oscilloscope
display,
with
all
of
the
features
of
a
modern
storage
oscilloscope;
