Hardware Reference
In-Depth Information
switch. In the 'AC' position the capacitor is inserted at the input, whereas
in the 'DC' position the capacitor is shorted. If 'GND' is selected the vertical
input is taken to common (0 V) and the input terminal is left floating. In order
to measure the DC level of an input signal, the 'AC-GND-DC' switch must
first be placed in the 'GND' position. The 'vertical position' is then adjusted so
that the trace is coincident with the central horizontal axis. The 'AC-GND-DC'
switch is then placed in the 'DC' position and the shift along the vertical axis
measured in order to ascertain the DC level.
Most dual-beam oscilloscopes incorporate a 'chopped-alternate' switch to
select the mode of beam splitting. In the 'chopped' position, the trace displays
a small portion of one vertical channel waveform followed by an equally small
portion of the other. The traces are thus sampled at a fast rate so that the resulting
display appears to consist of two apparently continuous traces. In the 'alternate'
position, a complete horizontal sweep is devoted to each channel on an alternate
basis.
Chopped mode operation is appropriate to signals of relatively low frequency
(i.e. those well below the chopping rate) where it is important that the display
accurately shows the true phase relationship between the two displayed signals.
Alternate mode operation, on the other hand, is suitable for high-frequency
signals where the chopping signal would otherwise corrupt the display. In such
cases it is important to note that the relative phase of the two signals will not
be accurately displayed.
Most modern oscilloscopes allow the user to select one of several signals
for use as the timebase trigger. These 'trigger source' options generally include
an internal signal derived from the vertical deflection system, a 50 Hz sig-
nal derived from the AC mains supply, and a signal which may be applied to
an 'external trigger input'. As an example, the 50 Hz trigger source should
be selected when checking for mains-borne noise and interference whereas
the external trigger input may usefully be derived from a processor clock
signal
when
investigating
the
synchronous
signals
present
within
the
PC
expansion bus.
Figure 13.25 shows the typical control layout of a modern dual-beam bench
oscilloscope.
Fault location
procedure
To simplify the process of fault location on a PC and associated expansion bus,
it is useful to consider the system as a number of interlinked sub-systems. Each
sub-system can be further divided into its constituent elements. Fortunately, the
use of a standard expansion bus makes fault finding very straightforward since
it is eminently possible to isolate a fault to a particular part of the system just by
removing a suspect board and substituting one which is known to be functional.
The following eight-point checklist may prove useful; the questions should be
answered
before
attempting to make any measurements or remove any suspect
boards.
1
Has the system operated in similar circumstances without failure? Is the fault
inherent in the system?
2
If an inherent fault is suspected, why was it not detected by normal quality
procedures?
3
If the fault is not considered inherent and is attributed to component failure,
in what circumstances did the equipment fail?
