28 Oscilloscopes
things, settings of channels not currently in use. Additionally,
when the instrument is under remote control (either RS232,
fitted as standard or IEEE 488.2, optional), the STATUS/LOCAL
button functions as a 'go to LOCAL' command, returning control
to the front panel.
Two lines of user-definable text can be displayed on the screen,
see Figure 4.11. This can provide useful additional information
on a screen shot when photographing the displayed traces. This
facility is accessed via a submenu called up after pressing the
UTILITY button, situated at the top of the screen above the
CURSOR button. Photographing the screen can conveniently be
carried out using the PM 9381/001 oscilloscope camera, illus-
trated in Chapter 4.
Other
facilities
So many facilities are provided by the microcontroller and
software, which monitor and control all aspects of the instru-
ment's operation, that they cannot be covered in full here, so a
representative selection is presented. The CAL function can be
called up by pressing the CAL button, situated to the right of the
AUTO SET button. This function makes fine adjustments to
input, trigger and timebase circuitry, to achieve high accuracy
even under extreme ambient conditions. Under normal labor-
atory conditions, weekly or even monthly calibration is adequate.
Note that the instrument should be allowed to warm up
thoroughly before calibration and that the CAL button must be
pressed for at least 2 seconds to initiate this function. A more
complete calibration procedure (advised annually or every 2000
hours' use) can be called up by a special submenu under the
MAINTENANCE menu.
One of the most important functions is AUTO SET, which can
be executed by pressing the AUTO SET button, located directly
above the STATUS/LOCAL button. This switches off any input
channels at which it detects no signal, and for the others selects
a suitable Y sensitivity setting with a.c. coupling- AUTO SET does
not work for very low signal frequencies. Additionally, input
impedance is set to 1 M~, trigger to positive edge triggering from
the channel with the lowest input signal frequency, main
Advanced real-time oscilloscopes 29
timebase only, with ALTernate or CHOPped display as most
appropriate, etc. These settings are the standard AUTO SET
default settings
for the given input signals,
not to be confused with
the standard default settings called up at switch-on, following the
self-test routine. Both sets of defaults can, however, be modified
by the user, to customize the instrument for his particular uses
and preferences.
Up to ten complete front panel set-ups can be stored in battery-
backed memory, to be recalled as required where a series of tests
is routinely carried out, such as in a production test department.
Settings can be saved, recalled, modified or cleared under the
SETUPS menu. This is activated by pressing the SETUPS button,
which is situated to the right of the CAL button. Once a suite of
setups has been saved, the AUTO SET button can be programmed
to act as a convenient 'recall next SETUP' key. This is done via the
AUTO SET submenu of the UTILITY menu. Alternatively, the
instrument can be commanded to the next front panel setup
when using one of the supplied probes fitted with a 'probe
command switch'.
The UTILITY menu is called up by pressing the UTILITY key,
which is situated to the right of the SETUPS key. This gives access
to five submenus (each with their own sub-submenus), including
AUTO SET, RS232 setup and the MAINTENANCE menu men-
tioned earlier (for service technicians only).
Back panel
The back panel carries the mains input connector, fuse holder and
the instrument type and serial number plates. There is also a
compartment to house the back-up batteries, and a useful storage
space for the mains lead when not in use. The 9 pin D type RS232
connector, for remote control of the instrument and fitted as
standard, is also mounted on the back panel.
In common with most advanced modern oscilloscopes, the
PM3094 economizes on front panel space by accommodating less
frequently used facilities on the instrument's back panel. These
include an optional GPIB! 1EEE-482.2 interface, and a row of five
AUX-sockets via BNC connectors, also optional. These auxiliary
sockets provide for Z modulation (an application for which is
30 Oscilloscopes
described in Chapter 5), an EXTernal TRIGger input, main- and
delay-timebase gate outputs and a Y-out signal. The timebase gates
are pulse signals of length equal to the corresponding timebase,
main or delayed, and may be used to trigger or stimulate external
circuitry. Thus if the timebase is allowed to free run (perhaps with
some HOLDOFF applied), a stable locked display of the response of
the circuitry to the stimulation can be observed. The Y-out signal,
derived from Channel 1, is a most useful feature and well worth
having. It provides an output of 20mV/div behind 500,,
representing a voltage gain of x l0 on the maximum Channel 1
input sensitivity of 2 mV/div. Thus if it is patched into the Channel
2 input, that channel's maximum sensitivity is increased to
200 bLV/div, at least for low frequencies. For high frequencies the
patch should be made with the shortest practicable length of good
quality 50 ~ coaxial cable, and Channel 2 input impedance set to
50 ~. The maximum Channel 2 sensitivity is then 400 bLV/div, with
a bandwidth of 200 MHz at-6 dB.
Options and accessories
A wide range of options and accessories is available, including
those described above, and others described below. The order
number PM3094/00n specifies the standard model, where n is a
single digit specifying the type of plug fitted to mains lead. The
options include European, N. American and UK types among
others. PM3094/40n specifies the addition of a GPIB/1EEE-482.2
interface while PM3094/73n denotes an instrument with the
extremely useful AUX-outputs described in the previous section.
The order number PM3094/93n specifies a model with both GPIB
and AUX options fitted. Accessories supplied as standard include
two PM9020/091 1.5 m long 10:1 passive probes. These probes
actuate the probe sensor arrangement on the input sockets of the
instrument, automatically adjusting screen scale factor readouts
to allow for the • probe attenuation. They also incorporate the
command switch mentioned earlier.
A rackmount kit is another option: this can be retrofitted
without any modifications to the instrument. The PM9381/001
oscilloscope camera kit is featured in Chapter 4, and a range of
a.c. and d.c. current probes is available, as is the PM8940/09n
Advanced real-time oscilloscopes 31
Figure 3.3 A good example of a recording oscilloscope, the Dash 16u is a I6
channel Data Acquisition Recorder. Its single ended or differential inputs (>60 dB
CMRR at 60 Hz) can be floated up to 250 V off ground. Each channel is sampled
a 200 ks/s, providing 20 kHz bandwidth. DSP-based filtering with a choice of low-
pass, high-pass, band-pass or notch can be selected to combat noise problems
(reproduced by courtesy of Astro-Med, Inc.)
isolation amplifier. Both current probes and isolation amplifiers
are featured in later chapters.
Mainframe plus plug-in oscilloscopes
Mainframe and plug-in oscilloscopes are designed for bench
operation rather than portable use. The well-known and long-
established Tektronix 7000 series mainframe plus plug-in oscillo-
scope systems are no longer manufactured. But many thousands
are still in use throughout the world and the manufacturer
supports each model in the range for eight years following the
date when it was discontinued. Consequently many will be
supported until after the year 2000, which explains the price that
they still command on the second user equipment market. Figure
8.19 shows an example of a mainframe oscilloscope.
The advantage of the mainframe plus plug-in format is
economy, since if a different facility is needed it can be had for the
cost of a plug-in, whereas otherwise a complete new oscilloscope
would be required. On the other hand, only one person at a time
can use the mainframe, so usually at any one time capital is tied
32 Oscilloscopes
up in various plug-ins sitting in a cupboard. Most large electronics
laboratories therefore sought to strike a balance, with some
mainframe oscilloscopes plus a variety of plug-ins for versatility,
and some stand-alone 'portable' scopes for economy. However,
the trend recently is for stand-alone/portable oscilloscopes to
become the norm, although a few manufacturers still offer
instruments in the mainframe/plug-in format.
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