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Review
Tektronix TDS 320
100MHz digital scope
In the last couple of years, digital scopes have
been redefined & presented with an ease of
use undreamt of even with analog scopes. The
Tektronix TDS 320 continues this process,
combining a simplified menu of control features
with a 500 megasamples/second sampling rate
and 100MHz bandwidth.
By LEO SIMPSON
The big challenge to scope manufacturers these days is how to combine
the ever-increasing performance and
potential complexity of features in a
package that is intuitively easy to use.
No longer do users want to refer to
thick manuals to find out how to make
a measurement – they want to do it all
simply by pushing some buttons on
the front panel.
Over last 18 months or so, we have
reviewed a number of digital scopes
and they have all had a different
approach to solving the conflicting
requirements of ease of use and
flexibility of use.
For its part, Tektronix has chosen
yet another approach – one which
makes extensive use of “soft” buttons
and pictorial menus.
First impressions
First impressions of the Tektronix
TDS 320 scope are that it is quite a
bulky unit, but one which is surprisingly light for its bulk. Overall dimensions of the unit, not including its
handle, are 325mm wide, 165mm high
and 470mm deep, including knobs and
rear projections. Its mass is only 6.8kg
which means that it is easily carried
with its large handle.
The front panel is uncluttered
although it does carry quite a lot of
buttons, when you count them all up;
there are 35 buttons and six knobs.
The layout is logical though and you
can clearly identify the main
knobs for vertical sensitivity
and timebase. To the left
of the vertical sensitivity
knob are but
tons to select
channels 1 and 2 and others
which become clear as soon
as you press them: Math, Ref
1 and Ref 2. Math gives you
the choice of CH1 + CH2,
CH1 - CH2 and CH2 - CH1, all
selectable via “soft” buttons
down the side of the screen.
I should explain that many
The Tektronix TDS 320 is a
2-channel digital scope with
main & delayed timebases,
500 megasample/second
sampling rate & 100MHz
bandwidth.
18 Silicon Chip
digital scopes nowadays make use
of these “soft” buttons whereby the
functions change depending on what
control menu is being displayed on
the screen. It actually sounds more
complicated than it is to use and it is
a highly practical way of providing
lots of features without having huge
numbers of buttons.
Naturally, there is an “Autoset”
button which allows you to sit back
while the scope rapidly makes all
the appropriate internal adjustments
to give an appropriate display of signals on the screen. It is the lazy way
of doing things but it makes a lot of
sense and you can then manipulate
the sensitivity and timebase controls
to show the waveform exactly as you
want it.
One very attractive feature of a digital scope such as this is a continuously
variable vertical sensitivity which is
calibrated. To bring this feature into
use on the TDS 320, you first press
the “Vertical Menu” button and then
press the “Finescale” soft button at the
bottom of the screen. Pressing any of
the menu buttons at the bottom of the
screen brings another menu into play,
down the righthand side of the screen,
and these menu choices are activated
by pressing the appropriate soft button
at the side of the screen.
However, when the “Fine Scale” option is pressed, the topmost knob (the
General Purpose knob) on the front
panel is activated, and it is signified
by a knob symbol in the top righthand
corner of the screen. As you vary the
knob, the actual gain setting is shown
at four places on the screen which
really is a bit of overkill. Of course,
once you select some other menu option, say from the “Horizontal Menu”,
the vertical gain setting is shown only
once, next to the appropriate channel
indicator on the screen.
Hence, along the bottom of the
screen you may have readings such
as “Ch1 1.66V Ch2 50mV M2.5µs
Ch1 ~ 33.2mV”. These indicate that
the vertical sensitivity for Channel
1 is 1.66 volts/div, for channel 2 it is
50mV/div, the main timebase setting
is 2.5µs/div, the trigger source is Channel 1 and with triggering on positive
slopes and for signals above 33.2mV.
As with other digital scopes with CRT
readouts, this obviates the need for any
scales on the controls themselves since
all the relevant settings are shown on
the screen.
Coming back to the Vertical Menu,
the sensitivity can be varied by the
“Volts/Div” knob from 2mV/div to
10V/div if you are using a x1 probe
and from 20mV/div to 100V/div if
you are using a Tektronix x10 probe.
The TDS 320 will recognise whether
you are using Tektronix probes which
have a third contact inside the socket
locking ring but it will default to the
gain setting for a x1 probe.
There is no probe menu to allow
you to tell the TDS 320 the settings of
a non-Tektronix switchable probe so
you have to resort to mental arithmetic
in that case.
When using the “Volts/Div” knob
the gain is varied in a 1/2/5 sequence
while in the Fine Scale mode the
gain is continu
o usly varied with
3-digit resolution, in steps ranging
“Delayed Runs 4.13722ms After
Main”. Whichever option you pick
from the side menu is then echoed
at the bottom of the screen together
with another option for Trigger Position. Pressing this soft button gives
you three options for trigger position:
10%, 50% and 90%.
This concept may seem a little odd
until you realise that with a digital
scope you can display part of the
waveform before the nominal trigger
point. In fact, the entire record of a
trace has 1000 sample points and only
the middle section of this record is
normally displayed. You can scroll
along this record by using the horizontal position control. Hence, the 50%
trigger option is in the centre of the
trace and is indicated by a “T” symbol
The high rate of sampling means that it is tops
at catching glitches which are undetectable
on other scopes. It can detect glitches a short
as 10 nanoseconds at all timebase settings
between 25µs/div and 2.5 seconds/div.
between 0.4% and 1%. For example, if the gain is in the range from
100mV/div to 200mV/div it is varied
in 1mV steps, while in the range from
200mV/div to 500mV/div it varies
in 2mV steps. This is in line with
the 8-bit vertical resolution of the
instrument. Vertical gain accuracy
is ±2%.
Timebase
The main and delayed timebases are
not able to be varied continuously but
they are adjusted in four steps per decade; ie, a sequence of 1/2.5/5 which is
adequate for just about all foreseeable
measurement situations. When you
want to make precise measurements
on waveforms you don’t need to vary
the timebase; you either use the vertical cursors or just call up one of the
many measurement options which
we’ll come to later. The timebase accuracy is ±0.01%.
Pressing the “Horizontal Menu”
button brings up two options at the
bottom of the screen and a number
of options down the righthand side:
Main Only, Intensified, Delayed Only
and then a fourth message such as
(although again, there is an option to
turn that off).
Triggering
There is also a Trigger Menu button
and pressing this gives two broad options of either edge triggering (positive
or negative slope) or video triggering
via the in-built sync separator. This
latter option enables the scope to be
triggered on field 1 or field 2 or the
lines of a composite video signal such
as PAL or NTSC. (Note that the instrument does not have a line selector).
Measurements
There are 21 automatic measurements available with the TDS 320 and
they are brought into play by pressing
the “Measure” button. This brings
up five options along the bottom of
the screen and pressing any of the
accompanying soft buttons brings up
options down the side of the screen.
For example, pressing “Select Measurement” brings up the first four of
the 21 measurements and these can
be paged through to pick the ones you
want. Each measurement option is accompanied by a little diagram which
July 1993 19
Review: Tektronix TDS 320 100MHz digital scope
perform volts and time related measurements using moveable vertical
and horizontal cursors. You can then
measure absolute volts, delta volts,
frequency and time difference.
Performance
A typical screen display from the TDS 320. In this case, one channel is shown
together with measurements of pulse rise time, fall time, & positive & negative
duty cycles. Note that the menu at the side of the screen has been cleared so that
the measurements can be displayed without obscuring the waveform.
shows just what is being measured.
This “pictorial approach” is used
extensively on the TDS 320 and is
very useful even for those who are
very familiar with scope measurements. Seeing the little diagrams
makes the measure
ment selection
quite unambiguous and would be a
boon for anyone not so familiar with
the English language or for students
leaning about scopes.
The instrument also provides
a running commentary about the
measurements. For example, it might
accompany a measurement of rise time
with a “low resolution” comment. This
means that you should select a faster
timebase speed. All 21 measurements
will be accompanied by comments
where applicable and again, this can
be most helpful, even to experienced
users.
One problem that can arise with
on-screen measurements is that they
are superimposed over the waveforms
and this can lead to a lot of clutter. Tektronix has thought of that too. If you
push the “Clear Menu” button, all the
measurement readings are transferred
to the area down the side of the screen.
Result: no clutter.
20 Silicon Chip
One aspect which could be argued
about relates to the selection and
removal of measurements from the
screen. As presented, you can select
up to four of the 21 possible measurements on the screen. If you want to
select an additional measurement, the
TDS 320 flashes up a message which
states that only four are allowed. You
then have to clear that message, push
the “Measure” button again to bring
up the measure menu and then push
“Remove Measurement”. You then
have the option of removing any or
all of the existing four measurements
after which you can select another
measurement.
On other brands of digital scopes,
the measurements are displayed on
the screen in a FIFO (first in, first out)
scheme; ie, the first measurement in
is the first to disappear off screen as
you select more measurements. That
has the beauty of simplicity but it can
mean quite a few button presses to
display the particular set of measurements you want.
Cursors
As well as the automatic measurements noted above, you can also
All of the foregoing has focused
on the user features of the TDS 320
without really mentioning its overall
performance. It really does have quite
remarkable performance with 500
megasample/second sampling rate,
giving a true 100MHz bandwidth
even for “single shot” mode. And the
high rate of sampling means that it is
tops at catching glitches which are
undetectable on other scopes. It can
detect glitches a short as 10 nanoseconds at all timebase settings between
25µs/div and 2.5 seconds/div.
That’s pretty amazing stuff and is
indicative of a level of performance
that was unthinkable in instruments
in this price range before the TDS
320 was released just a couple of
months ago.
Excellent manuals are provided
with the TDS 320 although most
users should seldom need to refer to
them. There is a large spiral bound
instruction manual, a 4-page reference
manual showing the menu maps and
controls, a 72-page spiral bound manual entitled “Basic Concepts” which
would be an excellent source for anyone learning about scopes and finally,
a 3-ring binder programming manual
which allows you to fully exploit the
GPIB and parallel printer interfaces
of the TDS 320 if you purchase that
option.
Perhaps I should make some comments about the optional GPIB and
printer interfaces. As with most other
scope manufacturers, Tektronix makes
the TDS 320 available without any
interfaces but to my mind, buying an
instrument such as this without the
interfaces means that you are not getting the full benefit of the product. We
had only a few days with the review
instrument and it did not have any
interfaces on it as it was an advance
sample.
Another option which could be very
useful is a thermal printer mounted in
a pouch on top of the scope.
Points for improvement
Any complex product such as this
always has facets which could be
improved and, in fact, Tektronix has
Conclusion
To conclude, the Tektronix TDS 320
is high-performance scope which is
deceptively easy to use. In some ways,
its ease of use conceals the power of
the instrument. This is a paradox that
comes about because in the past high
performance instruments of any type
have generally not been easy to use.
We should also emphasise that space
limitations and the brief time we had
the sample scope meant that we have
not been able to cover the full range
of features.
The price of the Tektronix TDS 320
is $4395 plus sales tax, while Option
14 (the GPIB and Centronics printer
interface) is $774 plus tax.
For further information on the TDS
320 and other digital scopes in the
range, contact Tektronix Australia Pty
Ltd, 80 Waterloo Road, North Ryde,
NSW 2113. Phone (02) 888 7066. SC
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a policy of continuous upgrades. With
this in mind, there are some points
which could be improved. Perhaps
the most noticeable is the fan which
is quite noisy. I mentioned this to the
Tektronix sales staff and they assured
me that this aspect would definitely
be improved.
So much for the hardware. All
the other points of note relate to
the software and could probably be
easily modified. For example, when
you select the “Fine Scale” option
for vertical sensitivity, it would be
more logical if the gain was then
con
tinuously varied by the Volts/
Div knob than by the general purpose
knob at the top of the control panel.
A probe menu would be useful too,
so that non-Tektronix probes can be
used. This could be a subset of the
“Coupling” menu.
Finally, when you have selected
an option which involves the general
purpose knob, a knob symbol appears
in the top right
hand corner of the
screen, as already noted. However, as
soon as you touch the knob, the symbol disappears even though you can
still use the knob while ever the same
menu is displayed on screen. In this
reviewer’s opinion, the knob symbol
should remain on screen while ever
the facility is available.
Tektronix has a policy of continuously upgrading the internal software
of their scopes so maybe some or all
of these quibbles will be addressed in
the near future.
July 1993 21
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