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TEST EQUIPMENT REVIEW
The Tektronix THS720 is
a lightweight, portable
instrument which
combines a powerful
2-channel 100MHz
oscilloscope and a
4000-count digital
multimeter in one
convenient package. It has
a liquid crystal display
and is very easy to drive.
By RICK WALTERS
The Tektronix
THS720 TekScope
This go-anywhere measuring system operates continuously from the
mains via a plugpack or for around
two hours from an inbuilt battery. A
spare 2.8 ampere-hour battery and a
battery charger which is capable of
recharging the battery in 1.5 hours are
also supplied. The TekScope, as well
as the charger, will operate from the
mains or from a 12V cigarette lighter
socket, which is an excellent idea for
a portable instrument.
The number of different functions
packed into this tiny package, which
measures 180 x 220 x 50mm (W x H
x D) and weighs only 1.5kg, is quite
amazing.
The major features of the oscilloscope are:
82 Silicon Chip
•
100MHz bandwidth with 20MHz
selectable;
• 500MS/s sample rate and 2500-point
record length;
• Separate digitisers for each channel;
• Waveform averaging and enveloping
with hardware peak detection;
• Digital Real Time digitising (up to
5X oversampling), sin(x)/x interpolation and peak detect acquisition to
limit the possibility of aliasing;
• Independently isolated input channels measuring up to 1000V RMS and
floating at 600V RMS (using P5102
probes);
• Cursors and up to 21 measurements
continuously updated;
• Simultaneous oscilloscope and me-
ter operation on the same or different
signals; and
• Pulse and video triggering capability.
The DMM features are:
• True RMS AC volts, DC volts, Ohms,
continuity and diode check functions;
• Auto or manual ranging;
• Data logger plot of meter measurements;
• Maximum, minimum, delta maxmin, relative delta and average statistics selectable;
• Bargraph indication for “analog
meter” style indication;
• Input can float to 600V RMS;
• Input over-voltage indicator.
It is apparent that great amount of
design effort has gone into this unit
as exemplified by the case support
assembly (see Fig.1). It normally lies
flat against the back of the case but
can be flipped out with a finger. By
swinging it up, the TekScope can be
hung on a hook.
If the hinged centre piece is pushed
out, it can be used to hang the unit on
the rung of a ladder or, if this centre
piece is clipped into a slot on the rear
of the case, it can support the TekScope
for bench use.
Pressing the on/off button, located
at the bottom left of the front panel,
causes the THS720 to do a self-check
of all functions. After a few seconds,
the message “Power-On self check.
PASSED . Press the CLEAR MENU
button” appears on the screen. When
the menu is cleared the instrument is
ready for use.
The unit initialises to the same
mode and settings as when it was
turned off. Pressing the SCOPE or METER button will select that function.
Let’s look at the scope features first.
An internal 1.2kHz square wave
output (its connections are located
under a flap on the righthand side of
the meter) is provided to allow you
to properly compensate the input
probes. This waveform is shown in
Fig.2 with the VERTICAL MENU displayed across the bottom. Pressing the
button below the menu item pops all
the options up.
Another useful feature is the AUTORANGE button. When any waveform
is fed to the TekScope, instead of
having to adjust the vertical sensitivity, the horizontal sweep rate and the
synchronising trigger point, a press of
the autorange button will usually give
you a rock steady trace at a suitable
amplitude.
As previously explained, the
changeover from SCOPE to multimeter
is just a matter of pressing the METER
button. The meter can be connected to
one circuit and the scope to another
and you can readily switch between
them. What is even more useful is the
ability to display the SCOPE function
on a full screen and (for instance) a DC
voltage which appears in the top right
corner next to the meter face.
As a practical example, you could
monitor the output fre
quency of a
VCO (voltage controlled oscillator)
Fig.1: a built-in
tilt stand folds out
and snaps into
place when not in
use. For benchtop
operation the tilt
stand locks in place
with the hinged flap.
It is hinged up to
hang from a nail or
extended to hang on
the rung of a ladder.
using the SCOPE probe and display
the waveform, with the frequency
readout on the righthand side of the
display. The VCO input voltage can
then be monitored using the DMM and
displayed at the top righthand side of
the readout (see Fig.3).
As mentioned in the specifications,
the meter circuitry and the SCOPE
inputs are each isolated from ground.
This allows you to monitor, for example, the mains voltage with the
DMM, while the SCOPE probe could
be looking at the ripple on a 5V supply
referenced to ground.
Controls
While the AUTORANGE button is
useful in obtaining a stable display,
it may not be quite what you require.
The auto setup can be overridden by
pressing the VOLTS/DIV, SEC/DIV or
TRIGGER LEVEL controls.
The VERTICAL controls consist of
a sensitivity rocker switch, a position rocker and a menu selector. The
VOLTS/DIV switch has a large sine
wave at the top and a smaller one at
the bottom. Pressing the top increases
the displayed amplitude; pressing the
bottom reduces it.
Similarly, pressing the top of the
POSITION rocker moves the trace
upwards while pressing the bottom
moves it down; all quite logical and
intuitive.
As you would expect, provision is
made for both channels, the stored
references and the Math display to
be turned on or off. Display selection
is effected by pressing the required
button. The display is turned off by
selecting it, then pressing the WAVEFORM OFF button.
The MENU button produces screens
similar to Windows “drop down”
menus, with the menu screen options
depending on the waveform selected.
For the input channels (CH1 and
CH2), you have the choice of selecting
the input coupling method (AC, DC
or ground), inverting the waveform,
setting the bandwidth and selecting
the type of probe (voltage or current)
as shown in Fig.2.
Using the Math function, you can
add both channels, subtract CH2
from CH1, subtract CH1 from CH2,
or multiply the two inputs. If one
channel was measuring voltage and
the other current, then the resultant
waveform would represent the power
being dissipated in the component
being measured. If either reference
channel (A or B) is selected, you have
the option to save CH1, CH2 or math
July 1996 83
Fig.2: this is the 1.2kHz waveform used to calibrate the
10:1 voltage probes. The Vertical MENU button has been
pressed to show the choices available. The previous menu
selection was the probe type and this is shown in inverse
lettering.
Fig.3: the VCO output waveform and frequency are shown
on the display, the input voltage to the VCO is shown next
to the meter.
Fig.4: a plot of the output of a regulated power supply
over a 4-minute period. The trace moves from right to
left, so the small negative spike that appears around 2.8
minutes actually happened 1.2 minutes after we started
the test. The readout on the RHS shows that the voltage
dipped to 26.44V at this time. The plateau which begins
at 2.6 minutes was actually 28.48V. The slight difference
in the two average figures is probably due to the fact that
the run was stopped before the full four minutes and the
figures were updated at different times.
Fig.5: a one Farad capacitor was charged to 3.96 volts and
discharged through a 1kΩ resistor. This time constant is
1000 seconds, or 16.66 minutes. As near as I can calculate
from an enlarged graph, this shows a time constant of
16.25 minutes. This indicates that the capacitor was about
2.5% below its nominal value.
waveform to a location from 1-10; ie,
60 locations in all.
The HORIZONTAL controls operate
in a similar manner to the VERTICAL
controls although instead of rocking
vertically they rock horizontally.
Pressing the right or left POSITION
arrow moves the trace in that direction.
The right side of the SEC/DIV button
with the expanded sinewave expands
the displayed waveform, while the
left side with the compressed sinewave increases the number of cycles
displayed on the screen. The MAG
button expands the display by a factor of 10.
The MENU button allows you to se84 Silicon Chip
lect the main or delayed timebase and
set the delay time. The main timebase
trigger has three preset positions at 10,
50 or 90% of the waveform period or
by using the TOGGLE button any period from 0-100% can be selected. The
delayed timebase can be set to start
any time from 2 nanoseconds to 50
seconds after the main sweep.
Trigger controls
The trigger controls, while they
appear simple, have a vast range of
options. The TRIGGER LEVEL toggle
moves the trigger point up and down
the waveform from 100% to 0%. The
SET LEVEL TO 50% button does just
that. All the other functions are accessed via the MENU button.
There are three trigger types you can
select from: Edge, Pulse and Video, all
of which can be from the CH1 or CH2
waveform. For edge triggering you can
select DC, HF reject, LF reject or noise
reject (DC low sensitivity) coupling.
The slope of the trigger can be selected for a positive going or a negative
going edge.
For pulses, you can select either
the positive going or the negative
going edge, as well as setting a pulse
width with the TOGGLE rocker. Once
this width is set, you can then elect
to trigger when the incoming pulse is
less than the set width, greater than
the set width, equal to the set width
(with a tolerance) or not equal to the
set width (with a tolerance).
For the video mode, you can select
any field, field 1, field 2 or lines. You
can also select scan rates between
15kHz and 65kHz with the TOGGLE
button. With all these options it is hard
to imagine a waveform that could not
be triggered.
While the controls we have covered
so far exist in some form on all analog
scopes, the following are mostly peculiar to the newer digital scopes:
The DISPLAY button lets you set the
screen display for dots or continuous
lines, set the contrast, turn the “T”
(which indicates the trigger point) on
and off, and show an XY or YT format
on the screen. It also allows you to
have a full graticule, a grid, crosshairs
or a frame for the display.
The CURSOR menu lets you move
vertical and horizontal reference lines
around on the waveform and these
can be used for the system to calculate
things like propagation delay.
Perhaps the most comprehensive of
all the menus is the MEAS(ure) menu.
This contains 21 different definitions
for the measurement of the displayed
waveform. These include, peak-topeak amplitude, true RMS over the
first cycle, frequency, duty cycle and
true RMS over the entire waveform.
If you need to carry out a particular
measurement on a routine basis, you
can set up the TekScope to make the
measurement and then save the setup
in one of 10 non-volatile memory
locations. This setup may be recalled
at any time using the location number
that it was saved in.
If you have a suitable printer, the
screen display can be sent to it via the
supplied RS232 cable when ever the
HARD COPY button is pressed. Naturally any previously stored waveforms
can be recalled and printed in this
manner. The printers supported are
Thinkjet, Deskjet, Laserjet and Epson
9 & 24-pin.
Using the Windows terminal program, the screen display or stored
waveforms can be transferred to an
IBM compatible PC. The formats supported are IMG, TIFF, PCX, BMP, EPS
and DPU411/412
So much for the oscilloscope; let’s
now look at the digital multimeter
specifications. First, the five DC voltage ranges cover from 400mV to 880V
with an accuracy of ±(0.5% reading
+ 5 counts), while the five true RMS
AC voltage ranges cover from 400mV
Above: BNC sockets are provided at the top of the TekScope for the two
oscilloscope inputs. These inputs are completely isolated from each other and
can be connected to sources at different potentials. The multimeter jack sockets
(see photo below) are mounted at the side of the instrument, together with the
serial input connector and probe compensation signal output (under the flap).
to 640V, with an accuracy of ±(2.0%
reading + 5 counts).
The six resistance ranges are 400Ω
to 40MΩ with an accuracy of ±(0.5%
reading + 2 counts), except on the
40MΩ range where it is ±(2.0% reading
+ 5 counts).
The DMM also features a diode test
function and a continuity tester which
emits a tone when the measured resistance is below 50Ω.
Once you have become familiar with
the SCOPE functions, the meter operation will be a breeze. When you press
the METER button, the five functions
listed above are available for selection.
By pressing DC and AUTORANGE the
meter will display the voltage. The
same selection procedure is applicable
to AC and Ohms.
A vertical analog bargraph (with
solid bars), situated at the righthand
side of the display, moves up and
down in sympathy with the signal
level, with open bars to indicate the
maximum and minimum values recorded. The maximum open bar can
be seen in Fig.4; the minimum open
bar is directly below it and is filled in
by the bargraph.
The data logger function could be
a very useful feature for many users.
It records the meter measurement
over a period of time just like a chart
recorder. This period can be set from
four minutes to eight days.
Two uses that immediately spring to
mind are monitoring variations in the
mains voltage and checking the stability of DC power supplies. To this end
we set up a power supply on the bench
and monitored its output voltage for
four minutes. The result can be seen
in Fig.4. The MEAS(ure) menu was set
to store the MAX, AVERAGE and MIN
voltage over that time.
From this graph, you can see where
the voltage dipped to the minimum
value of 26.44V at about 2¾ minutes.
The maximum voltage plateau is from
1-1½ minutes.
continued on page 93
July 1996 85
ASK SILICON CHIP
Got a technical problem? Can’t understand a piece of jargon or some technical principle? Drop us a line
and we’ll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097.
Battery capacity
meter wanted
I have been a regular reader of your
magazine for the past 4-5 years and one
of my interests involve model aircraft
which I regularly fly. As a consequence
of this, I have a vested interest in the
proper care of nicad batteries which
are used in the transmitter and receiver
packs.
To avoid memory effect, I regularly
discharge these packs using a nicad
discharger I have built using a circuit
from a past issue of your magazine.
This works well but it does have some
drawbacks.
What I would like is an indication of
the batteries’ capacity. This would involve a timer in the circuit with a readout display in minutes, showing the
time it took the batteries to discharge
to the endpoint voltage. A lot of the
commercially available nicad chargers/cyclers which are dedicated to
modelling have this feature. It would
also be nice to have variable discharge
currents to suit different capacity
batteries; eg, 500mA, 750mA and
1000mA.
Does SILICON CHIP plan to publish
such a circuit? I am sure that the
discharger circuit mentioned above
could be modified and expanded to
include a timer and variable current
discharge circuit. Of course, it would
need to have its own supply to power
the display when the batteries have
discharged. (J. C., Western Gardens,
Vic).
• We do not have any immediate plans
to produce a discharger or cycler with
a timer. However, we have published
your letter in order to gauge reader
interest in the concept. For your reference we did publish a “discharge
pacer” for electric vehicle batteries
(lead acid) in the July 1991 issue. This
was a fairly complex instrument which
indicated the percentage ampere-hour
capacity remaining in a rechargeable
battery as it was discharged.
would be a considerable development
time. Commercial units are complex
and involve a variable frequency output to give control over speed. Older
speed controls used cyclo-inverters
employing SCRs.
Howev
er, just recently an IC has
been released which we believe con
tains most of the control circuitry
needed for a variable frequency, variable voltage speed control. We will
investigate this chip and see if it can
be the basis of a speed control suitable for publication in SILICON CHIP.
But we are not promising anything at
this stage.
Variable speed for
induction motors
Leak amplifier
circuit wanted
Is there a circuit available for
controlling the speed of induction
motors? I have a wood lathe with a
2hp motor, and speed control without
the laborious rearranging of belts and
pulleys would be of great benefit. I’m
sure many other machines could also
benefit from easier speed control.
I believe commercial units are available but at around $1400 to $1700 the
control unit often exceeds the price
of the complete machine it is meant
to control. (W. S., Hallett Cove, SA.)
• We have not published a suitable
circuit and up until the last week or
so, we were not likely to since there
I am currently restoring a Leak Delta
70 amplifier. I think the HT rail may
be too high (75V). The preamp boards
are drawing too much current. I would
appreciate it if someone would send
me a circuit diagram with these voltages, etc. I will reimburse them for
their trouble.
As an aside, this is one for the Serviceman. I was given an Ibanez effects
unit from a large club in Sydney.
No-one could get it to work properly.
After much measuring of resistors,
capacitors, etc, I took it out in the sunlight and found what looked like a fine
piece of wire under the board lacquer.
TekScope Review .
Encouraged by this, we then
charged a one Farad capacitor to
3.96V and discharged it with a 1kΩ
resistor. The voltage was chosen
to ensure that the DVM would not
switch ranges as the capacitor discharged.
This graph can be seen in Fig.5.
The time constant for this combination is 1000 seconds; ie, the voltage
across the capacitor should drop to
37% of the initial value (1.46V) in
. . continued from p85
1000 seconds or 16.67 minutes. Similar recording functions are available
on the other ranges.
By now you must be wondering
how much this great little instrument is going to set you back. The
prices are as follows: THS710
(60MHz), $3195 + sales tax; THS720
(100MHz), $3795 + sales tax. These
prices include the TekScope, carry
case, two CRO probes, multimeter
leads, two batteries, the charger, and
RS-232 and power cables.
After using the TekScope for a
week I probably still have not explored all its capabilities and will
be very reluctant to hand it back to
Tektronix. I doubt if the vast majority of users would need, or even
want, any additional features to be
included.
I shall have to start working
on the boss to buy me one for my
SC
workbench.
July 1996 93
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