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The new digital oscilloscopes
are rapidly changing the way
we think about measuring
equipment. They offer a
stunning range of
measurement options, have
large bright screens and can
be hooked up to a computer
for data acquisition. We
recently took a look at
Yokogawa's model DL1100
and came away impressed.
By LEO SIMPSON
Yokogawa's 2-channel
100MHz digital CRO
okogawa's DLl 100 digital scope
really doesn 't look like any
scope you've used in the past.
It's fairly tall but not very wide or
very deep and it has a bigger than
normal screen. The unit's dimensions
are 204mm wide, 270mm high and
333mm deep. Its overall screen size is
140 x 110mm and the graticule area is
about 110mm wide by about 93mm
deep, which is larger than a conventional scope's screen. It weighs 8kg.
But look at all those buttons. And
only one knob. At first glance, there
are so many buttons that it is a little
off-putting. What do they all do? After you get over the first bit of culture
shock, you realise that quite a few of
Y
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SILICON CHIP
them have labelling which is more or
less self-explanatory. But to initially
display a waveform on the screen you
don't have to fiddle with all the screen
settings. You.just push the Auto Setup
button (just underneath the round
knob).
This brings up an "Auto Exec" legend up in the bottom lefthand corner
of the screen. Pressing the screen button immediately under the legend
then causes the scope to click relays
and things inside and within a few
seconds it brings up a stable display
together with the vertical sensitivity,
probe division ratio and timebase setting. Also displayed is the date and
time.
From there, you can then push the
"V/Div" button which allows you to
use the front panel knob to vary the
vertical sensitivity in the normal 1-25 sequence, (eg, lV/div, 2V/div, 5V/
div, etc). The range of sensitivity is
from 2mV/div up to 5V/div, assuming
a 1:1 probe. If you press the Input
button for either channel, you nominate the probe division ratio (1:1, 10:1
or 100:1) and the correct vertical sensitivity and probe ratio will be displayed in the top righthand corner of
the screen.
Now suppose that you want to vary
the timebase setting. To do so, press
the "Time/div" button and again you
can vary the settings in a 1-2-5 se-
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The amber screen is, as with other
digital scopes, a raster scanned tube
as used in computer monitors. Thus,
the display is made up of very fine
dots. Screen resolution is very high
although this is not covered in the
specifications. By close inspection
with a weaver's pick (a magnifying
glass used by printers to examine print
quality), we were able to determine
the horizontal resolution at 500 dots
(actually 501).
Proportionally then, vertical resolution must be close to 400 dots (actually 401). When related to screen size,
this order of resolution is roughly
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These printouts apply to the same waveform measurement. The tabulated
values on the left are printed out by asking for the measurement conditions. The
waveform on the right shows the fall time (20.7ns) of the square wave available
from the audio oscillator published in the January 1990 issue of SILICON CHIP.
Dot matrix screen
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quence from 10 nanoseconds per division (lOns/div) up to 50 seconds
per division. That last figure is not a
mistake. At the lowest timebase setting, the trace does take 50 seconds
per division and a total of 500 seconds (8 minutes and 20 seconds to
creep across the 10 divisions on the
screen.
This is just one aspect where this
digital scope is dramatically different
from conventional analog scopes. Why
would you want such long sweep
times? Well, why wouldn't you? There
are any number of occasions where it
would be nice to be able to measure
and record long term variations in
signals. Now you can do it and better
still, you can take a permanent record
with the inbuilt printer that sits in the
top of the unit.
'
:
equivalent to a VGA monitor. Suffice
to say though, the display is very fine
and most of the time you are not aware
of the dot structure.
Indeed, you have a bunch of options as to how the display is presented. Pressing the Display button
brings up a menu at the bottom of the
screen, with each menu option corresponding to one of the- six buttons
immediately below the screen. The
function of the softkeys naturally
changes for each menu and each function button that is pressed, so the
number of screen adjustable parameters runs into the many hundreds.
So back to the Display menu: this
has two options for interpolation, LINE
or SINE and the option for dot connection, ON or OFF. In addition, there are
menu selections for Grid Select and
INTensity adjust, which we'll come to
in a moment.
As we have already indicated, the
screen display is made up of dots and
so the channel traces are also dots,
with the number for a waveform cycle
being determined by the sampling rate
(up to 20 megasamples per second)
and the timebase frequency. Most of
the time though , you don't want to
look at a waveform which is just dots
so you choose one of the interpolation options which means "connect
the dots".
The LINE option connects the dots
with short vertical straight lines while
the SINE option uses an algorithm to
connect the dots still with vertical
lines but with a better approximation
to sine waveforms. In practice though,
unless the waveshape is critical and
you have keen eyesight, you won't be
able to tell the difference.
Now to the Grid Select menu; this
gives three possible graticule patterns:
Frame, Grid 1 and Grid 2. Frame just
encloses the screen area with an outline rule. Grid 1 divides the screen up
into vertical columns 11mm wide; ie,
1 division. Grid 2 is the conventional
crosshatch graticule pattern as shown
on all the screen photos accompanying this article.
Next, you have Tick and % Marker
on or off. "Tick" is (are?) the central
vertical and horizontal axes of the
graticule but divided into fifths; ie,
0.2div. "% Marker" are the conventional 0%, 10%, 90% and 100% horizontal rules that you would use when
measuring the rise and fall times of
square waves and pulses.
Finally, the INTensity option allows
you to vary the brightness of the dis- ·
played waveform, the grid and the
readout messages, either separately
or all together.
So you can see from all these functions, which are brought into play
just with the Display button, that there
are literally many hundreds of options you can play around with to
best measure and display the waveforms you are interested in.
On the rear panel of the instrument
are a pair of BNC sockets, one for a
trigger output and the other a TTL
signal used for when the DL 1100 is
configured as a GO/NO GO tester. Also
present are sockets for the IEE-448/
GP-IB (general purpose instrument
APRIL 1992
15
Worst case jitter can be easily captured on the DL 1100
using the Dynamic Accumulate mode. You just select the
period of time for which you want to accumulate data &
the jitter information is compiled on the screen.
bus) and RS-232 interface so that the
unit can be used for data acquisition
or remotely controlled by a computer.
One point about the fan should be
noted. If you are using the scope in a
quiet lab, the fan is noticeable and
can be irritating. On the other hand, if
you have computers, printers and
other fan-cooled gear around, you
probably won't notice the DLl 100 at
all.
Measurements
The "Measure" button brings into
play a lot of options. Pressing the
button brings up the first menu which
gives you options of AUTOmatic or
Manual functions. Pressing the AUTO
softkey then brings up a whole range
of on-screen measurements which are
superimposed directly over the displayed waveforms. This latter point
can be an irritation at times because
you may want to see the complete
waveform and all the relevant readings, without any interference between the two.
You can now use the softkeys to
further select which automatic measurements you want and those to ignore. If you select all the AUTO measurements, you will have displayed
readings for 13 waveform parameters,
including peak-to-peak, maximum
and minimum values, RMS, rise and
fall times and frequency.
If a waveform has a lot of noise or
jitter, you can go to the Acquisition
menu and select for normal, envelope, average smooth or decimate
16
SILICON CHIP
This photo shows a pulse waveform with accompanying
measurements for risetime & overshoot. Note that
sampling has been stopped to get a stable waveform on
the screen.
modes and these will either capture
or eliminate much of the hash, depending on what you want. You can
also use Window and Zoom modes to
extend the timebase by up to 1000
times to display glitches which may
be only 1 nanosecond long. Make no
mistake, this is a very powerful instrument.
Alternatively, you may want to
record and display all the jitter. In
this case, you call up the Accumulate
menu and you can display the jitter
over a range of times.
The Start/Stop button is also a very
useful facility. Say you are displaying
a pulse width modulated waveform
with a lot of motor commutator hash
on it. Such a waveform can be difficult to examine properly because the
hash causes it to jump around a lot,
even if it is properly triggered.
In this case, you have several options. Just pushing the Start/Stop button will freeze the display so you can
get a good look at it. Alternatively,
you might decide to go into short or
long single shot mode to capture a
picture of the waveform and store it
for later examination and reference.
And this is where the exceptional
storage capacity of this digital scope
really impresses. With a 1 millisecond time record, you can print it out
with a resolution of 1 nanosecond.
You should see the length of the printout - it runs for metres and metres.
You can use it to store and examine
long data trains, infrequent glitches,
you name it.
And to go back to that difficult PWM
waveform with motor hash on it, you
can always take the easy way out at
any instant and just take a printout. It
will also print out the scope's settings
(ie, those not already displayed on
the screen readout) such as trigger,
acquisition and sampling mode, sampling rate, data length and so on.
The question of sampling rate has
already been covered to some extent
and it varies according to the measurement conditions. Maximum sampling rate is 20 megasamples/second
(with two simultaneous phenomena).
This results in a maximum storage
(and display) bandwidth of 8MHz for
a single shot display and 100MHz
(-3dB point) for repetitively sampled
displays. The maximum memory size
is 32K words/channel for single shot
mode and 10K words/ channel in normal mode. Compared to competing
brands, this is quite an extensive storage capacity.
Settings saved
When you turn the scope off, it
automatically saves all your control
settings. In this way, if you have a
particular measurement setup, you
don't have to go through it all again
when next you switch on. On the
other hand, having all settings saved
can put a crimp on proceedings if you
weren't the last person to use the instrument. The machine may not be
able to do an "Auto Setup" in these
conditions and so the way out is to
continued on page 55
spite of the above results, I find myself leaning very heavily towards the
concept of a 50Hz controller. The resulting controller will be simpler and
much less expensive than a 2.5kHz
design.
In my experience, these are very
important points and as I have pointed
out above, the controller will spend
most of its time flat out anyhow.
Other approaches
Now let us discuss the ways other
designers have approached the problem. The first example is a simple
2.5kHz controller with no braking.
This controller is very smooth and
quite linear in operation. It has six
FETs which provide ample current
for most applications. A voltage tripler
provides 12.5V at the gates from the
4.8V Rx battery. It is a very nice little
controller.
I also have a circuit of European
origin using the least components I
have ever seen in any controller. One
wonders how well it works. This is an
opto-coupled unit to minimize noise
fed back into the Rx from the motor
drive circuit. It is fitted with a backEMF brake (dynamic braking) and
again one wonders just how well that
brake circuit works.
From bitter experience, I have
learned that the ON resistance of the
transistor across the motor must be
less than 100 milliohms for any braking effect to be achieved, which means
that it must be driven hard. It has no
voltage tripler and the drive voltage
for both the forward and braking FETs
is derived from the motor battery
which is in this case quite adequate,
being in the range of 10-35V. The disadvantage is that as the motor volts
fall, so do the drive and braking
voltages.
Noise is also a bigger problem as
the motor battery is coupled into the
drive electronics and so an optocoupler is almost mandatory. It was obviously designed with model aircraft
usage in mind, as a 7.2Vbattery would
not provide sufficient drive to turn
"From bitter experience, I have learned that
the on resistance of the transistor across the
motor must be less than 100 milliohms for any
braking effect to be achieved, which means
that it must be driven hard".
the FETs hard on. It is typically European in approach, showing concern
over feedback noise but unusual in
using 50Hz.
Another circuit uses 50Hz operation and has several clever features,
including braking. Separate decoders
drive the forward and braking FETs
so that the brake cannot come on
whilst forward is energized and vice
versa. If this did happen, it would
provide a dead short through the braking and forward FETs and destroy the
controller. The circuit also has a voltage tripler which provides heaps of
drive to both sets of FETs.
This unit has been designed specifically for cars and uses a battery
eliminator. The problem with battery
eliminators is that the Rx runs off the
motor drive batteries which eventu-
Yokogawa DL1100 Oscilloscope - continuedfromp.16
press the "Initialize" button. This
brings up an "Initial Exec" message
on the screen, prompting you to press
one of the softkeys (by the way, they're
called "softkeys" because their function changes with each new screen
menu).
You might wonder why you have to
press two keys to initialise the scope
when it would be easier to press one.
The same comment could go for the
Auto Setup routine . And for that matter, you might ask why the machine
could not initialise itself automatically at switch on.
The scope could undoubtedly have
ally go flat and thus all control is lost
- not good in an aircraft. This type of
Rx supply must also be filtered very
carefully if motor noise is to be kept
out of the circuit.
There are also reversing controllers
but these have a fundamental problem. The drive motor is included in a
bridge circuit (similar to the Rail power
controller featured in this month's issue) and thus there is double the volt-
been made to automatically initialise
itself at switch on but then there would
not have been the convenience of having the last used settings saved. And
the idea of making you press a soft
key after pressing a front panel button
stops you from accidentally wiping
out existing settings . If you do press
the wrong button and it brings up a
screen menu that you don't want, all
you do is press "Menu Off" and that
clears it. Pressing it again brings the
last menu back.
From the foregoing it should be clear
that the Yokogawa DLl 100 2 channel
100MHz digital oscilloscope is a
age drop across the FETs as there is
always one set of FETs on either side
of the motor. For this reason, reversing controllers are not popular with
the speed fraternity. They are, however, a must where total control over
the model is called for.
The final design
Note that none of these circuits has
all of the features considered desirable by the modern modelling fraternity so there is plenty of scope for
new designs. Drawing from the above ,
our proposed design is a now a little
firmer in that it will use 50Hz switching, dynamic braking, drive electronics working from the Rx battery, a
free-running voltage tripler and, as a
result of this battery isolation, no optocouplers.
SC
highly flexible and powerful instrument. It takes some time to become
familiar with all its features and use
them to the fullest. We had only a few
days with it but in that time we have
been very impressed. It is a fine instrument.
The DL 1100 is priced at $4900
which includes the GP-IB interface,
while the optional built-in thermal
printer is an additional $750, as is the
RS232 interface. These prices do not
include sales tax. For further information, contact Tony Richardson at
Yokogawa Australia Pty Ltd,
Centrecourt D3, 25-27 Paul Street
North, North Ryde, NSW 21_13. Phone
(02) 805 0699.
SC
APRIL 1992
55
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