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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 14 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- ~d -FEB- 199? ~Q:~7 T ICHJ =5v--~ DL1100 CONDITI ON REPORT Ver. 1. 00- 20H 10n s/ d AC P*lO : \ 2. TI ME/ di 11 I 10ns / d i vl SAMPLE RATE 3. TRIGGER 4. ACQUI SITION 5. SAMPLING 6. DATA l ENG TH +++ [IORMAL] ~MS ~~ InGc u r.: ?V RSHT: [REPEAT ' CE3Rl 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 ~ 17. 0V 1~- ~ ~ 8V - 1.0V 1 ., s: ]¼ '' :' : R!St, 20. 7n~ FAL L rD~ A --PER l KI D ------ -- ~~!R 1m ------ ------- - - '' ' : ' '' - 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 \ 10Msei P: -P MAX M JIM 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