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Equipment Review Tektronix TDS 544A colour oscilloscope Tektronix has long been regarded as one of the leaders in oscilloscope technology & it has confirmed its position with the release of the model 544A & 644A digitising oscilloscopes. We recently reviewed the 544A, a 4-channel model with 1 Gigasam­ple/second sampling rate, 500MHz bandwidth & a colour screen. By LEO SIMPSON New developments in digital scopes continue to come thick and fast but with the release of these colour scopes, Tektronix has changed the whole ball game. Just as colour has made a huge difference to the way in which we use computers then so it will be with oscilloscopes, particularly multi-channel models which display so much information on the screen. That really sums up the reason for having colour. If you are using a scope mainly just to display one or two chan­nels and you don’t use a lot of the on-screen measurement cap­ability of a modern digital scope, then you probably don’t need colour. But if you are displaying two or more channels plus a lot of on-screen information and perhaps even with FFT (spectrum analysis), then colour can make a world of difference. Consider for a moment the situation if you are displaying two channels on a typical CRT readout scope. As well as the traces themselves, the scope will usually display the vertical attenuator settings for both channels, timebase settings (main & delayed) and possibly also the trigger conditions. Possibly you will also have horizontal and vertical cursors and that usually implies voltage or time measurements too. And if you select other measurements as well, the screen can end up being a mass of confusion, particularly if some of the digital information is over-writing the traces. In a normal scope, the only way to reduce the confusion is to get rid of the digital on-screen information but with the Tektronix colour scopes you don’t have to. Not only is each channel trace displayed in a different colour but the digital information relevant to each channel is displayed in the same colour as the relevant trace. This makes an enormous difference in interpreting what is going on. Colour also lets you overlap traces and still be easily able to distinguish between them. This can be really helpful when you have pulse waveforms that are almost impossible to distinguish when the traces overlap. How the colour is added The Tektronix TDA 544A is a 4-channel 500MHz oscilloscope with a 1 Gigasample/second sampling rate. The addition of the colour display makes a dramatic difference to the way in which informa­tion can be shown on the screen. 8  Silicon Chip While the addition of colour to an oscilloscope may seem a radical enough feature in itself, the way in which it has been incorporated to these new scopes is even more radical. If you have read any of our reviews of the new digital scopes in Fig.1: this is a screen showing the Snapshot – demonstrating all the automatic measurements possible except for phase & delay (with respect to another channel). These measurements apply to the 48kHz sine waveform shown in Fig.2. the last two years or so, you will already know that most of these do not use a conventional cathode ray tube with electrostatic de­flection and PDA (post deflection acceleration) for fast writing speeds. Nor do digital storage scopes use expensive storage CRTs. Instead, all the analog signals fed to the channel inputs are sampled and converted to digital values. After that they are converted to be displayed on a raster-scanned CRT (cathode ray tube) in exactly the same way as on a computer monitor. In fact, some digital scopes can be connected to a VGA computer monitor to take advantage of a larger screen size. So you could be forgiven for thinking that Tektronix has incorporated colour into these new scopes by employing the equiv­alent of a VGA colour screen and whatever electronics are required to drive it. But you would be wrong. The Tektronix 544A (and 644A) does in fact employ a monochrome raster scanned CRT but the colour is added by a liquid crystal shutter in front of the screen. The CRT provides the video or luminance information while the LC shutter provides the colour. This is quite a differ­ent approach to that used by, say, LCD video projectors such as the Sanyo PLC-200P reviewed in the March 1993 issue of SILICON CHIP. Those units use a metal halide projector lamp, dichroic mirrors and three LCD panels to provide the red, green and blue pixel information. Fig.2: this is the waveform referred to in Fig.1 but shown with variable persistence. This is depicted as a spectral colour display with red showing the most frequently occurring parts of the waveform. Other colour persistence palettes are available, including greyscale. In these new scopes, they use the Tektronix patented Nucolor liquid crystal shutter. The shutter is an electrically switchable colour filter made up of two fast liquid crystal optical switches known as “pi cells” plus a combination of colour and neutral polarising filters. A colour screen is produced by having the CRT sequentially produce the red, blue and green video information on the screen while the LC shutter is switched to transmit red, blue and green respectively. Alternate video fields, viewed through the switched coloured filters, thereby create full colour images with a maximum of 256 colours. In more detail, a video frame for the Tektronix LC shutter has three fields – red, green and blue. The frame rate is 60Hz while the field rate is 180Hz. The horizontal scan rate is 91kHz. LC shutter advantages The advantages over a conventional triad or vertical slot shadow mask CRT include higher screen resolution, much greater contrast (up to 100:1), no convergence or purity problems and high colour saturation. In addition, the system is more rugged than a shadow mask tube. Of course, this is not the first time that liquid crystal shutters have been used to produced different coloured traces on a scope (Tektronix did it several years ago) but the Nucolour system is greatly refined and produces a much higher contrast than was achieved previously. If colour was the only new feature of the TDS 544A it would be most worthy of review but this scope is loaded with features that will make other scope manufacturers sit up and take notice and these are in addition to the awesome sampling rate of 1 gigasample/ second or the resulting bandwidth of 500MHz. It is a full 4-channel scope with sensitivity adjustment avail­able on each channel, from 1mV/div to 10V/div (or 10mV to 100V/div with 10:1 probes). Vertical sensitivity can be adjusted in the usual 1,2,5,10 sequence or continuously, using the Fine­scale softkey. The timebase is impressive, variable from 10 seconds/div to 500 picoseconds/div. That is a range of 2 to 1010! Vertical accuracy is quoted as ±1% while timebase accuracy is an in­ credible ±.0025%. While some recent digital scopes have tended to be smaller and lighter than their analog cousins, this new model from Tektro­nix is fairly bulky and heavy. Its overall dimensions are 420mm wide, 195mm high and 415mm deep. It weighs about 12.3kg, depend­ing on options. The screen size is 140 x 115mm, although the active screen is somewhat smaller than this. The scope has a very large fan on the side of the case and yes, it is fairly noisy although it is hard to see how that can be reduced. After all, the case is absolutely chocka-block with electronics. User interface One very attractive feature is the November 1993  9 This photo shows the Tektronix TDA 544A scope connected to a VGA monitor. While the reproduction may not fully show the dif­ference, the scope display is much sharper & has much better contrast. user interface, the system of menus and softkeys which make a complex instrument such as this easy to use. Without the system of softkeys (12 keys, below and to the left of the screen) it would have been impossi­ble to provide all the functions which are available. Tektronix has improved on the system which is used in the TDS 320 model (reviewed in the July 1993 issue of SILICON CHIP) by adding help screens for just about every function. These are displayed (white text on a blue screen) in much the same way as the help screens for the better software packages. So if you’re lost in the labyrinth of automatic test func­tions, just hit the HELP button followed by the function button you’re about to use and the screen pops up with an explanation. What a revelation! Triggering from everything Also very fancy is the selection of triggering functions you can have, particularly as far as video waveforms are con­cerned. The basic trigger choice is between edge triggering, logic triggering, pulse triggering or video triggering and as you might expect with an instrument of this calibre you can trigger on video line in a frame. Video formats supported are PAL, NTSC, SECAM and HDTV (including Japanese, US and European formats) but if you want 10  Silicon Chip something else such as CGA, VGA or something more exotic you just select FLEXFORMAT with one of the softkeys at the bottom of the screen. You can then program in the parameters of the video format you want: sync pulses, frame rate, number of fields, number of lines and so on. Thus, the TDS 544A can cover any video format, even those that have yet to be thought of. You can also trigger off any line in a video frame, using the numeric keypad or the select knob. Nor is the comprehensive video triggering necessarily the highlight of the seemingly dozens of triggering options. If you select pulse triggering, for example, you then decide to nominate the width of pulses to trigger on or ignore and you can also select glitch or “runt” triggering. Many readers will be familiar with glitch triggering and the TDA 544A can be programmed to specify the width and polarity (negative, positive or both) to accept or reject. The TDA 544A can trigger on glitches as short as two nanoseconds. Runt triggering So what is “runt” triggering? A runt is a pulse which is not up to scratch. Say you have pulse train in a circuit with an amplitude of 6V but every now and again the circuit fails to operate properly. You suspect it may be due to a pulse of insuf­ficient amplitude but with an ordinary scope that is all you can do – have your suspicions. With the TDA 544A, you can program it to look for the runt! You do this by programming in the thresholds which can be positive or negative. How can runts occur? One possibility is from an AND gate where two or more inputs change simultaneously. Finally, the TDA 544A has logic triggering whereby it can trigger on logic state (high or low, or for logic conditions which you define). For example, you could select an AND condition for the four input channels and the scope would then trigger on a true or false condition, again selected by you. You can also select for OR, NOR and NAND conditions. Measurement options As with many other digital scopes these days, the TDA 544A provides for a wealth of automatic measurement functions which can be brought into play by pressing the softkeys. Parameters such as frequency, period, risetime, fall time, duty cycle and so on are routine. All you need is a stable waveform and the relev­ant part of the waveform displayed. For example, the scope will not reliably measure frequency unless you have at least one cycle of the waveform displayed. And if it cannot measure the parameter reliably, it will tell you. But with so many measurement possibilities it can be a real pain trying to select the measurement you want, remembering that you can do these measurements for any or all of the four chan­nels. Tektronix has thought of that and by pressing the Snapshot softkey you can bring up all measurements which are possible for a channel, except for delay and phase. The screen shot of Fig.1 demonstrates this together with the relevant waveform in Fig.2. Variable persistence Among the many options for display is one called “variable persistence”. This is used to accumulate waveform dots which appear and disappear over time according to a decay constant which you can select. This can be useful for displaying the way in which a waveform varies over time. However, in this case the colour of the waveform varies depending on its frequency of occurrence. To explain this further, a typical sine waveform with superimposed noise will have a statistical mean waveform exactly corresponding to a sinewave but with deviations due to the noise. In effect, with variable persistence the waveform will “thicken up” due to the noise. However, on the TDA 544A the variable persistence is portrayed as a variation in colour from the most frequently occurring parts of the waveform to those that seldom appear. Thus, depending on the persistence time, you can readily see the effects of random noise, glitches and so on. Nor do you have to settle for one type of colour for variable persistence; you can have three colour palettes. The first of these is “Temperature” whereby the most often occurring waveform is in red ranging down to blue for the least. Or you can have a “Spectral” palette, whereby violet portrays the most common parts of the waveform ranging to red for the least. Or the third pos­ sibility is “Gray Scale” with white for the most down to light grey for the least. Waveforms displayed in variable persistence mode cannot be saved, as one of the Help screens points out, but you can save a printout – see Fig.2. Output options Today’s high end digital scopes cannot be regarded as com­plete unless they have comprehensive facilities for hard copy of waveforms and the ability to be part of a data acquisition sys­tem. To this end, the TDA 544A has a Centronics parallel inter­face (via DB25 socket), serial port, GPIB port and a socket for connection of a VGA colour monitor, to let you take advantage of a large colour screen. The latter is really good in teaching situations although the waveform resolution is not as good as from the scope itself, as you would expect. The TDA 544A also has its own floppy disc drive which you can use for waveform capture or printouts. It is a standard 3.5-inch 1.44MB drive with IBM DOS formatting. It means you can store waveforms for subsequent display on the scope or you can take the data and incorporate it into reports. That is what has been done for the screen shots in this article. The screens have been captured as EPS (encapsulated PostScript®) files and then taken straight into PageMaker® for the page composition. However, there are a wealth of other print formats that can be used including HPGL, TIFF, BMP, PCX and so on. You can also use a range of inkjet, laser, thermal and dot matrix printers. Tektronix Fig.3: a 48kHz sinewave displayed in Hi Res mode whereby the sampling rate is greatly increased to improve display resolution. Note the measurement menu at the right of the screen. Fig.4: this is the TDA 554A’s 1kHz calibration waveform depicted in Hi Res mode & showing one of the triggering menus. Note that triggering can be edge, logic, pulse or video. can even provide for colour printouts with one of their colour print­ers. Reviewing a complex product such as this really does place us in a quandary. No matter how long the review is, there are many features which will either be glossed over or omitted alto­gether. So what we are presenting is really just a brief review. We have not said anything about the FFT feature, programming and the very extensive programming manual. A typical demonstration by one of Tektronix’ sales engineers will take several hours and again, the demo will not show every feature. However, no matter how you look at it, the TDA 544A is a very impressive product which is at the leading edge of technolo­gy. None of this comes cheap of course and nor would you expect it to. Prices range up from around $15,000, depending on the op­tions fitted and supplied. The warranty is three years. For further information on the TDA 544A, contact SC Tektronix Australia Pty Ltd on (008) 023 342. November 1993  11