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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 SILICON CHIP BINDERS BUY A SUBSCRIPTION & GET A DISCOUNT ON THE BINDER (Aust. Only) These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers and are made from a dis­ tinctive 2-tone green vinyl that will look great on your bookshelf. ★ High quality. ★ Hold up to 14 issues (12 issues plus catalogs) ★ 80mm internal width. ★ SILICON CHIP logo printed in gold-coloured lettering on the    spine & cover. Yes! Please send me ________ SILICON CHIP binder(s) at $A14.95 each (incl. postage in Australia). NZ & PNG orders please add $5 each for postage. Not available elsewhere. Enclosed is my cheque/money order for $­__________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ Master Card Card No. Signature­­­­­­­­­­­­_________________________ Card expiry date______/______ Name _____________________________________________________ Street _____________________________________________________ Suburb/town __________________________ Postcode______________ SILICON CHIP PUBLICATIONS PO Box 139, Collaroy, NSW 2097, Australia. Phone (02) 979 5644 Fax: (02) 979 6503. ✂ 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