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Road testing the HP54601A 100MHz oscilloscope In the April 1991 issue, we previewed the new Hewlett-Packard 54600 series 100MHz oscilloscopes. We were quite impressed but first impressions are not enough to judge a new product like this. So now we present a more detailed review. By LEO SIMPSON Flying to Melbourne to see the latest oscilloscope from HP is all very well but to really come to grips with an instrument like this you need to spend many hours with it on the bench, using it in a wide variety of situations. It is only then that you can make a realistic assessment. Having had a model 54601A 4 channel unit on trial for a couple of weeks we can now present some conclusions. First, it is, as we thought, a dramatic improvement on all previous digital sampling scopes. Second, it is easier to use than just about any equivalent analog oscilloscope, with or without CRT readout. And third, it is the first of a new generation of oscilloscopes which will eventually make all but the most basic analog scopes obsolete and uneconomic. Having said all that, there are some applications where the analog oscilloscope still does a better job - we'll mention those later. First impressions of the 54601A are that it is compact, relatively light and essentially not much different from an analog scope apart from its large screen and buttons below the screen. Not including the handle and accessory case on top, it measures 317mm wide , 175mm high and 305mm deep. Weight is 6.4kg. The all plastic handle makes it very easy and comfortable to carry and it can be set up as a tilting bail. One clue that this is not a normal scope is the size of the screen which is quite large at about 130mm wide and 105mm deep. Active display area, not counting the space taken up by alphanumeric information at the top and bottom of the screen, is about 125 x 83mm. ' Compare that with a typical scope with a 105mm by 85mm screen (ie, 10 x 8cm) and an overall case depth of over 40cm. With the larger screen, you'd expect the HP unit to have a depth of about 50cm. The reason why it doesn't is that the HP does not use a This is the split screen mode for the HP 54601A. The upper trace runs with the normal timebase (tµs/div) while the lower trace is the delayed timebase (10ns/div). In this case, the pulse rise time is shown as 29.47 nanoseconds. 8 SILICON CHIP This waveform shows that the 54601A can display waveforms which are ostensibly well above its useable bandwidth. In this case, the signal is around 184MHz. Note that the display is clean (apart from the dots), bright and well spread out with a timebase setting of 2ns/div. On most analog scopes with equivalent bandwidth, this waveform would be impossible. At the high end, it runs to 2ns/div (10 times faster) while at the low end it creeps across at 5 seconds/division (25 times slower). At the very lowest speed, it takes 50 seconds for a waveform to be retraced across the screen. That's really slow but it gives an absolutely steady waveform, as you would expect from a raster scanned display. There is another big advantage from using a video monitor style tube and that comes about in terms of cost, life and reliability. Conventional analog scope tubes are very costly, especially those with PDA (post deflection acceleration) and they require expensive mu-metal shielding. Worse still, they may only last a few thousand hours before needing to be replaced. Against that, a cheap and readily available video monitor tube stacks up pretty well. Quiet fan While relatively unimportant when related to the high technology in this unit, we must comment on the fan - it is quiet. This is a nice change from the fans on many oscilloscopes (and computers) which are often tiny units which make an unseemly racket. Another nice feature is the soft vinyl bag on top of the unit for storage of accessories such as the probes. While HP and other upmarket scopes have had this as a feature for quite a few years, it is worth noting that it is very convenient. It certainly helps stop probes and the user manual from going missing. Driving it Amplitude modulated waveforms are easily displayed on the 54601A. This display shows a waveform with a frequency of about 65MHz and 400Hz modulation. normal scope tube. Instead, its tube is exactly the same as would be found in a small green screen computer monitor. It is not vector scanned like a scope (ie, with the waveforms exactly traced out by the electron beam) but is raster scanned like a conventional TV or computer video monitor. This means that the display is made up of very fine dots, (500 x 255 pixels). Because the 54601A has a raster scanned display, the trace or traces (up to four) are always constant in brightness. By contrast, in a conven- tional scope, the beam has to "write" at a faster and faster speed as the timebase is switched up (maximum, around 20 nanoseconds/div). So for high frequencies and very fast pulses, the trace brightness becomes very faint and difficult to see. At low timebase speeds, the traces become very flickery, until at the lowest speeds (0.2 seconds/div), the trace flares badly as it becomes a bright dot moving very slowly across the screen. With the 54600 series, the range of timebase speeds is a great deal wider. We've already noted in our April preview of this scope that it is easy to drive and having now spent a few weeks with it we can emphasise that point. But reading through the manual and then using the recommended procedures therein shows just how well thought out it is. Consider, for example , how you can make time (frequency, etc) and voltage measurements off the screen of an ordinary scope, provided the variable vertical gain and timebase controls are in their "calibrate" positions. Once you start using these vernier controls to get a better look at a waveform though, you don't know what your voltage and timebase settings are. On the 54600 series units you can engage a vernier facility for both the vertical sensitivity and timebase, via OCTOBER 1991 9 This is the sort of waveform you can expect when you push the 54601A right to the limit of its sensitivity. Here it is running at 2mV/div and there is really not enough signal for it to work properly in the Normal display mode. Selecting an "Average" display mode cleans up the signal but it then responds to changes much more slowly. Frequency and period measurements are a snack with the 54601A. This waveform was fed in, displayed using Autoscale, the Time button pressed and then the Freq, Period and Duty Cy softkeys pressed to display the readings at the bottom of the screen. one of the softkeys immediately below the screen. The difference here is that not only is the exact sensitivity and timebase setting displayed (eg , 4.76V/div, 184µs/div) on screen but automatic and manual measurements are still available. Some of the automatic measurements possible are illustrated in the photos. The "Autoscale" facility is a very worthwhile feature .You just feed in a signal, press the Autoscale button and the scope computes the optimum set10 SILICON CHIP tings (some upmarket analog scopes also have this feature). There are some conditions under which the facility won't work: frequencies below 50Hz; pulse waveforms with a duty cycle of less than 1 %; and signals with an amplitude of less than about l00mV RMS, depending on frequency. For those conditions, pressing Autoscale will bring up an on-screen message saying "No signal found" even though, in some cases, the display will appear on screen! There are some traps involving Autoscale too . Say you want to view a small signal on Channel 1 and you are feeding a sync signal into Channel 4. You can set up the display manually and get a stationary waveform but if you then accidentally press Autoscale the scope will pick Channel 4, the stronger signal, as the one to be displayed. Even then, after momentarily kicking yourself, the situation is easily fixed. Just press the Setup button, to bring up a new Softkey menu and then press "Autoscale Undo". This reverts the scope to your previous settings. You can also save your screen setup and there are two trace memories which can be recalled at any time so that you can compare a previous waveform (displayed at half brightness) with those presently on the screen. Rise and fall times Making rise and fall time measurements with a conventional delayed timebase scope can be very tricky but with this HP unit, dare we say it, it is dead easy. With a signal being fed in, you push the Main/Delayed button, select Delayed timebase operation and you immediately get a split screen display. You can see this in one of the photos. At the top is the waveform · being measured while below is the expanded/delayed timebase display. For a quick measurement of rise and fall times, you need only press a few buttons. Push the Time button and it brings up the "Time Measurements" menu on the bottom of the screen (for Frequency, Period and Duty Cycle). Press the "Next Menu" button and it brings up RiseTime, FallTime etc. You can then get a reading for both rise and fall time by just pressing the two buttons required but if the times are very short (ie, in nanoseconds). then you have to go a bit further. At the top right hand corner of the screen, is the legend RUN, STORE or STOP, depending on the Storage mode in use. To the left of that is a legend showing which channel is the trigger source and what is the trigger slope; normally positive. For measuring rise time, it is best to use the positive slope which then shows the pulse rising edge on the delayed screen. To measure the fall time, you need to display the trailing edge on the delayed screen. This is easily done by ELECTRONIC WORLD Riston negative acting precoated circuit board Single Double sided sided 76mm x 127mm $2.60 $ 3.25 100mm x 160mm $5.70 $ 7.20 127mm x 152mm $5.70 $ 7.20 152mm x 254mm $10.70 $12.95 254mm x 304mm $20.50 $24.80 Instructions supplied with PCB This photo shows how easy waveform voltage measurements are with the HP54601A. It's all done by simply pressing a few buttons. pushing the Slope/Coupling button which then allows you to select positive or negative slope triggering. To get the most accurate figure, you need to run the delayed timebase so that the expanded wavefront (or trailing edge) spreads over a couple of divisions or more. Then, if you press for positive slope coupling, you'll get an accurate figure for the rise time and an approximate figure for the fall time, expressed symbolically as less than or equal to, say, lO0ns. Then, as you select for negative slope coupling, you get an accurate figure for fall time and an approximate figure for the rise time. All these measurements and a great many more besides, are much easier and quicker to run through on the scope than they are to describe in this text. By now then, you should have the idea that the 54600 series machines are particularly ' well thought out and easy to use. As an indicator of that, all the photos accompanying this article were taken before we had a chance to familiarise ourselves with the user manual; we weren't using the instrument to the best of its capabilities at that stage! Special situations At the start of this review, we mentioned that analog scopes could still do a better job in certain applications and there are at least two that we canthink of. First, where you need absolute waveform fidelity on the screen, the dot waveform structure of the HP is something of a drawback. In most cases though, this probably does not apply and the 54600 display will be entirely adequate. It is very good at picking up very fast glitches in wavetrains and better than normal scopes where you generally have to focus very carefully and then use a viewing hood in order to avoid missing very faint transients. A second situation entails low signal levels, say below 50mV peak to peak. Here there is really not enough signal for the digital to analog converter to process properly and the resulting display is only approximate and made up of a series of very short horizontal lines rather than dots. You can improve this situation by changing the Display mode to Average rather than Normal. This averages the display over a number of sweeps which may be selected at 8, 64 or 256. This changes the display back to a dot structure but now it is slow to update and very slow if you pick the 256 average. Interestingly, even here the 54600 does pretty well because even though it may not show a really good waveform (and that can be a real drawback in some measurement situations), its own noise level is generally better than typical analog scopes and so there is less noise on the screen. Well, as you've guessed by now, we're pretty impressed and we think that this is the precursor of all future oscilloscopes. The fact that it costs less than equivalent analog scopes is a sure indicator of that. 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Telephone: (03) 723 3860 (03) 723 3094 Fax: (03) 725 9443 Disposals bargain store at: 27 The Mall, South Croydon, Vic 3136 Telephone: (03) 723 2699 OCT0BER1991 11