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S/1,ICO/li -CBIP • J ~· ♦ o. ~ } . ~'. . , . The Philips PM3394 digital/analog scope While more than half the world wide sales of oscilloscopes are now of digital storage models, there is still a big demand for analog performance. Recognising this, Philips has recently released a new range of analog/digital scopes which combine the best of both worlds. By LEO SIMPSON We had the chance to review the top model of the range, the Philips PM3394. This is a 200MHz 4 channel scope with a digital sampling rate of 200 megasamples/second. It also was fitted with the "Math+" option and the GPIB/IEEE488.2 option, making it a very powerful measurement instrument, as we shall see. In the last 12 months or so, there has been an accelerating trend towards digital-only oscilloscopes with raster scanned cathode ray tubes; ie, borrowing from the technology of computer video monitors. These have the advantage oflarge bright displays and compact size. On the debit side, there are some signal measurement situa- tions where the analog scope is definitely preferred and this is mainly to do with low signal levels and those where waveform fidelity is all important. Clearly, a raster scanned digital scope cannot provide true waveform fidelity - the good old vector scanned cathode ray tube can, can't it? Well , in fact, that statement is not quite true and it is only when you see the combination of the two different scope types that you fully realise that both digital and analog displays can reveal or conceal details of a signal waveform. In effect, there is a very good argument in favour of this type of combination instrument. The Philips PM3394 uses a conventional PDA (post deflection acceleration) tube with an acceleration voltage of 16.5kV. The tube viewing area .0CTOflEH 1992 73 is 8 x 10cm and all settings can be displayed on screen. To the right of the screen is a column of six blue buttons which are softkeys - their function changes depending on which control menu is displayed on the righthand side of the screen. On the lower half of the control panel are buttons which provide the normal control features which you would expect to associate with each of the four vertical inputs; ie, sensitivity, coupling (AC, DC or GND) and triggering (positive or negative slope). For each input channel, there are two buttons to control the sensitivity. Pressing the down or up button causes the sensitivity to change in a 1-2-5 sequence from 20mV/div to 50V/div if a 10:1 probe is connected or from 2mV/div to 5V/div if a 1:1 probe is connected. The type of probe connected is automatically compensated for if Philips 9000 series probes are used or it can be switched, via the probe menu, if a non-Philips probe is used. In addition, if you push both the up and down buttons simultaneously for a moment, you can then vary the sensitivity continuously from 20mV/div to 125V/div, with 3-digit resolution, if a 10:1 probe is connected. The facility of a calibrated continuously variable input attenuator is very useful and the ability to go to 125V/divmeans that AC mains voltages can easily be measured and displayed. Interestingly, if you are in the digital mode, you can have a vertical input sensitivity of much more than 20mV/div. By calling up the Display menu on the screen, you can call for vertical magnification of up to 32 times. This equates to a vertical input sensitivity of 625 microvolts per division. Of course, measuring signals at these low levels means that quantising noise becomes an appreciable part of the displayed signal. However, you can then get around part of that problem by using an averaging (AVG) mode for the signal display. But we are getting ahead of ourselves in presenting this review. On the upper half of the control panel are the controls for the main timebase, delayed timebase and trigger selection. The timebase can be varied in 1-2-5 steps from 500 milliseconds per division up to 20ns per division and this can be increased by a factor of 10 to 2ns/div. In the digital 74 SIUCO .\' CIIIP The Philips "Touch Hold & Measure" probes supplied with the PM3394 scope are a useful innovation. When the scope is in digital mode, pressing the Command button on the probe freezes the waveform on the screen together with the principal measurements. By the way, the "T" symbol on the other probe button is an earth symbol. Pushing that button grounds the probe. mode, on the other hand, the time base can run much slower, down to 200 seconds per division. At this minimum rate, it would take 2000 seconds, or 33 minutes to scan the screen once! Triggering facilities The triggering facilities are especially comprehensive. Pressing the Trigger button brings up a menu on the screen which allows you to select between "edge", logic and TV modes. The "edge" mode is conventional triggering and you are able to select AC or DC coupling, low frequency or high frequency reject and positive or negative edge triggering. In TV mode, you can select field 1 or field 2 or lines and then you can go on to select the actual line number to be displayed. Video systems supported are HDTV (1050, 1125 or 1250 lines), NTSC, PAL and SECAM. While the PM3394 is a combination digital/analog scope, all the controls mentioned have been associated with the analog mode of operation although clearly, they have the same function when the unit is in digital mode. Autoset button Perhaps the most useful button of all is on the top lefthand corner of the control panel and it is the Autoset button. When all else fails and you can't make any sense of a test setup you are doing, you can always restore sanity by pressing the Autoset button. This causes the scope to measure the signals present at the four inputs and then select timebase and vertical attenuator settings which give a useable display. From that point you can then alter settings to proceed to the measurement you want. Digital control facilities All the digital scope facilities are brought into play by the row of buttons along the top of the control panel . and the "soft" buttons down the righthand side of the screen. In general, the buttons along the top call up function menus on the lefthand side of screen and these can then be selected and varied using the soft buttons. For example, say you have a square wave on the screen and you wap.t to measure its principal parameters such as voltage and frequency. The first step is to push the "measure" button which brings up the first "MEASURE" menu on the screen. Pushing one of two soft buttons brings up "MEASl" or "MEAS2" menus on the screen. These a-llow you to display readings of voltage such as DC, RMS, MIN, MAX, PKPK (peak to peak), Low, High, overshoot, and preshoot. You can also display time parameters such as frequency, period, pulse duration, rise and fall times and duty cycle. All of these measurements are scrolled through by rotating the track knob These two scope photos show the same signal displayed in analog (left) & digital (right) modes (although with slightly different attenuator settings). Note that the analog trace is slightly blurred due to noise superimposed on the signal. Again, these two shots show the differe_nces between the analog (left) and digital displays. The analog trace shows the blurring effect of noise while the digital trace shows glitches which are otherwise invisible. By the way, the PM3394 has the facility for inserting user text comments on the screen, as seen here. This is very useful when taking scope photos as a permanent record. which doubles as one of the cursor controls at other times. Touch hold probes This brings us to the alternative method of making measurements which is unique to the Philips PM3300 series of scopes when teamed with the PM9000 series of probes. These probes provide two facilities whkh can be very useful and these are accessed via the UTILities button. One is "Autoset" which enables the appropriate vertical input sensitivity to be set for the signal being displayed. The other is "Touch, Hold & Measure" which allows the signal being displayed to be frozen on screen together with readings for DC level, RMS value, peak to peak value and frequency. This is done by pushing the "Command" button on the probe once. Pushing it once again reverts the scope to the previous display mode. Also on the probe is another small button with a "T" marking which you might think was meant for the "touch and hold" facility. That's what we thought but it is actually a grounding button and the "T" is the European symbol for earth. That threw us for quite a while until a Philips sales representative put us right. Regardless of that, the touch and hold facility is a really good idea and particularly useful where you have difficulty making connection to an instrument but wish to store and measure the signal. Of course, you can do the same thing by saving and storing a waveform so that its parameters can be measured and analysed but the touch and hold probe is such a convenient idea. We think it is sure to catch on with other scope makers. Picture quality The major difference between analog scopes and digital storage scopes is the way in which they present signal waveforms on the screen. With analog scopes, the signal waveform is repeatedly scanned across the screen so that what you see depends on the sweep speed. At low sweep speeds it is safe to say that the waveform displayed on the screen is essentially very close to the waveform being measured, after allowing for non-linearities in the CRT's deflection system. At higher sweep frequencies, the waveform displayed on the _screen is also close to that being measured except that any high-frequency noise superimposed on the signal will tend to show up as a slight blurring or thickening of the trace. By contrast, on digital scopes, the same waveform is subject to a one-off sampling process each time the screen waveform is updated. This means that the waveform is likely to have a spikier appearance than if it was displayed on an analog scope. The spikes will partly be due to the discrete quantising steps but also partly due to the superimposed noise on the waveform. On a raster scanned digital scope you also have the problem that the display is made up of discrete dots and depending on the nature of the signal (and the keenness of your eyesight), these fine dots will be more or less evident. In the case of the Philips PM3394 scope though, the display process is essentially the same in digital or analog mode. You may wonder how this can be but remember that this scope uses a conventional vector scanned cathode ray tube (CRT) - ie, the electron beam is moved across the screen by the signals applied to the horizontal and vertical deflection plates. In the digital mode, signals pass through the input attenuators and are then processed by the analog to digital conversion circuitry.. The result-. ing digital signals are either stored to memory and further processed or are fed to the digital to analog conversion circuitry where they are converted back to analog signals to be displayed on the screen. Because of this additional conversion process, the screen display is not made up of dots. (Although, just to confuse the issue, one of the display menu options is a dot waveform). Not only is the display a continuous waveform but the very high sampling rate of 200 megasamples per second means that waveform fidelity in the digital mode is very good. In most measurement situations, the only real difference between waveforms displayed in analog and digital modes is that in digital mode the trace will . O CTOBER 1992 75 be brighter, sharper and will show more hash. And as noted above, the hash will partly be due to noise and spikes on the signal and partly due to the quantising process. If you then switch to the "Acquire" m enu and select averaging, anywhere between 2 and 4096 samples, you can clean up the waveform as much as you want. However, the more samples you select, the slower will be the response of the displayed waveform to changes in the signal. In practice, we found that selecting an average of 8 samples was a good compromise, giving excellent waveform fidelity and quick response to signal changes. In fact, for most of the work done in the SILICON CHIP lab, we think this digital + average would be the preferred mode. One reason why the waveform fidelity is so good in this mode is that it increases the effective vertical resolution from 8 bits to 16 bits. And even if you do stay in the digital mode most of the time, it is always possible to flick back to the analog mode. at any time just by pressing the digital button; pressing it again flicks you back to digital mode. In this way you can check the differences in the display and check for the presence of glitches. Math+ option An overview of the comprehensive autofitatic measurement functions was given above but these tend to pale when you consider th e power of the "Math+" option. This involves a separate card with its own processor and a great deal of ancillary circuitry. In effect, it turns the PM3394 into a powerful signal analyser. And this is on top of the standard mathematical functions in all the PM3300 series. These comprise Add, Subtract, Multiply and Digital Filter. Typically, the Add function can be used to add a displayed signal to that of another input channel or a stored signal. The same applies to the Subtract and Multiply functions. For example, you could use the Multiply function to compute the product of voltage and current signals and hence display the power waveform. The Digital Filt er provides a selectable low pass filter in which the lowest corner frequency is inversely proportional to th e timebase fre- quency. This function allows you to display a signal and then show its shape after the signal has passed through the filter. The filter corner frequency (-3dB point) is displayed on the screen at the same time. The additional functions provided by the Math+ option are Integrate, Differentiate, FFT (Fast Fourier Transform) and HST (Histogram). The first two are more or less self-explanatory and can be easily demonstrated using a square wave signal; integration of a square w ave gives a triangular waveform while differentiation gives rise to an impulse waveform with positive and negative spikes corresponding to the positive and negative going edges of the square wave. For those not familiar with FFTs, this function effectively turns the scope into a spectrum analyser with a signal dynamic range of 50dB. In the FFT mode, the signal is processed into its fundamental and harmonics and these are displayed in the frequency domain; ie, amplitude versus frequency. One of the photos accompanying this review shows the classic FFT of a square wave with the odd harmonics displayed with decreasing A unique family of full function pocket-sized digital rnultirneters. ■ LARGE LCD DIGITS SAFETY TESTER ■ SAFETY TEST LEADS ■ FUSED 10A CURRENT JACK ■ INPUT WARNING BEEPER ■ DM5XL DM10XL DM15XL $71.00 $98.00 $120.00 Al l prices plus 20 % sales tax. JE ST LUCIA ELECTRONICS 24 Campbell St. Bowen Hills Q 4006. Tel: (07) 252 7466 Fax (07) 252 2862 ECONOMIC ELECTRON/CS: 22 Campbell St. Bowen Hills Q 4006. P.O. Box 481, Fortitude Valley 4006. Tel: (07) 252 3762. Fax (07) 252 5778. SOUTHPORT ELECTRONIC SHOP: Shop 1, 10 Welch St. Southport Q 4215. Tel: (075) 32 3632. Fax: (075) 51 0543. 76 SILICO N CHIP STOCK SALE!! FANS 80 x 80 x 25mm 240VAC .. ... $15.00 120 x 120 x 38mm 240VAC . $15.00 MISCELLANEOUS The interior of the Philips PM3394 is quite different from units we have seen in the past. It has a large moulded plastic chassis and large double-sided printed circuit boards which are packed with surface mount components. The CRT is fully screened and the outer case of the unit is vinyl coated aluminium. The large coils at the rear of the tube are the delay lines for the vertical amplifiers. amplitude against increas ing fre quency (ie, 1F + 1/3F3 + 1/5F5 + ). To identify the frequency, you just move a vertical cursor along the signal trace and you get a readout of the frequency and its amplitude, down to -50dB. FFT analysis can be applied to input signals and to stored signals alike and is particularly useful for analysing one-shot signals. When you consider that the FFT covers the whole frequency range of the instrument (ie, up to 200MHz in the case of the PM3394), this is a very powerful and cost-effective option indeed. HST (Histogram) is an unusual feature which displays the voltage distribution of a signal against time. For example, a perfect square wave will have a histogram which indicates that the signal is high 50% of the time and low 50% of the time. Real world signals have a much more complicated voltage distribution and the HST facility can reveal a lot of information which has previously been unavailable from normal scopes. Although this has been a relatively long review of the PM3394, it cannot hope to cover the instrument's full range of features. We were very impressed with this oscilloscope and predict that it will sell well in the years to come. It has a good range of features encompassed in its digital and analog display modes and its Math+ option is very worthwhile; in fact, some buyers are likely to purchase the PM3394 almost for this option alone. The prices of the four models in the PM3300 series are as follows. Top of the range is the PM3394 with 4 input channels, 200MHz bandwidth and 200MS/s sample rate and priced at $10,919. Next down is the PM3392 with the same bandwidth and sample rate and 2 plus 2 in put channels ( ie, 2 channels with full attenuators and two with switched sensitivity) and priced at $10,079. Then there is the PM3384 with 100MHz bandwidth, 200MS/s sample rate, four input channels and priced at $9239. The PM3382 variant has the same ban dwidth and sample rate and 2 plus 2 channels and is · priced at $8399. Three options are applicable to the whole range. The Math+ option is $840 while th e GPIB/IEEE488 .2 card and the 32Kb memory card are also $840 each. These prices do not include sales tax. For further information, contact Philips Test & Measurement Division, 25-27 Paul St North, North Ryde, NSW 2113. Phone (02) 888 8222. 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