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TEST EQUIPMENT REVIEW The Tektronix THS720 is a lightweight, portable instrument which combines a powerful 2-channel 100MHz oscilloscope and a 4000-count digital multimeter in one convenient package. It has a liquid crystal display and is very easy to drive. By RICK WALTERS The Tektronix THS720 TekScope This go-anywhere measuring system operates continuously from the mains via a plugpack or for around two hours from an inbuilt battery. A spare 2.8 ampere-hour battery and a battery charger which is capable of recharging the battery in 1.5 hours are also supplied. The TekScope, as well as the charger, will operate from the mains or from a 12V cigarette lighter sock­et, which is an excellent idea for a portable instrument. The number of different functions packed into this tiny package, which measures 180 x 220 x 50mm (W x H x D) and weighs only 1.5kg, is quite amazing. The major features of the oscilloscope are: 82  Silicon Chip • 100MHz bandwidth with 20MHz selectable; • 500MS/s sample rate and 2500-point record length; • Separate digitisers for each channel; • Waveform averaging and enveloping with hardware peak detection; • Digital Real Time digitising (up to 5X oversampling), sin(x)/x interpolation and peak detect acquisition to limit the possibili­ty of aliasing; • Independently isolated input channels measuring up to 1000V RMS and floating at 600V RMS (using P5102 probes); • Cursors and up to 21 measurements continuously updated; • Simultaneous oscilloscope and me- ter operation on the same or different signals; and • Pulse and video triggering capability. The DMM features are: • True RMS AC volts, DC volts, Ohms, continuity and diode check functions; • Auto or manual ranging; • Data logger plot of meter measurements; • Maximum, minimum, delta maxmin, relative delta and average statistics selectable; • Bargraph indication for “analog meter” style indication; • Input can float to 600V RMS; • Input over-voltage indicator. It is apparent that great amount of design effort has gone into this unit as exemplified by the case support assembly (see Fig.1). It normally lies flat against the back of the case but can be flipped out with a finger. By swinging it up, the TekScope can be hung on a hook. If the hinged centre piece is pushed out, it can be used to hang the unit on the rung of a ladder or, if this centre piece is clipped into a slot on the rear of the case, it can support the TekScope for bench use. Pressing the on/off button, located at the bottom left of the front panel, causes the THS720 to do a self-check of all functions. After a few seconds, the message “Power-On self check. PASSED . Press the CLEAR MENU button” appears on the screen. When the menu is cleared the instrument is ready for use. The unit initialises to the same mode and settings as when it was turned off. Pressing the SCOPE or METER button will select that function. Let’s look at the scope features first. An internal 1.2kHz square wave output (its connections are located under a flap on the righthand side of the meter) is provided to allow you to properly compensate the input probes. This waveform is shown in Fig.2 with the VERTICAL MENU displayed across the bottom. Pressing the button below the menu item pops all the options up. Another useful feature is the AUTORANGE button. When any waveform is fed to the TekScope, instead of having to adjust the vertical sensitivity, the horizontal sweep rate and the synchro­nising trigger point, a press of the autorange button will usual­ly give you a rock steady trace at a suitable amplitude. As previously explained, the change­over from SCOPE to multi­meter is just a matter of pressing the METER button. The meter can be connected to one circuit and the scope to another and you can readily switch between them. What is even more useful is the ability to display the SCOPE function on a full screen and (for instance) a DC voltage which appears in the top right corner next to the meter face. As a practical example, you could monitor the output fre­ quency of a VCO (voltage controlled oscillator) Fig.1: a built-in tilt stand folds out and snaps into place when not in use. For benchtop operation the tilt stand locks in place with the hinged flap. It is hinged up to hang from a nail or extended to hang on the rung of a ladder. using the SCOPE probe and display the waveform, with the frequency readout on the righthand side of the display. The VCO input voltage can then be monitored using the DMM and displayed at the top righthand side of the readout (see Fig.3). As mentioned in the specifications, the meter circuitry and the SCOPE inputs are each isolated from ground. This allows you to monitor, for example, the mains voltage with the DMM, while the SCOPE probe could be looking at the ripple on a 5V supply referenced to ground. Controls While the AUTORANGE button is useful in obtaining a stable display, it may not be quite what you require. The auto setup can be overridden by pressing the VOLTS/DIV, SEC/DIV or TRIGGER LEVEL controls. The VERTICAL controls consist of a sensitivity rocker switch, a position rocker and a menu selector. The VOLTS/DIV switch has a large sine­ wave at the top and a smaller one at the bottom. Pressing the top increases the displayed amplitude; pressing the bottom reduces it. Similarly, pressing the top of the POSITION rocker moves the trace upwards while pressing the bottom moves it down; all quite logical and intuitive. As you would expect, provision is made for both channels, the stored references and the Math display to be turned on or off. Display selection is effected by pressing the required button. The display is turned off by selecting it, then pressing the WAVEFORM OFF button. The MENU button produces screens similar to Windows “drop down” menus, with the menu screen options depending on the wave­form selected. For the input channels (CH1 and CH2), you have the choice of selecting the input coupling method (AC, DC or ground), inverting the waveform, setting the bandwidth and selecting the type of probe (voltage or current) as shown in Fig.2. Using the Math function, you can add both channels, subtract CH2 from CH1, subtract CH1 from CH2, or multiply the two inputs. If one channel was measuring voltage and the other current, then the resultant waveform would represent the power being dissipated in the component being measured. If either reference channel (A or B) is selected, you have the option to save CH1, CH2 or math July 1996  83 Fig.2: this is the 1.2kHz waveform used to calibrate the 10:1 voltage probes. The Vertical MENU button has been pressed to show the choices available. The previous menu selection was the probe type and this is shown in inverse lettering. Fig.3: the VCO output waveform and frequency are shown on the display, the input voltage to the VCO is shown next to the meter. Fig.4: a plot of the output of a regulated power supply over a 4-minute period. The trace moves from right to left, so the small negative spike that appears around 2.8 minutes actually happened 1.2 minutes after we started the test. The readout on the RHS shows that the voltage dipped to 26.44V at this time. The plateau which begins at 2.6 minutes was actually 28.48V. The slight difference in the two average figures is prob­ably due to the fact that the run was stopped before the full four minutes and the figures were updated at different times. Fig.5: a one Farad capacitor was charged to 3.96 volts and dis­charged through a 1kΩ resistor. This time constant is 1000 sec­onds, or 16.66 minutes. As near as I can calculate from an en­larged graph, this shows a time constant of 16.25 minutes. This indicates that the capacitor was about 2.5% below its nominal value. waveform to a location from 1-10; ie, 60 locations in all. The HORIZONTAL controls operate in a similar manner to the VERTICAL controls although instead of rocking vertically they rock horizontally. Pressing the right or left POSITION arrow moves the trace in that direction. The right side of the SEC/DIV button with the expanded sinewave expands the displayed waveform, while the left side with the compressed sinewave increases the number of cycles displayed on the screen. The MAG button expands the display by a factor of 10. The MENU button allows you to se84  Silicon Chip lect the main or delayed timebase and set the delay time. The main timebase trigger has three preset positions at 10, 50 or 90% of the waveform period or by using the TOGGLE button any period from 0-100% can be select­ed. The delayed timebase can be set to start any time from 2 nanoseconds to 50 seconds after the main sweep. Trigger controls The trigger controls, while they appear simple, have a vast range of options. The TRIGGER LEVEL toggle moves the trigger point up and down the waveform from 100% to 0%. The SET LEVEL TO 50% button does just that. All the other functions are accessed via the MENU button. There are three trigger types you can select from: Edge, Pulse and Video, all of which can be from the CH1 or CH2 waveform. For edge triggering you can select DC, HF reject, LF reject or noise reject (DC low sensitivity) coupling. The slope of the trigger can be selected for a positive going or a negative going edge. For pulses, you can select either the positive going or the negative going edge, as well as setting a pulse width with the TOGGLE rocker. Once this width is set, you can then elect to trigger when the incoming pulse is less than the set width, greater than the set width, equal to the set width (with a toler­ance) or not equal to the set width (with a tolerance). For the video mode, you can select any field, field 1, field 2 or lines. You can also select scan rates between 15kHz and 65kHz with the TOGGLE button. With all these options it is hard to imagine a waveform that could not be triggered. While the controls we have covered so far exist in some form on all analog scopes, the following are mostly peculiar to the newer digital scopes: The DISPLAY button lets you set the screen display for dots or continuous lines, set the contrast, turn the “T” (which indi­cates the trigger point) on and off, and show an XY or YT format on the screen. It also allows you to have a full graticule, a grid, crosshairs or a frame for the display. The CURSOR menu lets you move vertical and horizontal reference lines around on the waveform and these can be used for the system to calculate things like propagation delay. Perhaps the most comprehensive of all the menus is the MEAS(ure) menu. This contains 21 different definitions for the measurement of the displayed waveform. These include, peak-topeak amplitude, true RMS over the first cycle, frequency, duty cycle and true RMS over the entire waveform. If you need to carry out a particular measurement on a routine basis, you can set up the TekScope to make the measure­ment and then save the setup in one of 10 non-volatile memory locations. This setup may be recalled at any time using the location number that it was saved in. If you have a suitable printer, the screen display can be sent to it via the supplied RS232 cable when ever the HARD COPY button is pressed. Naturally any previously stored waveforms can be recalled and printed in this manner. The printers supported are Thinkjet, Deskjet, Laserjet and Epson 9 & 24-pin. Using the Windows terminal program, the screen display or stored waveforms can be transferred to an IBM compatible PC. The formats supported are IMG, TIFF, PCX, BMP, EPS and DPU411/412 So much for the oscilloscope; let’s now look at the digital multimeter specifications. First, the five DC voltage ranges cover from 400mV to 880V with an accuracy of ±(0.5% reading + 5 counts), while the five true RMS AC voltage ranges cover from 400mV Above: BNC sockets are provided at the top of the TekScope for the two oscilloscope inputs. These inputs are completely isolated from each other and can be connected to sources at different potentials. The multimeter jack sockets (see photo below) are mounted at the side of the instrument, together with the serial input connector and probe compensation signal output (under the flap). to 640V, with an accuracy of ±(2.0% reading + 5 counts). The six resistance ranges are 400Ω to 40MΩ with an accuracy of ±(0.5% reading + 2 counts), except on the 40MΩ range where it is ±(2.0% reading + 5 counts). The DMM also features a diode test function and a continui­ty tester which emits a tone when the measured resistance is below 50Ω. Once you have become familiar with the SCOPE functions, the meter operation will be a breeze. When you press the METER button, the five functions listed above are available for selection. By pressing DC and AUTORANGE the meter will display the voltage. The same selection procedure is applicable to AC and Ohms. A vertical analog bargraph (with solid bars), situated at the righthand side of the display, moves up and down in sympathy with the signal level, with open bars to indicate the maximum and minimum values recorded. The maximum open bar can be seen in Fig.4; the minimum open bar is directly below it and is filled in by the bargraph. The data logger function could be a very useful feature for many users. It records the meter measurement over a period of time just like a chart recorder. This period can be set from four minutes to eight days. Two uses that immediately spring to mind are monitoring variations in the mains voltage and checking the stability of DC power supplies. To this end we set up a power supply on the bench and monitored its output voltage for four minutes. The result can be seen in Fig.4. The MEAS(ure) menu was set to store the MAX, AVERAGE and MIN voltage over that time. From this graph, you can see where the voltage dipped to the minimum value of 26.44V at about 2¾ minutes. The maximum voltage plateau is from 1-1½ minutes. continued on page 93 July 1996  85 ASK SILICON CHIP Got a technical problem? Can’t understand a piece of jargon or some technical principle? Drop us a line and we’ll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097. Battery capacity meter wanted I have been a regular reader of your magazine for the past 4-5 years and one of my interests involve model aircraft which I regularly fly. As a consequence of this, I have a vested interest in the proper care of nicad batteries which are used in the transmitter and receiver packs. To avoid memory effect, I regularly discharge these packs using a nicad discharger I have built using a circuit from a past issue of your magazine. This works well but it does have some drawbacks. What I would like is an indication of the batteries’ capacity. This would involve a timer in the circuit with a readout display in minutes, showing the time it took the batteries to discharge to the endpoint voltage. A lot of the commercially available nicad chargers/cyclers which are dedicated to modelling have this feature. It would also be nice to have variable discharge currents to suit different capacity batteries; eg, 500mA, 750mA and 1000mA. Does SILICON CHIP plan to publish such a circuit? I am sure that the discharger circuit mentioned above could be modified and expanded to include a timer and variable current discharge circuit. Of course, it would need to have its own supply to power the display when the batteries have discharged. (J. C., Western Gardens, Vic). • We do not have any immediate plans to produce a discharger or cycler with a timer. However, we have published your letter in order to gauge reader interest in the concept. For your reference we did publish a “discharge pacer” for electric vehicle batteries (lead acid) in the July 1991 issue. This was a fairly complex instrument which indicated the percentage ampere-hour capacity remaining in a rechargeable battery as it was discharged. would be a con­siderable development time. Commercial units are complex and involve a variable frequency output to give control over speed. Older speed controls used cyclo-inverters employing SCRs. Howev­ er, just recently an IC has been released which we believe con­ tains most of the control circuitry needed for a variable fre­quency, variable voltage speed control. We will investigate this chip and see if it can be the basis of a speed control suitable for publication in SILICON CHIP. But we are not promising anything at this stage. Variable speed for induction motors Leak amplifier circuit wanted Is there a circuit available for controlling the speed of induction motors? I have a wood lathe with a 2hp motor, and speed control without the laborious rearranging of belts and pulleys would be of great benefit. I’m sure many other machines could also benefit from easier speed control. I believe commercial units are available but at around $1400 to $1700 the control unit often exceeds the price of the complete machine it is meant to control. (W. S., Hallett Cove, SA.) • We have not published a suitable circuit and up until the last week or so, we were not likely to since there I am currently restoring a Leak Delta 70 amplifier. I think the HT rail may be too high (75V). The preamp boards are drawing too much current. I would appreciate it if someone would send me a circuit diagram with these voltages, etc. I will reimburse them for their trouble. As an aside, this is one for the Serviceman. I was given an Ibanez effects unit from a large club in Sydney. No-one could get it to work properly. After much measuring of resistors, capacitors, etc, I took it out in the sunlight and found what looked like a fine piece of wire under the board lacquer. TekScope Review . Encouraged by this, we then charged a one Farad capacitor to 3.96V and discharged it with a 1kΩ resistor. The voltage was chosen to ensure that the DVM would not switch ranges as the capacitor discharged. This graph can be seen in Fig.5. The time constant for this combination is 1000 seconds; ie, the voltage across the capacitor should drop to 37% of the initial value (1.46V) in . . continued from p85 1000 seconds or 16.67 minutes. Similar recording functions are available on the other ranges. By now you must be wondering how much this great little instrument is going to set you back. The prices are as follows: THS710 (60MHz), $3195 + sales tax; THS720 (100MHz), $3795 + sales tax. These prices include the TekScope, carry case, two CRO probes, multimeter leads, two batteries, the charg­er, and RS-232 and power cables. After using the TekScope for a week I probably still have not explored all its capabilities and will be very reluctant to hand it back to Tektronix. I doubt if the vast majority of users would need, or even want, any additional features to be included. I shall have to start working on the boss to buy me one for my SC workbench. July 1996  93