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BUILD THIS LOW-CAPACITANCE SCOPE PROBE By HERB FRIEDMAN What's that - your budget won't extend to a low-capacitance x10 scope probe? You can build this one using junk-box parts. Even if you're measuring well down into the bass audio frequencies, an oscilloscope's display of a complex waveform - such as a square wave - will not be accurate unless the input to the scope is made through a low-capacitance test probe. In the June 1989 issue of SILICON CHIP, we published an in-depth discussion on the why's and wherefore's of low-capacitance oscilloscope probes so there's no sense in repeating what's already been said. Suffice to say that, as a general rule, all AC scope measurements should be made using a low-capacitance xl0 test probe. Also, as a general rule, you're probably best off with a low-C xlO scope probe that's specifically designed for your scope. But what if you can't afford to buy one? Simple! You build it using readily available, budget-priced parts. From input to output, the prototype low-C xlO probe shown here can cost as little as $10. And by building your own, you get the extra advantage of precisely matching your scope's vertical input connector: anything from a modern BNC connector to a somewhat older banana plug, to an ancient microphone-type plug connector. Whatever your scope needs, just hang it on the end of the probe's cable. The circuit of a low-capacitance c, PROBE ,:. [!J . ..... T ~ PLUG COAX CABLE =----e 1 CLIP~ LOW CAPACITANCE PROBE FIG.I: ALTHOUGH A LOW-CAPACITANCE probe is a simple device, it plays a vital role in obtaining accurate oscilloscope traces. 76 SILICON CHIP FIG.2: A PLASTIC HANDLE, a metal shield, a probe tip and a small piece of perforated board are required to make the scope probe. Matching holes in the handle and metal shield provide access to Cl. FIG.3: THE RESISTOR SHOULD be placed to the side of CL Do not run the resistor across the top of the capacitor. FIG.4: THE POINTER INDICATES where the wiring board's solder lug is tack-soldered to the inside of the· shield tube. probe, shown in Fig.1 , is certainly simple enough because it essentially consists of three components: Rl, Cl and PL1. Rl can be any kind of ¼ W or ½ W resistor with a value of about 9 megohms. The actual value isn't all that critical; it can be 9.2Mfl or 8.8Mfl if you happen to have one of these values in your parts box. Trimmer capacitor Cl can be anything that physically fits inside the handle. The unit specified in the parts list is cheap and readily available. As already mentioned, PLl is whatever connector is need- ed to match your scope's vertical input. The problem is, however, to assemble Rl and Cl in a shielded probe - one that does not pick up extraneous hum and noise because of capacitance-coupling to the user's hand. On the other hand, even though the probe is shielded, it should be insulated from the user's hand to ensure safety. So you will need some form of plastic handle, a probe tip for the handle, an internal shield, a perforated wiring board, and a solder lug that can be connected as a common ground. Be sure to use perforated wiring board (the stuff without the copper strips). Don't use Veroboard or stripboard as the copper strips have too much stray capacitance. Just where you scrounge all the parts is up to you. The internal shield, for example, could be easily make up from a discarded tin can and should be about 10mm in diameter and about 80mm long. The PARTS LIST 1 9Mfl resistor, see text 1 trimmer capacitor (Jaycar Cat. RV-5706 or RV-5708 , DSE Cat. R-2930) 1 plug connector to match scope input, see text 1 piece of pert board 1 metal shield, see text 1 plastic handle 1 small alligator clip 1 solder lug 1 probe tip assembly, see text 1 2-metre length of coaxial cable, RG-58A/U or RG-58C/U FIG.5: YOU MUST CUT A NOTCH in the plastic handle s6 that the ground test lead from the shield tube can exit from the front of the probe. FIG:6: THE BNC CONNECTOR at the top provides shielding all the way into the scope. On the other hand, a banana plug causes a break in the shield which exposes the inner conductor to possible hum and noise pickup. perforated wiring board should be trimmed so that it slides neatly inside the shield. Construction Holes must be drilled in both the handle and the shield to allow access to trimmer Cl. Take extreme FIG.7: A FULLY SHIELDED CABLE will provide the rock-steady square wave display shown in (a). A break in the display, such as caused by a banana plug, can cause the trace smearing shown in (b).Closer examination of the smear in (c) shows a 60Hz (USA) mains noise signal superimposed on the desired signal. AUGUST 1989 77 FIG.8: THE PARTS FOR THE CRO probe could be fitted into this commercial probe case which is available from Jaycar for $12.95. Alternatively, a suitable case could be fashioned from a short piece of plastic conduit. FIG.9: THE TWO HALVES OF the Jaycar probe case simply clip together. There's more than enough room to accommodate all the parts. care when drilling the holes in the handle and the shield because they are prone to snag on the drill bit. You must create some kind of safe drilling jig when working with the handle and the shield. Fig.2 shows the probe-assembly components after the handle and the shield are drilled. A 6mm hole should be drilled in the metal shield about 8mm from one end and a matching hole drilled about 18mm from the front of the plastic handle. Fig.3 shows how the ground lug is bolted to one end of the board. Bend this lug upwards through 90° so that the board can slide through the metal shield. Position the lug so that it is inside the tube and exactly flush with one end. Mark the opposite end of the board at the end of the metal tube and cut the board exactly on the line. Install Cl on the board so that it is centred 8mm from the cut edge. You will have to enlarge the ex78 SILICON CHIP isting board holes so that the trimmer's lugs just barely squeeze through. Using Fig.3 as a guide, install a PC pin on each side of Cl; then install Rl between the pins so that it lies alongside Cl. Do not position Rl across the top of Cl. Notice from Fig.3 that' one of Rl 's leads at the front PC pin is left at fu]l length. This extra length will pass into the probe's tip during final assembly. Connect Cl to the two PC pins on the bottom of the board using short, direct lengths of wire. The connecting cable is 1-2 metres of conventional coax. To prevent constant flexing from snapping the centre conductor, use one of the cable types having a stranded centre conductor, such as RG-58A/U or RG-58C/U. Avoid RG-59 cable because its centre conductor is solid. The probe's ground is the solder lug on the wiring board. Unbraid the cable's shield, twist the strands tightly into a pigtail and solder the pigtail to the lug. This done, connect the cable's centre conductor to the nearest PC pin that supports Rl. Finally, slip the shield tube over the board until the back end is exactly flush with the solder lug. Force or bend the lug against the inside of the shield, then rotate the shield so that it's hole is exactly opposite Cl and tack solder the lug to the inside of the shield. Fig.4 shows the details. Using a knife or a 3mm drill bit as a router, cut a slot for the grounding test lead in the front of the plastic handle, 180° opposite to the 6mm hole. This done, solder a 15cm length of stranded insulated wire to the front of the metal shield. Once again, this should be diagonally opposite to the adjustment hole. Now slide the assembly into the handle so that the holes for Cl are in alignment and the ground test wire flows out the slot. Terminate the ground lead with a small alligator clip. The next step is to assemble the probe tip. It's up to the constructor to devise a suitable probe assembly. For example, you could use parts from a discarded multimeter test probe. The probe tip assembly should be mounted on a plastic end piece and the resistor lead soldered directly to the probe tip. This end piece can be secured to the handle using small retaining screws. Alternatively, you could house the entire project in a ready-made probe case which is currently available from Jaycar (Cat. HB6400; $12.95). FIG.to: ADJUST Cl USING an insulated tool for optimum squarewave scope display. banana jacks for the vertical input connections then PLl must be a banana plug. Adjustment (b) (a) FIG.11: IMPROPER ADJUSTMENT of Cl will produce the rounded leading edge shown in (a) or overshoot of the leading edge as shown in (b). Ideally, PLl should be a BNC connector but some scopes may require the use of a banana plug connector. Bear in mind that, as shown in Fig.6, a BNC connector provides full shielding all the way from the probe to the scope's input, while a banana plug actually breaks the shield at the connector and will allow some noise pickup. For example, on a square wave signal, a BNC connector gave a rock-steady trace as shown in Fig.7a. In Fig.7b, PLl was a banana Garbage Reminder ctd from page 52 To set the unit, all you need to do is insert the batteries at the time you want the LED to begin flashing on your garbage collection day or days. The LED will flash for 7 hours and 12 minutes unless stopped using the BIN OUT switch. Normally the reminder would be set to start the LED flashing at about 5pm or 6pm. It does not matter which day the batteries are inserted since the days are selected using the DIP switches. Day 1 is the day that the Reminder is set, day 2 the next day and so on. For example, if the Reminder has its batteries inserted on Wednesday at 5pm, day 1 is Wednesday and day 7 is Tuesday. If your garbage night is Sunday and Wednesday, the required settings are switches 1 and 5 on. All other switches are off. Got it? Good. Now get that garbage out! ~ plug that was connected to a BNCbanana adaptor. Notice that noise (hum) pickup has caused the trace to "smear" . Fig.7c is the same trace as Fig.7b but a faster camera shutter was used to show that the "smear" is actually another signal (60Hz mains signal leaking through the broken shield at the banana plug) that is superimposed on the square wave. So if possible, maintain shielding throughout the entire assembly. Of course, if your scope only has To adjust the probe, set the scope's vertical input for DC. Then, using any known good square wave as the signal source - lkHz is almost a universal standard - use an insulated alignment tool to adjust Cl for the precise square wave shown in Fig.7a. Improper adjustment of Cl will result in a rounding of the signal's leading edge (Fig. 11 a) or overshoot of the signal's leading edge (Fig.llb). Refer to the scope probe article in the June issue to learn why Cl 's adjustment does what it does. Finally, be sure to allow for the x 10 factor when using the low-C xlO probe. For example, if the scope indicates that a signal is 1.1 volts p-p, then the actual value is 11 volts p-p. ~ Copyright 1989, Gernsback Publications. Reprinted with permission from January 1989 Radio-Electronics. 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