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WIDEBAND, ACTIVE DIFFER OSCILLOSCOPE Using your oscilloscope to examine and measure high speed and high frequency circuits can be tricky if you use only the usual passive test probes supplied. Here’s a design for a high performance, active differential probe which costs much less than commercially available active probes. It has very little circuit loading and usable bandwidth of more 80MHz. The differential probe can be powered by any convenient USB – in this case, the Agilent ’scope has a USB input which is more than capable of supplying the <40mA required. If your ’scope doesn’t have a USB socket, you could use a computer, laptop or even a USB plugpack. The trace shown on the oscilloscope is actually the output of transformer T1 on the SiDRADIO PCB (SILICON CHIP, October 2013), as measured by the differential probe (not the point shown in the photo). 40  Silicon Chip siliconchip.com.au By JIM ROWE RENTIAL PROBE D o you know what is inside the ‘passive’ test probes supplied with most oscilloscopes, and how to use them to make reasonably accurate measurements at high frequencies? If not, have look at the excellent article on this topic by Doug Ford in the October 2009 issue of SILICON CHIP. Doug explains how complex these probes can be and how many factors can result in their performance falling away, espe- cially at high frequencies. In addition they tend to disturb operation in the circuit being tested, making it difficult to make proper measurements. It’s because of the shortcomings of passive probes that some of the big manufacturers produce ‘active’ probes to provide a much higher input resistance together with a much lower input capacitance. Originally, active probes used valves (vacuum tubes) at their input but then when semiconductor technology came along, JFETs and MOSFETs made it possible to make active probes that were much smaller and easier to use. It also became feasible to make ‘differential’ active probes, which overcame some of the remaining drawbacks with conventional ‘single ended’ active probes. (More about these shortly.) The big problem with commercial active probes is their price tags. Even the single-ended type can set you back well over $700, while the differential type can cost over $2000 apiece – more than most of us paid for our digital scopes! In short, the only way that most of our readers are likely to be able to use an active probe with their scope is to build one. Yet the last DIY active probe to be described in Australia was way back in the September 1989 issue of ELECTRONICS Australia – 25 years ago. That design is now very dated. This new active, differential scope probe design takes advantage of modern surface-mount components to deliver a high level of performance and it fits inside a compact case. Best of all, it can be built for much less than the cost of any currently available commercial active probes. We estimate that you should be able to buy all of the components and build it for about a quarter of what you’ll pay for the cheapest commercial active probe presently available. Why differential? Before we start describing the new probe and how it works, perhaps we should look at why a differential active probe tends to be better than a single-ended one. A single-ended active probe is certainly a big improvement over most passive probes, offering high input resistance combined with very low input capacitance. It tends to cause lower disturbance to the circuit under test, particularly at high frequencies – where the higher input capacitance of a passive probe causes increased circuit loading. The high frequency and transient response of the probe-plus-scope combination also tends to be better and smoother, due to better compensation and fewer reflections in the cable between the probe output and the scope input. But there can still be problems when you’re making HF measurements with a single-ended active probe. These problems are mainly associated with the ‘ground clip lead’, which is used to make the connection between the probe’s input and the earthy or ‘cold’ side of the circuit under test. As you can see from Fig.1A, even when the ground clip lead is quite short, it can introduce enough inductance (Lg) to reduce the effective signal voltage appearing at the actual input of the probe at high frequencies. So the frequency response of the probe tends to droop at high frequencies, reducing the measurement reliability. As well, the ground lead inductance can interact with the probe’s input capacitance (Cin), resulting in resonances siliconchip.com.au September 2014  41 R FFE BU P AM TO SCOPE INPUT +Vsig/2 Rin TIP Cin L TIA EN ) FER =1 DIF MP (A A GROUND LEAD (MAY BE OPTIONAL) Lg POSITIVE TIP Cin Rin Rin Vcom Cin Lg Vsig GROUND LEAD R FFE BU P AM 2) TO SCOPE INPUT 50 + (A= – +Vsig – (–Vsig) = 2Vsig (50 TERM. AT SCOPE END) R FFE BU P AM (A=2) –Vsig/2 NEGATIVE TIP A SINGLE-ENDED PROBE B DIFFERENTIAL PROBE Fig.1: Comparing a ‘single ended’ active probe (A) with a differential active probe (B). With a single ended probe the ground lead inductance Lg can cause problems at high frequencies, but a differential probe solves these problems. at specific high frequencies. This can not only result in the probe producing unwanted loading on the circuit being measured but can also produce spurious ‘peaks and dips’ in the measurements. It is possible to minimise these problems by replacing the ground lead with a very short ‘ground blade’, providing a somewhat lower inductance than the usual 100mm-or-so long ground lead and clip. Many of the commercial singleended active probes come with this type of ground blade as an accessory. But a better solution is to change over to a differential probe, as shown in simplified form in Fig.1B. As shown, the differential probe has two tips and is designed to measure the signal difference between the two – rather than the signal between either probe tip and ground. In fact the ground lead (or blade) is really only used to tie the circuit under test’s ground to that of the probe and scope, to keep the voltages being measured within the probe’s common mode input range. This means that if there is no sig- nificant voltage difference between the two grounds, the ground lead or blade may be regarded as optional. Inside the differential probe there are two virtually identical input buffer amplifiers (one from each tip), each of which feeds one input of a third amplifier, the differential amplifier. This is where one of the two signals is subtracted from the other to send only the ‘difference’ signal out to the scope input. This subtraction cancels out any ‘common mode’ signal present at both probe tips, leaving only the ‘difference Specifications An active differential probe for oscilloscopes, housed in a compact handheld case and operating from +5V DC, derived from any convenient source such as a USB port on a PC or digital oscilloscope. It provides tip area illumination via a white LED and a choice of two switched gain settings: 1:1 or 10:1. Input coupling: AC Input resistance, each probe tip: 1MΩ nominal on the 1:1 range (1.0023MΩ); 10MΩ nominal on the 10:1 range Input capacitance, each probe input socket to ground: 3.15pF approximately (So capacitance tip-to-tip is approximately 1.6pF) Maximum DC voltage at probe tips: ±45V, both ranges Maximum AC voltage input before overload, both probe tips: 2.0V peak-to-peak (700mV RMS) on the 1:1 range, 20.0V peak-to-peak (7.0V RMS) on the 10:1 range Output impedance: 50Ω (Needs an output cable of 50Ω characteristic impedance, terminated in 50Ω at the scope end) Bandwidth (probe + output cable and termination): 25Hz - 80MHz +0.2dB/-3dB, both ranges 60Hz - 50MHz +0.2dB/-0.5dB, both ranges 150Hz - 40MHz +0.2dB/-0.3dB, both ranges Overall transmission gain/loss: On 1:1 range, 0.0dB ±0.6dB On 10:1 range, -20dB ±1.0dB Current drain from 5V DC supply: Less than 40mA 42  Silicon Chip siliconchip.com.au This photo, close to life size, shows the Active Differential Probe in its handheld instrument case. It’s a comfortable fit in the hand while applying the probe to the circuit under test. MEASURED RESPONSE (1:1 RANGE) signal’ – the signal between the positive and negative probe tips, which is what we are trying to look at and measure. As the common mode signal is essentially equal to the voltage VCOM at the probe’s ground terminal, this explains why any voltage difference developed across the ground lead or blade inductance LG is no longer a problem. It’s simply cancelled out. Before we leave Fig.1, you may be wondering why we’ve shown the output of the differential probe as having an amplitude of 2Vsig. Won’t this cause a calibration problem, by giving the probe a gain of 2? Not really, because as shown in Fig.1B, there’s a ‘source termination resistor’ of 50Ω fitted in series with the probe output. This is to match the characteristic impedance of the probe’s output cable (normally 50Ω). Then at the scope end of the same cable, another 50Ω shunt resistor is used to ensure that the cable is terminated correctly at that end too, to avoid reflections and consequent complications (like peaks and dips). And the combined effect of the two termination resistors is to introduce an attenuation factor of 2:1 – bringing the overall signal gain of the probe and cable back to unity. 1F capacitor in parallel with a 10nF capacitor. This combination has been chosen to give a lower input corner frequency of less than 30Hz, together with the smoothest possible upper frequency response. Following the DC blocking capacitors the signals each pass through 27Ω overload protection resistors, before reaching the gates of input buffer transistors Q1 and Q2. These are BSS83 N-channel MOSFETs designed especially for operating from a 5V supply voltage. We’re using them as near-unity gain wideband source followers, to give high input impedance combined with the lowest possible input capacitance. The gates of both Q1 and Q2 are biased to +4.3V via the 1MΩ resistors. This bias level is chosen to provide a ‘half supply voltage’ (+2.5V) level at the sources, which are direct coupled to the following ICs. The bias voltage is The probe’s circuit Now refer to Fig.3, which shows the complete schematic of our new probe. The two probe input tips plug into CON1 and CON2 at left, from where they each pass to the end contacts of S1a and S1b – the two sections of range switch S1. Depending on the setting of S1, they each pass into the two input buffer amplifiers directly or via a series 9.0MΩ input divider resistor comprising three 3.0MΩ 1% resistors in series. Then each signal passes through a DC blocking capacitance comprising a +1.5 +1.0 +0.5 0dB –0.5 –1.0 –1.5 –2.0 –2.5 –3.0 10k 15 20 30 40 50 70 100k 150 200 300 500 700 1M 1.5 2 3 4 5 7 10M 15 20 30 40 50 70 100M INPUT FREQUENCY (kHz/MHz) Fig.2: The upper frequency response of the differential probe, as measured on the 1:1 range. At the LF end it rolls off quite smoothly below 150Hz, with the -3dB point at around 25Hz. siliconchip.com.au September 2014  43 330 A TIP ILLUM +5V K LED1 10F  (WHITE) K + TIP 3.0M 3.0M MMC 2.7k 1F 3.0M 27 OPTIONAL GROUND LEAD & CLIP 10:1 MMC +4.3V RANGE SWITCH K 8 D G A Q2 BSS83 100nF AD8038ARZ BSS83 S *B 6 47F 10nF MMC MMC 1.0k 1.30k 1.0k 3 2 IC1– IC3: AD8038ARZ +5V 7 IC3 4 10nF +2.5V D MMC SM5819A, SS16 A MMC S* G 10nF 3.0M SM5819A OR SS16 7 IC1 470 27 3.0M +5V 47F MMC 1.0M 1F MMC 3.0M 1.0k 15k S1b MMC 4 MMC 3 2 7 IC2 6 1.0k 1.0k 4 1.0k 47F 1.30k MMC 4 1 (SUBSTRATE) ACTIVE DIFFERENTIAL RF SCOPE PROBE derived via the 2.7kΩ + 15kΩ voltage divider, with a 10F bypass capacitor to provide filtering. and Q2 – while also providing the current drive capability to feed the inputs of difference amplifier IC3. The two 47F capacitors connecting the 1.0kΩ ‘lower feedback’ resistors to probe earth are to maintain the LF response. IC3 is also an AD8038ARZ device, configured so that the positive-tip input signal is fed to its positive input (pin 3) while the negative-tip signal is fed to the negative input (pin 2). The four 1kΩ resistors and 47F + 10nF bypass capacitors ensure that IC3 does perform the desired subtraction 10:1 attenuator 44  Silicon Chip POSITIVE TIP OUTPUT CABLE TO INPUT OF SCOPE/DSO UT E TP OP OU SC ) TO (50 on c p hi .c om .a u DC R 5V WE PO N w.silic E CO ww AL PROB I L TI SIHIPIFFERSECNOPES C TIVE DSCILLO O 0 AC R FO – IN PU x1 The 1MΩ gate biasing resistors also provide the main component of input resistance for both input channels, when switch S1 is in the ‘1:1’ position. Then when S1 is moved to the ‘10:1’ position, they form the lower elements in the 10:1 input dividers (in conjunction with the 9.0MΩ series resistors). After passing through input buffer transistors Q1 and Q2, the two input signals pass through amplifiers IC1 and IC2. These are AD8038ARZ wideband amplifiers, specified for operation from a single 5V supply and with a bandwidth of better than 150MHz (for a gain of 2.0). Incidentally, we also looked at several other devices, including the AD818, MAX4414ESA and OPA356 but none performed as well as the AD8038ARZ. So the 8038 it is! We are using them here as buffer amplifiers with a gain of 2.3, NEGATIVE TIP to compensate for the small loss in the input source followers Q1 T x1 SC 2014 470 10F – TIP K A 3 2 S1a SMA SKT (STRAIGHT, END ON) LED1 +2.5V S 1.0M 1:1 CON2 MMC MMC SMA SKT (STRAIGHT, END ON) D1 10nF D * G 10nF 10F MMC Q1 BSS83 MMC CON1 100nF + IN PU T OPTIONAL GROUND CLIP LEAD (CLAMPED TO THE FERRULE OF EITHER TIP PLUG) of the two signals, so a ‘2Vsig’ difference signal appears at its output (pin 6). The two paralleled 100Ω resistors at the output of IC3 provide the 50Ω ‘source termination’ for the cable connecting the probe’s output at CON3 to the scope input and the paralleled 100F and 100nF capacitors provide DC blocking. LED tip illumination Finally, LED1, located at upper left is included to illuminate the area right in front of the probe’s tips, to make connections easier. Many of the up-market commercial active probes also provide this ‘tip illumination’, be5V POWER CABLE cause when you are (FROM USB SKT making measurements in ON DSO, PC OR high frequency circuits PLUG PACK) you’ll almost certainly be using very short tips on the probe itself. This means that the probe body will not only shield the immediate area of the circuit being tested from a light source, but will also tend to block your view as well. In other words, it’s a very worthwhile feature and one which was easily provided at low cost. siliconchip.com.au 6 F1 1A L1 100H Parts List – Active Differential Oscilloscope Probe POWER VBUS (FAST BLOW) GND 1 2 3 4 5 1 ABS instrument case, 114 x 36 x 24mm CON4 USB MICRO TYPE B SOCKET 10nF MMC 10F MMC 100nF MMC CON3 100 100 100F MMC OUTPUT TO SCOPE INPUT SMA SKT (STRAIGHT, END ON) TERMINATE OUTPUT CABLE IN 50 AT SCOPE END Fig.3: The probe’s full circuit schematic. All components except range switch S1 and LED1 are SMD devices. The whole probe runs from a +5V DC supply which means that it can be powered via virtually any standard USB port, such as the one on the front of many recent-model digital scopes, a USB port on your PC – or if neither of these are available, one of those low-cost ‘USB charger/power pack’ devices you can pick up for less than $15 (preferably not a dodgy “el cheapo” from China!). Since the total drain of the probe is less than 40mA, this should be well within the capability of most USB ports on DSOs and PCs. CON4 is used to bring the +5V DC power into the probe. This is a USB micro type-B socket, which allows you to use a standard ‘USB type A-plug to USB micro type-B plug’ cable (as used to hook up tablet PCs and mobile phones to a PC or charger) to provide the probe with power. 100H inductor L1 is used to filter the +5V input and remove any noise from the USB port or charger, while fuse F1 and diode D1 are used to protect against reversed-polarity damage. These components do nothing if the 5V supply is connected with the correct polarity but if the polarity should be reversed for any reason, D1 will immediately conduct and cause F1 siliconchip.com.au 2 1 1 1 3 PCBs, 103 x 26mm, code 04107141 & 04107142 100H SMD inductor, 1.6A rating (L1) 1A SMD fuse, 0603 fast acting (F1) DPDT/DIL slide switch, raised actuator (S1) SMA socket, end launch, PCB edge mtg (CON1,2,3) 1 Micro USB type B socket, SMD (CON4) 8 Self-tapping screws, 6G x 5mm long Semiconductors 3 AD8038ARZ SOIC8 video amplifier (IC1,2,3) 2 BSS83 MOSFETs, SOT-143 SMD pkg (Q1,2) 1 3mm white waterclear LED (LED1) 1 60V 1A Schottky diode, DO214AC SMD pkg (D1) (Hammond 1593DTBU element14 code 187-7372) (Murata 48101SC) (Cooper Bussman 0603FA1-R) (TE Connectivity ASE 2204) (Emerson Connectivity 142-0701-806 or Multicomp 19-70-4-TGG) (FCI 10103594-0001LF or Molex 105017-0001) (RS Components order code 523-6872) (element14 order code 108-1312) (SS16 or SM5819A) Capacitors 1 100F MLCC, SMD 1210, X5R dielectric 6.3V rating 3 47F MLCC, SMD 1210, X5R dielectric 6.3V rating 4 10F MLCC, SMD 1210, X7R dielectric 16V rating 2 1F MLCC, SMD 1206, X7R dielectric, 50V rating 3 100nF MLCC, SMD 1206, X7R dielectric 50V rating 6 10nF MLCC, SMD 1206, X7R dielectric 50V rating (Code 107) (Code 476) (Code 106) (Code 105) (Code 104) (Code 103) Resistors (all 0.125W 1%, SMD 1206) 6 3.0MΩ 2 1.0MΩ 1 15kΩ The codes shown here 1 2.7kΩ are the two most common 2 1.30kΩ but there are others! If in 6 1.0kΩ doubt, check all SMD 2 470Ω resistors with your 1 330Ω multimeter as you would 2 100Ω any doubtful resistor. 2 27Ω (Code 3M0 or 3004) (Code 1M0 or 1004) (Code 15K or 1502) (Code 2K7 or 2701) (Code 1K3 or 1301) (Code 1K0 or 1001) (Code 471 or 470R) (Code 331 or 330R) (Code 101 or 100R) (Code 270 or 27R) to ‘blow’ – protecting both the probe circuitry and the 5V source from significant damage. Construction All of the probe circuitry and components are fitted onto a PCB measuring 103 x 26mm (code 04107141). This is designed to fit inside one half of a small handheld ABS plastic case, with a screening PCB of the same size (code 04107142) fitted into the other half of the case. The case itself measures only 114mm long, 36mm wide and 24mm high, so it can be held in your hand very comfortably. In fact, the case has been designed to house hand-held equipment such as this. It comes from NB: not all SMD capacitors are marked. If in doubt, measure! Hammond Manufacturing. The small SMA sockets (CON1 and CON2) used for connection of the probe’s input tips are mounted at one end of the case, along with the white LED1, which illuminates the tip. Two sockets are mounted at the other end, SMA output socket (CON3) along with CON4, the USB micro B socket for the probe’s 5V DC power. All of the components used in the probe are mounted directly on the main PCB and all but two of the components are SMDs (surface-mountdevices). The two through-hole exceptions are slide switch S1 and LED1. Switch S1 is mounted under the PCB and LED1 is mounted above it with its September 2014  45 ACTIVE DIFFERENTL SCOPE PROBE UPPER SHIELD PLATE 4 330 Q2 2 1 27 BSS83 100nF 1.0k 47F top of the board. Your PCB assembly should now be complete, with all that remains being to connect the shield PCB copper to the ground copper on the main PCB. This can be done using a short length of light hookup wire – baring a few millimetres at each end so that the ends can be soldered into the ‘via’ holes at the rear of each PCB, as shown in Fig.4. Preparing the case Now prepare the case. This involves drilling three 7mm holes in the removable ‘front’ end panel (for CON1, CON2 and LED1), together with another round hole in the ‘rear’ end panel for CON3. Then there’s an 8 x 3mm rectangular hole to be cut in the rear end panel as well (for access to CON4), and finally a 10 x 7.5mm rectangular hole cut in the bottom half of the case (which becomes the top) for clearance around S1 and access to its actuator. The location and size of all of these holes is shown in Fig.5. You might also want to make a ‘dress’ front panel, to give your probe a professional look and TOP L 1 2.7k 10nF K F1 SS16 1A 10nF 1.0k 10nF 100  C 2014 410 2 HC 1 5 1 100 100nF 1.0k 1.0k 100 1.30k 100F help in using it. Artwork for a dress front panel is also shown in Fig.7. You can make a photocopy of this, (or you can download it from siliconchip.com.au and print it), hot laminate it (or use self-adhesive book cover film) for protection and then attach it to the front panel using double-sided adhesive tape – after cutting it to size and also cutting out the clearance holes for the case assembly screws and S1. Assembly Now slip the front end panel of the case over CON1, CON2 and LED1 at the ‘front’ end of the main PCB, and the rear end panel over CON3 at the rear end of the PCB. Then lower the complete main PCBplus-end-panels assembly down into the bottom half of the case (which becomes the top), with the two end panels passing into the moulded slots and S1 passing down through its matching slot. Once this main board assembly is down as far as it will go, you can secure it firmly in position using four 5mm long 6G self-tapping screws – mating (NOTE: BECOMES TOP OF PROBE) TOP (REAR END PANEL) 6 A 7.75 PWR IN 104107141 41L1 70140 8038A IC3 10F 100H CON4 4800S 10F L1 A MURATA 10nF D1 A D B 7.75 A 6.5 L L C 7.5 8 1.75 3 6 L OUT 3 1.0k 10F 15k 10F 47F 1.0k 1206 1206 10nF 1 WIRE CONNECTING SHIELD PCB WITH GROUND C ON 2014 MAIN PCB 04107141b CON3 1F 3.0M 1 Q1 1.30k IC1 8038A 4 1F 47F IC2 8038A S1 3.0M (BOTTOM HALF OF CASE – TOP VIEW) (FRONT END PANEL) 1206 1206 IN– 3.0M 10:1 (UNDER) A 100nF 470 1:1 LED1 ACTIVE DIFFL SCOPE PROBE 3.0M 10nF 27 BSS83 2 3 1.0M 1.0M K TIP 1206 1206 IN+ CON1 3.0M 3.0M 470 (SHIELD BOARD – FITS INSIDE UPPER HALF OF CASE) CON2 leads bent forward by 90° so the LED’s body can protrude through the ‘front end’ of the case between the two input sockets. The component overlay diagram of Fig.4 shows the location of all components, together with their orientation. When assembling the PCB, use a finetipped soldering iron – preferably one with temperature control. We suggest fitting the components to the PCB as follows: first fit USB micro socket CON4, taking great care when soldering its five very small contacts at the rear. Then mount the resistors and capacitors, followed by fuse F1 (which is very tiny). Then fit diode D1, Mosfets Q1 & Q2 and the three ICs. Next, fit the three SMA sockets (CON1, CON2 and CON3), which slide onto the front and rear edges of the PCB, with their centre pin resting on (and soldered to) the centre pad at the top of the PCB. Their ‘side prongs’ solder to the matching pads on each side, on the top and bottom of the PCB. Inductor L1 comes next, followed by LED1 on the top of the PCB and switch S1 underneath it in the position shown. When you are fitting LED1 make sure you mount it vertically with the underside of its body about 13mm above the top of the PCB. After the leads are soldered they can both be bent forward (left) by 90°, so the LED can protrude from the centre hole in the case front end panel. Finally fit slider switch S1. This is in a 6-pin DIL package, which mounts under the PCB with its pins coming up through the matching holes. Make sure you push the switch body firmly against the underside of the PCB before you solder its pins to the pads on the 10.0 31.5 HOLES A: 7.0mm DIAM. HOLE B: 3.5mm DIAM. HOLE C: 3.0 x 8.0mm HOLE D: 7.5 x 10.0mm 5.25 (ALL DIMENSIONS IN MILLIMETRES) Fig,5: drilling and cutout details of the Hammond Manufacturing “Hand Held Instrument Case”, shown 1:1. The only slightly difficult holes are the cutouts for the USB socket on the rear end panel and the switch on the lower half of the case. 46  Silicon Chip siliconchip.com.au Fig.4 (left): the component overlay for the main PCB with the shield board (which contains no components) above. It is connected to the main board by the short link as shown. The main board fastens to the bottom of the case, which becomes the top, while the shield is secured to the top of the case, which becomes the bottom! Below is a same-size photo of an early prototype main PCB, actually mounted in the case. Take no notice of the “AD818” labelling – we actually used AD8038s as shown on the PCB overlay. with the holes in the moulded standoffs underneath. Then the shield PCB can be fixed into the other half of the case, using another four of the same screws. The final assembly step is to invert the case half with the shield PCB and lower it down over the half with the main PCB, so that each end panel slips into the moulded slots as before. Then you can upend it and fasten it all together using the two countersinkhead self tappers supplied and your active differential probe should be complete. Making the probe tips The simplest way to make ‘basic’ probe tips for the project is probably to base them on an SMA male connector, as shown in Fig.6. This is the way I made the probe tips you can see in the photos, basing them on an Amphenol Connex type 132113 SMA plug; only the plug body and the centre contact are used – the crimping sleeve and PTFE spacer are not needed. The steps in making the tips are shown overleaf. The actual tips are 20mm lengths of 1mm diameter nickel plated steel wire, cut from a large paper clip. You might like to make a second pair of tips, fashioned in the same way but with longer lengths of wire – say 30mm The two ends of the case, with their drilling/cutouts to suit the three SMA sockets, USB socket and white LED. long – with a ‘crank’ in the centre to allow their tip spacing to be adjustable. This would be done simply by loosening their plug bodies and then rotating the tips as needed to set the tip spacing before tightening them again. Ground clip lead As mentioned earlier, a ground clip Here’s how it all goes together – the main PCB and the shield PCB screwed into their respective case halves. The SMA connectors and USB socket poke through the case ends. siliconchip.com.au September 2014  47 Fig.6: MAKING A BASIC PROBE USINGAN ANSMA SMA PLUG MAKING A BASIC PROBE TIPTIP USING (SCALE: 2x ACTUAL SIZE) BODY OF SMA PLUG CENTRE CONTACT 20mm LENGTH OF 1mm DIAM. NICKEL PLATED STEEL WIRE (CUT FROM A LARGE PAPER CLIP) GROUND TO A POINT AT FAR END 1 THE FOUR COMPONENTS YOU’LL NEED (SMA PLUG’S CRIMPING SLEEVE & PTFE SPACER ARE NOT NEEDED) 9.0mm LONG SECTION OF 3.0mm OD, 1.0mm ID PTFE DIELECTRIC FROM A LENGTH OF COAXIAL CABLE Close-up of ground clip construction. 2 APPLY FLUX TO THE BLANK END OF THE WIRE, PUSH IT INTO THE REAR OF THE SMA PLUG’S CENTRE CONTACT AND SOLDER. The close-up photograph above shows the idea. By the way you don’t have to make the ground clip lead particularly short, because its inductance is not critical when you are using a differential probe. So feel free to make it any convenient length. Other Uses 3 WHEN IT HAS COOLED, PUSH THE CENTRE CONTACT AND WIRE INTO THE REAR OF THE SMA PLUG’S BODY UNTIL THE 0.8mm DIAMETER CENTRE PIN EMERGES FROM THE FRONT CENTRE OF THE INSULATING PLUG BY 2.0mm AND ITS WIDENING SHANK JUST BECOMES VISIBLE 4 FINALLY, PUSH THE LENGTH OF DIELECTRIC DOWN THE WIRE AND INTO THE REAR OF THE SMA PLUG’S BODY AS FAR AS IT WILL GO. YOUR PROBE TIP WILL NOW BE COMPLETE. 48  Silicon Chip matter which one). Then a 3mm hole is drilled in the centre of the flat sections of the clamp, so a 6mm long M3 screw and nut can be used to attach the solder lug of the ground lead, while at the same time fastening the clamp to the plug ferrule. + INPUT x10 – INPUT x1 lead is often not necessary when you are using a differential probe of this kind. However, you might like to make one up, so it will be available in situations where you may need it – or at least to see if it has any effect. An easy way to make a suitable clip lead is to connect a suitable clip to one end of a length of flexible insulated hookup wire and then fit a small solder lug to the other end. The solder lug can then be attached securely to a small clamp made of thin brass sheet and bent into a ‘P’ shape with an inner loop diameter of 4.5mm, so it will slip over the ‘crimp ferrule’ of one of your probe tip plugs (it doesn’t A differential probe can also be handy for measuring signals which are relative to other voltages in a circuit. Both signals must be within the probe’s common mode input range and given that the probe is AC-coupled, you will only get the AC component of that signal. For example, if you have a circuit with a signal that’s relative to a ‘half supply’ rail, there may be ripple or signal injected into this rail. So using the differential probe would allow you to see the signal with this unwanted component removed. Many scopes can perform this function using ‘math’ mode but that requires the use of two of your precious scope inputs and the result is generally a lot better when the subtraction is performed in the analog domain. With this method, the circuit ground can remain earthed, allowing easy simultaneous measurement of the signal. ACTIVE DIFFERENTIAL PROBE FOR OSCILLOSCOPES SILICON CHIP www.siliconchip.com.au SC OUTPUT TO SCOPE (50 ) 5V DC POWER Fig.7: same-size front panel(FRONT artwork to photocopy and glue to the PANEL ARTWORK) hand-held instrument case for a professional finish. siliconchip.com.au