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Add a digital ’scope to your test bench for the price of a large pizza! By PETER SMITH Do you own a computer with a sound card? If you do, then all you need is this simple project, a little spare time and some free software to build your own ultra-low cost digital oscillo­scope – and more. The sound card in your computer is useful for a lot more that just recording and playing audio tracks. With the right software, you can have a virtual electronics lab full of digital test & measurement tools that won’t crowd your bench or break the bank! Sounds too good to be true? Admittedly, the sound card is an audio device, so the “virtual” test instru58  Silicon Chip ments will be limit­ed to work within the audio spectrum. They also lack some of the goodies that are available on their physical counterparts, such differential inputs and direct (DC) coupling – but the price is right! This project will enable you to use your PC as a digital oscilloscope, spectrum analyser and signal generator. Other more specialised instruments are also available in software form, such as signal processors, loudspeaker analysers and enclosure design­ ers, radio demodulators and decoders, and so on. If you work in education, are new to electronics or would simply like to learn about digital instruments, then this project is for you. A sound background In basic terms, a PC sound card provides an interface bet­ween the analog world and the digital internals of a PC. Signals appearing on the sound card inputs are first coupled to an analog multiplexer/mixer and then piped to an A-D (analog-to-digital) converter. Depending on your application www.siliconchip.com.au Fig.1: simplified block diagram for a typical PC sound card. software, the resultant “stream” of digitised data from the A-D converter may be further manipulated (filtered, enhanced, etc), transported elsewhere (eg, to the Internet) or just saved as a file to the hard disk. During playback, the reverse process occurs. The digitally encoded audio data is converted back to analog format by the sound card’s D-A converter, then filtered, amplified and fed to the loudspeaker and/or line output sockets. For the technically curious, a simplified block diagram of a typical sound card is shown in Fig.1. As you can see, there’s a little more to it than we’ve described. Analog and digital audio from a range of sources can be mixed and level-shifted along both the input and output signal paths. Software-based instruments that provide stimuli, such as sound generators, utilise the sound card’s D-A converter and analog output circuitry. Generally, sound card outputs can di­ rectly drive external circuitry, so no additional hardware is required. By contrast, instruments that need to acquire data, such as oscilloscopes, do so via the sound card’s analog input circuitry and its A-D converter. Software is then used to interpret the digital data stream and generate a graphical waveform display similar in appearance to conventional CRT-based (analog) oscillo­scopes. All that’s left to do then, is to apply the signals to be examined to the sound card’s inputs in suitable form. And that’s where the hardware part of our project comes in. Getting attached This simple adapter circuit provides a simple oscilloscope-like interface between the signals we wish to measure CHOOSING SOFTWARE This adapter circuit is basically designed to allow you to connect test probes to your PC’s sound card. Once the signals are in, software does the rest. There are many digital instrument software packages avail­able via the Internet, either as freeware or shareware. Our feature article on page 7 has a rundown on the some of the more popular packages that you can use. www.siliconchip.com.au August 2002  59 Parts List 1 PC board, code 04108021, 125mm x 62mm 1 plastic instrument case, 129 x 67 x 42mm (L x W x H) (Altronics H-0203) 2 single-pole 12-position PCmount rotary switches (S1, S2) 2 knobs to suit above 1 M205 500mA fast-blow fuse 1 M205 in-line fuseholder (DSE P-9962) 4 M3 x 10mm pan head screws (to attach shield) 8 M3 nuts 11 M3 flat washers 1 M3 star washer 1 M3 solder lug 1 2m length medium-duty figure-8 cable 1 80mm length light-duty hook-up wire 1 75mm length (approx.) tinned copper wire for links 1 PC board pin (“matrix” pin) Semiconductors 2 TL071CP JFET-input op amp ICs (IC1, IC2) 1 TC7660HCPA (Microchip Technology) or ADM660N (Analog Devices) 120kHz voltage inverter IC (IC3) (Farnell 703-655) 2 1N751A 5.1V 0.5W zener diodes (ZD1, ZD2) 4 1N4148 small-signal diodes (D1 - D4) 1 3mm high-efficiency red LED (LED1) and the line input on the sound card. Although we could connect our test probes directly to the sound card’s input, we’d be limited to measuring signals of just 0-2V peak. Not only that, but the card’s input would “load down” high impedance circuits such as op amp inputs and the like. To overcome these problems, the adapter provides a fixed high (1MΩ) input impedance, as well as a 6-stage attenuator to allow signals of up to 10V peak to be measured. And with a x10 oscilloscope probe, the range is extended to 100V peak. In addi­tion, an op amp stage amplifies the input by a factor of 10, thereby significantly improving the 60  Silicon Chip Capacitors 1 220µF 16VW PC electrolytic 2 100µF 16VW PC electrolytic 2 10µF 16VW SMD tantalum (surface mount) 2 0.1µF 100V MKT polyester 4 0.1µF 50V monolithic 2 56pF 50V ceramic 2 18pF 100V ceramic (Farnell 236-950) Resistors (0.25W, 1%) 2 1.5MΩ (Farnell 336-701) 2 1MΩ 2 3kΩ 2 200kΩ 2 1kΩ 2 150kΩ 2 470Ω 4 100kΩ 1 330Ω 2 27kΩ 2 100Ω 4 20kΩ 2 10Ω Connectors 2 horizontal PC-mount BNC sock­ets (Altronics P-0529) 1 3.5mm sub-miniature PC-mount stereo socket (Altronics P-0096) 1 2.5mm PC-mount DC socket (Altronics P-0621A) 1 2.5mm cable-mount DC plug 1 15 pin male ‘D’ connector with backshell Miscellaneous Shielded stereo cable for connection to sound card (3.5mm plug to 3.5mm plug); 125 x 62mm sheet of stiff cardboard/elephantide or lightgauge aluminium for shield (see text); osc­illoscope probes. signal-to-noise ratio when measuring low-level signals. How it works Fig.2 shows the complete circuit diagram of the adapter. There are three main sections, labelled “Channel 1”, “Channel 2” and “Power Supply”. As the two channels are identical, we’ll only describe channel 1. Signals applied to the BNC connector (CON1) are AC-coupled to the input circuitry via an 0.1µF capacitor. A string of resis­tors to ground along with an 18pF capacitor provides the neces­sary high input impedance (1MΩ). In conjunction with rotary switch S1, these resistors also function as a voltage divider for input signal attenuation. In all, six ranges are provided, with the topmost position passing the signal through to op amp IC1 without attenuation. To protect the op amp (and therefore the sound card) input, signal levels are clamped by D1 and D2 to within 0.6V of the positive and negative supply rails. The 1kΩ resistor shown to the left of the diodes limits the current through D1 and D2, while the 470Ω resistor limits the current into the op amp’s non-in­verting input (pin 3). Zener diodes ZD1 and ZD2 also form part of this protection scheme. Because the impedance of the supply rails is quite high, they could easily be driven above their nominal values by a large input excursion. ZD1 and ZD2 prevent this from happening by breaking down above 5.1V. This scheme also protects the inputs when power is not applied to the adapter. Input protection is limited to ± 100V maximum. This allows for times when you are measuring a level above 10V using the x10 attenuation of your probe but forget to slide the atten­ uation switch from x1 to x10. Don’t be tempted to poke around in high voltage equipment (live mains circuits, for example) – you will certainly “smoke” the adapter and perhaps your PC and your­self into the bargain! Op amp IC1 (TL071) is a high input-impedance, low-distor­t ion amplifier designed for audio work. In this circuit, it is configured for a gain of 10, with a frequency response of about 100kHz. The 100Ω resistor in series with the output provides short circuit protection and isolates the op amp from the cable and sound card input capacitance. To keep costs to a minimum and eliminate the need for yet another plugpack, we decided to power our project directly from the PC’s +5V supply rail. As luck would have it, the +5V rail is accessible via the sound card’s joystick port connector, usually situated right beside the audio input/ output sockets. Power enters the adapter via a standard 2.5mm DC socket. A little “brute-force” filtering is then applied using a 220µF ca­pacitor followed by a low-pass RC filter formed by the combina­tion of a 10Ω resistor and a 100µF capacitor. www.siliconchip.com.au Fig.2: this is the complete circuit diagram for the adapter. It consists of two switched attenuator channels which drive op amp output stages IC1 & IC2. Power (+5V) comes from the PC games port, with IC3 (a charge-pump voltage inverter) generating a -5V rail. www.siliconchip.com.au August 2002  61 now slide all the way into the case. That done, you can complete the case preparation by drill­ing and filing the required holes in the lid and sides. The easiest way to get everything to line up properly is to photocopy the templates in Fig.6, cut them out and tape each one to the indicated faces of the case. You can then centre-punch directly through the templates to get accurate targets for drilling. Always start with a small drill size and work up to the required size in several stages. The larger holes can be finished off using a tapered reamer. Board assembly Fig.3: follow this diagram when installing the parts on the PC board. Note that the two 10µF SMD (surface mount) capacitors adjacent to IC3 are installed on the copper side of the board, as shown in one of the photos. The TL071 op amps (IC1 & IC2) require both positive and negative supply rails. The negative rail is obtained by inverting the +5V rail using a charge pump voltage inverter (IC3). We chose a TC7660H device for this job because its 120kHz switching fre­quency is well above the audio spectrum. In addition, we’ve used surface-mount capacitors in the pump circuit to reduce radiated noise that could otherwise easily find its way into the high im­pedance attenuation networks. The -5V (nominal) output on pin 5 of the inverter is cleaned up using a second low-pass filter, which removes most of the ripple and noise. Finally, high frequency decoupling of the 5V rails is provided using four 0.1µF ceramic capacitors. Preparing the case Before mounting any components on the PC board, you will need to perform some minor surgery on the case internals (assum­ing that you are using the recommended case). Initially, the PC board should fit neatly inside the lip of the case but will rest on top of the integral guides. If it’s a little oversized, then trim the board to fit using a fine file. Next, cut away all of the guides with sidecutters or a sharp knife so that you’re left with reasonably smooth internal surfac­es. The PC board should Table 1: Typical PC Sound Card Specifications Frequency response ..................................................................20Hz - 20kHz Signal to noise ratio ...............................................................................>90dB Total harmonic distortion .........................................01% <at>1VRMS into 10kΩ Line-in impedance ...................................................................................47kΩ Line-in sensitivity .................................................................................. 2V P-P CD audio-in impedance ...........................................................................50kΩ CD audio-in sensitivity .......................................................................... 2V P-P Microphone-in impedance .......................................................................600Ω Microphone-in sensitivity ..........................................................10-200mV P-P A-D & D-A resolution .............................................................................16 bits Sample rate ........................................................................................ 4-48kHz Output power (speaker-out) ........3W into 32Ω (6W into 4Ω on some mod­els) 62  Silicon Chip Using the overlay diagram of Fig.3 as a guide, begin by installing the three wire links and all the resistors. Follow with the capacitors, noting that the electrolytic types are polarised and must be oriented as shown. The two 10µF tantalum capacitors are miniature surface-mount devices that need to be mounted on the solder (copper) side of the board. The mounting area must be well tinned, clean and free of excess solder. Position the banded (positive) end as shown in Fig.3 and solder the device in place using a fine-tipped iron. After soldering, use your meter to check for solder bridges between pads, as they can be difficult to spot with the naked eye. Install the diodes (D1-D4, ZD1, ZD2) next, aligning the cathode ends (mark­ ed with a band) as shown. IC1, IC2 & IC3 can go in next and again, orientation is important. These are static-sensitive devices, so it’s a good idea to wear an earthed antistatic wrist strap and to use a soldering iron with an earthed tip. Once they’re in, install the four connectors (CON1-4) and the GND pin. Before soldering, ensure that they’re seated squarely against the surface of the PC board. The two rotary switches (S1 & S2) are next on the list. Before installation, they need to be reconfigured to limit their rotation from the default of 12 positions to just six positions. To do this, remove the nut, washer and locking ring. Notice how the tab on the locking ring can be inserted into one of 10 holes, numbered 2-11. Re-insert the tab in the number “6” hole and check that you have six possible shaft positions. Repeat this procedure for the secwww.siliconchip.com.au This is the completed PC board assembly, ready to be attached to the lid of the case. Note the metal shield which is mounted on the copper side of the board using machine screws and nuts. The inset at top left shows how the two 10µF SMD capacitors are installed. ond switch and then solder them into position. Once again, make sure that they are seated firmly against the PC board surface. The last component to be mounted is LED1 (the power indica­tor). Slip the LED into place with the flat (cathode) side aligned as shown in Fig.3 but don’t cut the leads short or solder it just yet. Next, remove the nuts and washers from the rotary switches, leaving the locking rings in place, and fit the case lid. That done, turn the assembly upside-down and manoeuvre the LED into its hole in the lid. Ideally, the shoulder of the LED should be slightly proud of the inside surface of the lid. Now solder and trim the leads. hold on the cable to prev­ent stress on the solder joints. Making the power cable Testing the power cable Fig.4 shows the wiring for the power cable. You can see that we’ve opted to fuse the +5V rail right at the source, using an in-line fuse. This provides an extra measure of safety should the tip of the DC plug accidentally contact something that it shouldn’t! To protect the cable and provide effective strain relief, use a couple of layers of heatshrink tubing or insulation tape on the cable at the point where it passes through the backshell clamp. The clamp needs to have a firm Don’t be tempted to skip this step! Before connecting the cable, use your multimeter to verify that the positive and nega­ tive wires are not shorted together. Next, plug the cable into the joystick port and with your multi­ meter set to “DC Volts”, carefully measure the voltage at the DC plug. The tip (or “cen­tre”) of the plug should measure +5V (±0.25V) with respect to the outer shell. If you measured +3.3V instead of +5V, then unfortunately you have one Table 2: Resistor Colour Codes             No. 2 2 2 4 2 4 2 4 1 2 2 www.siliconchip.com.au Value 1.5MΩ 1MΩ 200kΩ 100kΩ 27kΩ 20kΩ 3kΩ 470Ω 330Ω 100Ω 10Ω 4-Band Code (1%) brown green green brown brown black green brown red black yellow brown brown black yellow brown red violet orange brown red black orange brown orange black red brown yellow violet brown brown orange orange brown brown brown black brown brown brown black black brown 5-Band Code (1%) brown green black yellow brown brown black black yellow brown red black black orange brown brown black black orange brown red violet black red brown red black black red brown orange black black brown brown yellow violet black black brown orange orange black black brown brown black black black brown brown black black gold brown August 2002  63 losses in the voltage inverter circuitry and the ±5% margin on the +5V rail, the negative rail should fall within approximately -5V to -4.55V. Finally, rotate S1 and S2 to position “6” (fully clockwise) and measure both op amp outputs. They should be with­ in a few millivolts of the ground rail. Shield’s up Fig.4: these diagrams show how to make the power supply cables. Note that the cable at right is only necessary if your games port supplies +3.3V instead of +5V. of the few late-model cards that provide this lower, non-standard voltage on the game port connector (so much for backward compatibility!). In this case, you will need to delve into your PC’s internals to get access to the +5V rail. A spare disk drive power connector is a convenient connection point. Fig.4 also shows the wiring details for this alternate power supply connection scheme. Basic checks Before we’re ready to connect the stereo cable and launch the software, we need to perform a few quick DC voltage checks on the completed board. The following measurements are all with respect to ground. Simply connect the negative lead of your multi-meter to the ground point provided by the PC board GND pin (between CON2 & CON4) and use the positive lead to make each measurement. Apply power and check that you have +5V (±0.25V) at pin 8 of IC3, pin 7 of IC1 and pin 7 of IC2. Next, check for -5V at pin 5 of IC3, pin 4 of IC1 and pin 4 of IC2. Note that with the The metal shield is exactly the same shape and size as the PC board. It can be made from a thin sheet of tinplate or by gluing aluminium foil to a piece of stiff cardboard or elephantide insulation material. 64  Silicon Chip The adapter’s high input impedance makes if susceptible to radiated noise in its immediate environment. Typically, the 240V AC mains and your PC’s monitor are the worst noise generators. To minimise noise pick-up, the adapter could be installed in a metal case but to keep costs to a minimum, we’ve presented the finished project in a plastic instrument case instead. We achieved good results without a metal enclosure by fitting a shield (or “ground plane”) to the underside of the PC board. The shield is exactly the same dimensions as the PC board and can be fashioned from a variety of materials. We glued ordi­nary heavy-duty aluminium cooking foil to one side of a sheet of elephantide material and then cut out the required shape with kitchen scissors. Any thin conductive material should be suitable but ideally, it should be insulated on one side so as not to short protruding component leads to ground. An old scrap of blank single-sided PC board material would also be a good choice. To fix the shield to the underside of the board, first insert an M3 x 10mm screw in the corner hole closest to IC1. This screw will be used as the ground connection point, so place a star washer and solder lug under the head before winding up a nut from the copper side of the board. That done, fit screws and nuts to the remaining three corners, then invert the board and place flat washers on all four screws. Next, with the conductive side facing away from the PC board, slide the shield over the screws (you remembered the holes, right?), install another four flat washers and wind on the remaining nuts. Make sure that all component leads are well clear of the shield and use your meter to verify that the shield makes good electrical contact with the lug. To finish the job, connect the solder lug to the ground pin (between www.siliconchip.com.au The DC power socket and the output socket (for the sound card) are accessed through matching holes in the rear panel. CON2 & CON4) using a short length of light-duty hook-up wire. Signal generator cable Well, that completes the hardware that you’ll need to use with the oscilloscope and spectrum analyser software. If you’d also like to use the sound generator included with many software packages, then the only additional requirement is a simple cable – see Fig.5. All analog signals from your sound card are AC-coupled to their output sockets, hence the need for the termination resis­tors. Be sure to insulate all connections and use insulated crocodile clips or probes. Quantifying measurements Most digital instruments provide some degree of input level (gain/attenuation) selection. Add to this the range switches on the adapter, and it can all seem a little confusing! Just how do you determine the magnitude of your measurements? The 2V positions on the adapter’s range switches pass the measured signal without any change in level. Ranges below this point provide amplification (gain) of the input signal, whereas ranges above provide attenuation. The table included on the front panel (see Fig.6) lists a multiplier, or scale factor, that can be used to calculate the actual signal level. For example, with 8.5V input to the adapter and a switch position of 10V, the voltage applied to the sound card input will be 8.5 x 0.2 = 1.7V. Let’s try that in reverse. If your digital oscilloscope is set to 500mV/div and the waveform peak measures 1.5 divisions, then the voltage at the sound card’s input must be 750mV. So, if the Fig.5: this cable can be used if you’d also like to use the signal generator instrument included with many software packages. MINI SUPER DRILL KIT IN HANDY CARRY CASE. SUPPLIED WITH DRILLBITS AND GRINDING ACCESSORIES $61.60 GST INC. www.siliconchip.com.au August 2002  65 The completed adapter is shown here fitted with two oscilloscope test probes, plus the power supply and sound card cables. adapter range switch is set to 500mV, then the actual applied voltage is 1/4 x 750mV = 187.5mV (or 132mV RMS). Note that if you set your oscilloscope to read 2V/div, then the adapter switch positions now directly reflect what you see on the screen. With the adapter switched to 200mV, you’re reading 200mV/div; and at the 500mV setting, you’re reading 500mV/div, etc. Digitally accurate? It’s important to be aware of the limitations of your new digital instruments before relying on them for serious work. In practice, the resolution and The PC board is installed by first inserting the BNC connectors through their holes and then flexing back the rear of the case slightly as the back of the board is lowered into position. 66  Silicon Chip accuracy of any measurement system that relies on a sound card depends on the characteristics of the card itself. Table 1 lists the specifications of a typical sound card. The frequency response of the card will also be the band­ width of the digital instruments (’scope, multi­ meter, spec­trum analyser, etc). This assumes that you’ve set the sampling rate to maximum (usually either 44kHz or 48kHz). This also means that it you measure signals above 22kHz, the results will be inaccurate. That’s because the sam­pling rate must be at least double the signal frequency. A sound card’s 16-bit A-D converter can measure 65,535 dis­crete voltage levels, so with a 2V span it has low µV resolu­tion. However, this doesn’t mean that your digital instruments will be able to measure signals in the µV range! In practice, the PC power supply, sound card, cable and adapter all add a certain amount of low-level noise (called the noise “floor”), so that the smallest voltage you’ll be able to measure accurately will be in the mV range. Our prototype showed less than www.siliconchip.com.au 1mV RMS noise but this will almost certainly be different on your system. Most software includes at least rudimentary calibration for the line-in socket. You’ll need a sinewave signal generator and multimeter for some, while others utilise their inbuilt digital signal generators and a line-out to linein loop cable for the task. Be sure to check the documentation for details, as methods vary considerably. If you wish, you can include the adapter in the signal loop during calibration to improve overall accuracy. Be sure to set the rotary switches to the 2V positions for 0dB gain. The maximum voltage that can be applied to the sound card’s line-in socket is 2V P-P, or about 1.4V RMS. In practice, we found that our Sound­ Blaster Live card began clipping at just over 1V RMS. To ensure accurate measurements, it’s a good idea to use the ’scope to check for clipping before switching to other in­struments such as the spectrum analyser. Staying alive To wrap up, a word of warning about measurement techniques is in order. Be aware that the ground (0V) line of a PC’s power supply is connected to mains earth. Because the adapter is effec­tively an extension of the PC circuitry, it’s BNC connectors are also at mains earth potential. This doesn’t cause a problem if the circuit you’re probing is floating (ie, isolated from earth). If, however, the circuit has a return path to earth, then be sure to connect your probe’s ground clip to a point that’s also at earth poten­tial. If the chosen point is above earth potential, then current will flow around an earth “loop”. If the potential difference is high, the results can be disastrous! A good example is the prim­ary side of any off-line switchmode power supply. Connecting a probe ground clip to most points in one of these suckers will generate more fireworks than New Year’s Eve on the Sydney Harbour Bridge! Some readers would undoubtedly point out that this problem could be overcome by floating either the circuit under test or the test equipment itself (eg, by lifting the earth or by using an isolation transformer). Our advice is simple – don’t do it! Seek advice from an experienced technician if you’re not sure what you’re SC doing! www.siliconchip.com.au Fig.6: here are full-size artworks for the front panel (top), the front and rear panel drilling templates and the PC board pattern. August 2002  67