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Roadies’ by John Clarke Test Signal Generator This test oscillator is ideal for testing balanced and unbalanced inputs on professional sound equipment. It’s small, rugged, very portable and easy to use. It’s powered by a single cell and is built to withstand use in a ‘roadie’ environment. Its frequency is fixed, but the output signal level is adjustable. S ound reinforcement systems in public venues typically have a set of 3-pin XLR (eXtension Line Return) sockets providing a connection point for microphones. Instruments usually connect via a DI (Direct Input) Box or using an unbalanced lead. Over time, these connections can become unreliable or go faulty. Problems that can occur include bad connecting leads, poor XLR socket connections, broken wires or shorts. Finding where the problem is located may be difficult. That’s because the pathway from the XLR socket to a mixer can be long and can pass through separate patch boxes before finally making its way to a mixer. There are many ways of tracing faults. You can simply use a microphone or instrument as a signal source and test for sound from the loudspeakers or headphones at the mixer. But then you need to have somebody standing there speaking into the microphone or playing the instrument while you trace the fault; not exactly ideal. It’s much easier to use a test oscillator as the signal source. This oscillator provides a signal level that is constant and continuous. That makes it easier to get on with the job of finding the trouble spot. Our Roadies’ Test Signal Generator is a small unit that’s powered from a lithium button cell. The housing is diecast aluminium so it can take 24 some punishment; the only exposed parts are the outlet socket and a potentiometer knob for adjusting the signal level. The oscillator output is around 440Hz (‘A’) – not so high that it’s irritating, but high enough that it can be clearly heard over background noise. There is no on/off switch as such, since it is switched on automatically when a jack is plugged in, as happens in much professional audio equipment. This eliminates the possibility that it can be accidentally left on after it is unplugged, or accidentally switched on when it is jostled, draining the cell of all its power. Two versions We have produced two versions of the Roadies’ Test Signal Generator. One uses surface-mount components so that the PCB is smaller and is housed in a more compact enclosure. If you prefer using throughhole components instead, then you can still build it; but you will need to use a larger case. Practical Electronics | June | 2021 Features and specifications Rf Rin C1 C1 C1 • Generates 440Hz sinewave at 0-1.2V RMS (adjustable) • Single-ended or impedance-balanced output via a 6.35mm jack socket • Auto on/off switch • Powered by a lithium button cell • 60 hours of use from a single cell (3.5mA current draw when on) • Compact and rugged • Easy to build (two versions depending on constructor skill level) IC1 R1 R1 R1 TRADITIONAL PHASE-SHIFT OSCILLATOR Fig.1: a traditional phase-shift oscillator uses three RC high-pass filters in the feedback loop of an op amp (or similar amplification device) with sufficient gain for oscillation to start up and then be maintained, but not so much gain that the output becomes squared off. Circuit basics The circuit uses a simple phase-shift oscillator based on op amps. These op amps can run from 1.8-6V and have a railto-rail output, so they are ideal for use with a 3V cell. They can provide a sufficient output signal level of around 0.7V RMS, even when the cell has discharged to 2V. Fig.1 shows the configuration of a typical phase-shift oscillator. This often uses three identical resistor-capacitor (RC) highpass filters, in conjunction with an inverting amplifier IC1. The gain of the inverting amplifier is made sufficient so that oscillation will start at power-up and is maintained. With the correct amount of gain, the op amp output signal is a sinewave. Too much gain will cause the op amp to produce a squared-off waveform, with the tops of the sinewave clamped at the op amp maximum output. So these oscillators require the gain to be calibrated for correct operation. That can be troublesome, especially when the supply voltage changes, as can happen in a batterypowered oscillator. The phase-shift oscillation frequency is given by: 1 / (√6 × 2 × R1 × C1) Circuit details The complete circuit is shown in Fig.2. The oscillator section is the components around IC1a at upper-left. You can see that this is a little different than what is shown in Fig.1; we are using RC low-pass filters and the amplifier is not set at a predetermined gain. Instead, it is operated in open-loop mode, providing the maximum gain available from the op amp. This means that the gain is more than sufficient for oscillation to start and to be maintained. The op amp output swings fully to the supply rails, so the waveform at IC1a’s output is almost a square wave. But there is a sinewave at the inverting input of op amp IC1a (pin 2), as this is the output signal after passing through the three low-pass filters. This is the reason for choosing low-pass filters instead of high-pass. Oscillation normally stabilises at a frequency when there is a total phase shift of 180° through the three filter stages. This, along with the 180° phase shift provided by inverting amplifier IC1a, gives the overall 360° shift required for oscillation. Anyway, that’s the theory; but in our circuit, the frequency is lower than expected. For our circuit, the theoretical oscillation frequency is √6 / (2 × R × C), where R is 6.8kΩ and C is 100nF. In this case, √6 is in the numerator and not denominator due to our use of low-pass filters. This works out to 573Hz. However, we measured the actual oscillation frequency at 448Hz, and simulation shows that S1 Vcc (3V) 470 10k Vcc/2 100nF 6.8k 100nF 6.8k 100nF 6.8k 100nF 8 2 3 IC1a 1 4 D2 1N4148 Vcc/2 ~440Hz LED K 10k A A A K K A LED1 10k 6 3V BATTERY D1 1N4004 A  K D3 1N4148 IC1b 7 S1: MICROSWITCH OPERATED VIA CON1 1 F VR1 10k LEVEL 3(5) 8 2(6) IC2a (IC2b) 1(7) 150 CON1 (6.5mm JACK SOCKET) 1nF Vcc/2 1N4004 K A 100nF K A K 100 F IC1, IC2: MCP6002 OR MCP6272 5 180k 1N4148 POWER 100 F 1k POWER NOTE: IC SECTIONS AND PIN NUMBERS IN BRACKETS ARE FOR THROUGH-HOLE VERSION 10k RING 10k TIP SLEEVE 5(3) 6(2) IC2b (IC2a) 7(1) 150 CHASSIS 4 SC ROADIES’ Roadies’ TestTEST Signal Generator SIGNAL GENERATOR 2020 Fig.2: our circuit uses a slightly unusual phase-shift oscillator with three low-pass filters in the feedback path and diodes D2 and D3 to limit the output swing to around 1.4V peak-to-peak. The signal is taken from input pin 2 of IC1a, as this is a sinewave, and amplified by op amp IC1b before being attenuated by VR1 and then fed to output socket CON1. Practical Electronics | June | 2021 25 The XLR-to-6.35mm lead we made up to suit this project (see Fig.8) also serves to turn it on and off: a tiny microswitch is activated when ever the plug is inserted in the socket. Scope1: this is the output waveform with VR1 adjusted so the output just started clipping. It measures 448Hz and 1.0V RMS. The waveform is a relatively clean, undistorted sinewave. IC2b (IC2a in the through-hole version) provides a buffered Vcc/2 output, also via a 150Ω resistor. This connects to the ring terminal of the jack socket. When there is no signal, with VR1 wound fully anticlockwise, both the tip and ring are at Vcc/2. Since the whole circuit is powered from a 3V cell, it floats with respect to any outside reference voltage, so this voltage can be grounded within the equipment being fed. it is nominally 435Hz (the difference can be explained by component variation). The LTspice circuit simulation file we used to determine this is available for download from the June 2021 page of the PE website. The discrepancy between these figures and the calculated 573Hz value is due to IC1a switching into full output satuBalanced and unbalanced connections ration, which slows down its low-to-high and high-to-low Oscilloscope trace Scope1 shows the output waveform with transitions, as it takes extra time for the op amp to come VR1 adjusted so the output just started clipping. It measures out of saturation. 448Hz and 1.0V RMS. The waveform is a relatively clean, The signal level from IC1a is clamped to a nominal ±0.6V undistorted sinewave. about half supply (Vcc÷2) by back-to-back diodes D2 and D3. The output is impedance-balanced, ie, the ring terminal The 1kΩ resistor limits the current from the op amp output impedance is the same as the tip output impedance. It is when the diodes conduct. not a true balanced output where the tip and ring have This arrangement provides a relatively constant signal complementary signal swings. level regardless of changes in the supply voltage. That However, the impedance-balanced output still provides can vary from 3V with a new cell, down to 2V when it is good common-mode signal rejection at receiving equipdischarged. ment, cancelling noise and hum pickup that’s common in The half-supply rail (Vcc/2) is formed by a 10kΩ/10kΩ both balanced leads. voltage divider across the supply, bypassed with a 100µF For unbalanced lines, the ring connects to the sleeve and capacitor. The non-inverting input to IC1a is also tied to this so the signal is from the tip connection. More information Vcc/2 supply. The signal therefore swings above and below on this configuration is available at: http://bit.ly/pe-jun21-bal this reference voltage. For a balanced connection to the test signal oscillator, With a nominal 1.2V peak-to-peak swing from pin 1 of IC1a, ideally you should have a lead with a stereo jack plug at after passing through the filters, we get a 78mV peak-to-peak one end and an XLR at the other. signal at pin 2 of IC1a. This is amplified by a factor of 19 by op amp IC1b, giving 1.48V peak-to-peak, or 525mV RMS. The signal is then AC-coupled to level control potentiometer VR1. The lower portion of VR1 connects to the Vcc/2 reference, so that there is no DC voltage across the potentiometer. IC2a (IC2b in the through-hole version) amplifies this by a factor of two, so the maximum output can be up to 1.2V RMS, with just over 1V RMS available before clipping. This signal goes to the tip terminal of the jack socket. Note IC2a’s (IC2b) output includes a series 150Ω resistor for isolation, so that the op amp isn’t prone to oscillation with capacitive loads. That’s extra protection for the already stable op amp (MCP6002), which has a typical 90° phase margin with a resistive load and a The through-hole PCB mounts upside-down on the diecast case lid... which 45° phase margin with a 500pF capaci- becomes the base! Its power LED, output socket and level control all poke tive load. If the MC6272 is used instead, through holes drilled in the side of the case. The panel label can be used as a the resistive load phase margin is 65°. template for hole locations. 26 Practical Electronics | June | 2021 LED1 470 100 F CR2032 10k SILICON CHIP BUTTON CELL HOLDER 100 F CON1 01005201 C 2020 REV.B 10k 1 IC2 CUT OFF + 150 150 S1 VR1 GND TEST OSCILLATOR A k TOP OF SMALL PCB BOTTOM OF SMALL PCB 3x 100nF 10k 3x 6.8k 180k 10k 1k 4004 1 IC1 4148 2x 100nF 4148 10k 1 F D2 D3 D1 Fig.3: here’s the PCB overlay diagrams for both top and bottom of the SMD version PCB, with a matching photo (of the top side) which also shows the microswitch to turn power on when the 6.35mm plug is inserted. Note the area of the 6.35mm socket which must be shaved off to clear the button cell holder (in red). Also shown is the case with the short ground lead in place – this is essential to prevent hum when you touch the case. It connects to the ‘GND’ terminal on the PCB. 1nF The jack tip should connect to pin 3 on the XLR, and the ring to pin 2. The sleeve would connect to the pin 1 of the XLR plug. Such cables are readily available or you can make one up, as per Fig.8. For an unbalanced output, a mono jack plug to mono jack plug lead can be used. This automatically connects the ring to the sleeve within the jack socket. As mentioned earlier, power is from a 3V button cell. Diode D1 provides reverse polarity protection as the diode will conduct with the cell inserted backwards. This can usually only happen if the cell holder itself is fitted the wrong way around on the PCB. Construction The smaller SMD version of the Roadies’ Test Signal Generator is built on a PCB coded 01005201 which measures 47 × 47mm. This mounts in a 51 × 51 × 32mm diecast aluminium box. The through-hole version is built on a PCB coded 01005202 which measures 86.5 × 49.5mm. It fits in a diecast box measuring 111 × 60 × 30mm. Figs.3 and Reproduced by arrangement with SILICON CHIP magazine 2021. www.siliconchip.com.au The smaller SMD version is held in place by its input socket and level control, with a hole drilled through the case for the power LED to poke through. The panel label can be used as a template for hole locations. Also shown here is the card ‘insulator’ to ensure none of the components or solder joints can short out to the case. Any type of card, or even thin plastic, is adequate. Practical Electronics | June | 2021 4 are the PCB overlay diagrams for the two versions. Both boards are available from the PE PCB Service SMD version assembly For the surface-mount version, many of the parts are on the underside of the PCB. In this case, begin construction by installing the SMDs on both sides of the PCB. They are relatively large, so they are not difficult to solder using a finetipped soldering iron. Good close-up vision is necessary so you may need to use a magnifying lens or glasses to see well enough. Be sure that the ICs are oriented correctly before soldering all their pins. For each device, solder one pad first and check alignment. If necessary, adjust the component position by reheating the solder joint before soldering the remaining pins. If any of the pins become shorted with solder, solder wick can be used to remove the solder bridge. The capacitors are usually unmarked except on their packaging. The resistors are marked with a code as shown in the parts list. Diodes D1-D3 are through-hole parts. These are mounted and soldered form the underside of the PCB, with the leads trimmed flush on the top side. Take care to orient each correctly before soldering. Now move on to the combined assembly instructions below. Through-hole assembly For the through-hole PCB, start with the resistors and diodes, then fit the ICs, orientated as shown. We don’t suggest that you use sockets as the ICs could fall out if the unit is 27 SILICON CHIP MCP6002 A K LED1 10kW GND TEST OSCILLATOR 180k IC2 MCP6002 100nF 1 10kW 10kW 100mF 1mF NP 10kW IC1 150W S1 150W 470W 10k 6.8kW 1kW 6.8kW 100nF C 2020 REV.B 01005202 D3 + 100mF 4148 6.8kW CON1 D1 100nF 100nF CELL1 CR2032 D2 4148 4004 BUTTON CELL HOLDER 1.0nF 100nF 1 VR1 10kW Fig.4: and here’s the through-hole overlay and photo for those who aren’t comfortable soldering SMDs! dropped or kicked. Next, fit the MKT capacitors, which are not polarised. Combined assembly Now mount the electrolytic capacitors. Two of these are polarised, so they must be installed with the longer leads towards the + sign on the PCB. Next, mount the cell holder with the orientation shown, followed by potentiometer VR1 and jack socket CON1. Note that for the surface-mount version, a small section of the plastic case of the jack socket for CON1 needs to be cut off, so that it does not foul the cell holder. Fig.3 shows where to cut at 45°; this can be done with a sharp hobby knife. Switch S1 is a microswitch, mounted so that the lever is captured under the front ring contact of jack socket CON1. Before soldering it, check that the switch is open-circuit between its two outside pins when there is no jack plug inserted, and closed when a plug is inserted. The lever may require a little bending so that the switch works reliably. For the through-hole version, mount LED1 so its body is horizontal and located so that the centre is in line with the centre of the CON1 hole, as shown. Make sure the leads are bent so the anode (longer lead) is to the right. The surface-mount PCB has LED1 arranged vertically, with the top of the dome 21mm above the top of the board. Case assembly We are using the lid as the base of the case for both versions. This gives a better appearance and also means that we can replace the lid screws with M4 nylon screws (after tapping the holes to M4) to act as feet. Changing the cell requires removing the PCB. That’s not too difficult, and we don’t expect the cell will need changing for years with intermittent use. Expect over 60 hours of usage from a good cell. We have provided front-panel artwork for both versions and many of the drilling positions on the diecast boxes. These are shown in Fig.6 and Fig.7; they can also be downloaded as a PDF file from the June 2021 page of the PE website. The hole for the 6.35mm jack socket is 11mm, the potentiometer hole is 7mm and the LED hole is 3mm in diameter. The panel artworks show the positions. 3-PIN XLR PLUG 1 2 3 For the surface-mount version, the LED hole is on the top of the case. With this version, drill the holes at an angle so that the pot shaft and jack socket can be inserted more easily. The LED will need to clear the box edge without affecting its position. Countersinking the inside of the LED hole will make it easier to locate the LED as the PCB is inserted into the case. Both versions require a solder lug to ground the case. For the through-hole version, this is located on the lid but is away from the underside of the PCB. You need to drill a 3mm hole for this, plus four for the PCB mounting posts. The PCB is located centrally across the width of the lid, but the front edge is positioned so it is only 3mm back from the lid edge, so that the pot and jack socket are against the case edge when assembled. We used countersunk screws for the standoffs and solder lug screws, and if you do the same, these holes will require countersinking on the outside of the case. Add a star washer against the solder lug before tightening the nut. Then solder hookup wire to one end to the solder lug and solder the other to the GND terminal on the PCB. The through-hole version uses a GND PC stake fitted to the underside of the board to connect this wire. On the surfacemount version, the wire solders to the top of the PCB directly to the GND pad. The surface-mount version should have an insulator made from some stiff card added between the PCB and case lid (see Fig.5). This prevents possible shorting between the two. SLEEVE Fig.8: if you don’t have a jack plug to XLR cable, here SC is how to make one. Use shielded stereo or 2020 balanced microphone cable. 28 TIP RING 6.3mm STEREO JACK PLUG Practical Electronics | June | 2021 Parts list – Roadies’ Test Signal Generator Parts common to both versions Insulator template for surface mount PCB Fig.5: make this insulating panel from thick card and insert it between the SMD PCB and case lid. As mentioned, M4 nylon screws are ideal for mounting the lid. Tap each hole with an M4 tap before securing the lid with these screws. Alternatively, you could use the mounting screws supplied with the case, and add small stick-on rubber feet. Panel labels The front panel labels can be made using overhead projector film with the printing as a mirror image, so the print will be between the enclosure and film when affixed. Use projector film that is suitable for your printer (inkjet or laser) and glue with clear neutral cure silicone sealant. Squeegee out the lumps and air bubbles before the silicone cures. Once cured, cut out the holes through the film with a hobby or craft knife. The potentiometer shaft is held in place using its washer and nut, while the 6.35mm jack socket is secured using the supplied washer, plastic dress piece and dress nut. Testing and modifications You can test the oscillator using a multimeter set to measure AC volts and connected to the output between the tip and ring connections of a stereo jack plug. Note that the output can produce clipping if the signal level is near maximum, so bring the level back a little for a clean sinewave. The output frequency can be changed by altering the values of the three 6.8kΩ resistors in the low-pass filters or changing the values of the + Power (with jack plug inserted) SILICON CHIP Outlet Parts for surface-mount version 1 double-sided PCB coded 01005201, 47 x 47mm from PE PCB Service 1 diecast aluminium case, 51 x 51 x 32mm [Jaycar HB5060] 1 M3 x 6mm countersunk screw (solder lug mounting) 1 M3 nut and star washer Semiconductors 2 MCP6002-I/SN or MCP6272-E/MS op amps, SOIC-8 (IC1,IC2) [RS Components Cat 6283598 or 6674492] Capacitors (all 50V X7R SMD, 3216/1206 size) 1 1µF ceramic 5 100nF ceramic 1 1nF ceramic Resistors (all 0.25W SMD, 1% 3216/1206 size) 1 180kΩ (code 1803) 5 10kΩ (code 1002) 1 1kΩ (code 1001) 1 470Ω (code 4700) 3 6.8kΩ (code 6801) 2 150Ω (code 1500) Parts for through-hole version 1 double-sided PCB coded 01005202, 86.5 x 49.5mm from PE PCB Service 1 diecast aluminium box, 111 x 60 x 30mm [Jaycar HB5062] 4 M3 x 6mm pan head screws (PCB to standoffs) 5 M3 x 6mm countersunk screws (lid to standoffs and solder lug mount) 1 M3 nut and star washer 4 M3 tapped x 6.3mm standoffs 1 PC stake Semiconductors 2 MCP6002-I/P or MCP6272-E/P op amps, DIP version [RS Components Cat 403036 or 402813] (IC1,IC2) Capacitors 1 1µF 16V NP PC electrolytic 5 100nF MKT polyester 1 1.0nF MKT polyester Resistors (all 0.25W, 1%) 1 180kΩ 5 10kΩ 3 6.8kΩ 1 1kΩ three associated 100nF capacitors. Smaller values will provide a pro- HOLE SIZES: Power LED: .......3mm Outlet Socket: ...11mm Level pot: ...........7mm SILICON CHIP 1 470Ω 2 150Ω portionally higher frequency; larger values, a lower frequency. Roadies’ Test Signal Generator .. Power + (With jack plug inserted) Roadies’ Test Signal Generator Level 1 panel label (see text) 1 CR2032 PCB-mount button cell holder 1 CR2032 cell 1 6.35mm stereo switched jack socket (CON1) [Jaycar PS0195, Altronics P0073] 1 C&K ZMA03A150L30PC microswitch or equivalent (S1) [eg Jaycar SM1036] 1 9mm 10kΩ linear pot (VR1) 1 knob to suit VR1 4 M4 x 12mm nylon screws (for mounting feet – replace supplied case screws) 1 solder lug 1 90mm length of green hookup wire 1 1N4004 diode (D1) 2 1N4148 diodes (D2,D3) 1 3mm LED (LED1) 2 100µF 16V PC electrolytic capacitors + Outlet . . . . . . . . Level . + . . min max Fig.6 and Fig.7: front-panel artwork for both versions of the Roadies’ Test Signal Generator. As mentioned in the text, the artwork can be photocopied and used as a drilling template. (Download from the June 2021 page of the PE website.) Practical Electronics | June | 2021 29