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3-Spot Low Distortion Sinewave Oscillator This sinewave oscillator is ideal for testing audio equipment & loudspeakers. It provides three switch-selectable spot frequencies at 100Hz, 1kHz & 10kHz, with levels up to 2V RMS & less than 0.004% distortion. By DARREN YATES Sinewave oscillators are among the toughest circuits to get working well. There are many circuits around which use a couple of transistors and produce a sinewave with about 1% distortion which may be OK for some applications. However, when it comes to producing very clean (minimal distortion) sinewaves, the circuits really start to thin out. 60  Silicon Chip There are several reasons for this. Oscillators are basi­ cally amplifiers with positive feedback. For a square­ wave oscil­ lator, the basic rule is “more positive feedback, please!” but for sinewave oscillators, a more controlled method is required. Sinewave oscillators come in many shapes and forms but the one characteristic they have in common is that they require a precise amount of positive feedback to obtain the cleanest wave­ f orm possible. The most common sinewave oscillator circuit is probably the Wien Bridge configuration. An example of this type of circuit using an op amp is shown in Fig.1. As you can see, it uses two RC time constants to pro­vide positive feedback, one in series between the output and the non-inverting input (R1 & C1) and the other in parallel between the non-inverting input and ground (R2 & C2). These positive feedback components set the frequency of oscillation. In order for this circuit to oscillate, the theory states that it must have an overall gain of three, as set by the nega­ tive feedback components between the C1 R1 AMPLIFIER R2 C2 R3 LG1 Fig.1: typical Wien bridge oscillator circuit. The light globe (LG1) in the feedback network stabilises the output amplitude. output and the inverting input (R3 and L1). This would give a pure sinewave with no dis­tortion at all. But like most things in electronics, the perfect isn’t possible so in order for the circuit to keep oscillating, the gain must be slightly greater than three. And this causes other problems. The first of these is that because the circuit uses posi­ tive feedback, any gain above that just required for oscillation will cause an increase in output amplitude. This increase causes even further increases in amplitude and before you know it, you’ve got a lovely squarewave staring at you from the CRO! This in turn leads to a second problem – increased distortion. The most common solution is to use some non-linear element, such as a light globe, to regulate the amount of gain. As shown in Fig.1, the globe is connected in the negative feedback path of the circuit. When the circuit begins to oscillate, the output voltage increases which causes an increased current flow through the globe. The good thing about globes is that they have a positive thermal coefficient (PTC) which means the more current you try to pump through them, the more their resistance increases. This increased resistance counteracts any tendency for the output amplitude to rise by reducing the gain of the circuit. In other words, if the output amplitude goes up, the re­sistance of the globe also goes up, which reduces the gain of the circuit and thus brings the amplitude back under control. This technique is used in countless low-distortion sinewave oscillator circuits. Its main drawback is that a globe does not have an instantaneous response, so if you change frequency, the output amplitude will “bounce around” for a short period until a new equilibrium is established. Another problem is that while we now have a very stable waveform in terms of output voltage, the non-linearities of the lamp filament introduce distortion into the waveform. One way to reduce this distortion is to simply filter the output signal to remove the unwanted harmonics. Since we are only interested in one particular frequency, a “brick wall” filter (ie, a low-pass filter with a very steep cutoff) can be used to remove the un­ wanted harmonics and hence reduce the distortion. The project presented here uses both these techniques and can be switched to produce one of three output frequencies – either 100Hz, 1kHz or 10kHz. It provides up to 2V RMS output into a 600Ω load with a distortion figure of less than .004%. Circuit details Fig.2 shows the complete circuit details for the Low Dis­tortion 3-Spot Oscillator. It is based on three identical circuit topologies, each with an oscillator and filter, the only dif­ference between each section being the necessary changes in com­ponent values to obtain the desired frequencies. The reason for using three separate oscillators to generate the three frequencies is to reduce the required switching to a minimum. For example, we could have used just one oscillator to produce all three frequencies but then switching would be re­ quired for the frequency determining components. This extra switch­ing would inevitably lead to large transients when the frequency was switch­ed and the overall envelope stability would not be as good. For ease of understanding, we shall explain only one sec­tion but note that all three work in exactly the same manner. Looking at the 100Hz (top) section, IC1a and IC1b form a modified Wien bridge oscillator. Its frequency of operation is set by the 0.1µF capacitors and the 15kΩ resistors in the posi­tive feedback loop and follows the standard Wien bridge formula: F = 1/(2πRC). IC1b is connected as an inverter to drive the negative feedback network of IC1a; ie, it drives lamps LG1 and PARTS LIST 1 PC board, code 01110941, 158 x 100mm 1 front panel artwork 1 zippy box, 195 x 113 x 60mm 1 3-pole 3-position rotary switch (S1) 1 SPDT toggle switch (S2) 1 3.5mm socket 1 RCA panel-mount socket 2 knobs to suit 1 10kΩ log potentiometer (VR5) 1 12-way length of Molex pins 1 16VAC plugpack 6 12V DC switch replacement globes (Jaycar Cat. SL-2636) 4 rubber feet Semiconductors 7 LM833 dual low-noise op amps (IC1-4, IC6-8) 1 TL072 dual op amp (IC5) 1 7812 3-terminal regulator 1 7912 3-terminal regulator 2 1N4004 diodes (D1,D2) 2 OA91 germanium diodes (D3,D4) 1 5mm red LED (LED1) 3 100Ω 5mm horiz. trimpots (VR1-VR3) 1 10kΩ 5mm horiz. trimpot (VR4) Capacitors 2 470µF 25VW electrolytics 2 100µF 16VW electrolytics 9 0.1µF 63VW MKT polyester 3 .015µF 63VW MKT polyester 5 .01µF 63VW MKT polyester 3 .0015µF 63VW MKT polyester 5 .001µF 63VW MKT polyester 3 150pF ceramic Resistors (0.25W, 1%) 9 47kΩ 2 10kΩ 9 36kΩ 1 2.2kΩ 1 27kΩ 1 1kΩ 9 24kΩ 1 560Ω 9 15kΩ 3 68Ω Miscellaneous Light duty hook-up wire, light-duty speaker cable, machine screws & nuts, washers. LG2. In effect, IC1a and IC1b drive the feedback network, including the lamps, in bridge mode. This effectively halves the voltage swing at the output of both op amps and the result is an oscillator with a quick settling time. December 1994  61 0.1 15k 7 36k 24k 68  0.1 0.1 5 0.1 6 IC1a LM833 VR1 100  15k .015 47k IC2a 5 LM833 7 +12V .015 .015 36k 24k 6 47k 0.1 0.1 8 2 2 3 -12V E .01 15k 5 .01 6 7 36k 24k 68  .01 .01 IC3a LM833 VR2 100 15k .0015 47k IC4a 5 LM833 .0015 47k 36k 24k 6 7 .01 .01 +12V 8 2 6 IC7b 5 LM833 .01 7 -12V B 1kHz OSCILLATOR LG4 F 1 IC3b 4 G .001 15k 5 .001 6 7 36k 24k 68  .001 .001 IC5a TLO72 VR3 100  15k 150pF 47k 6 7 IC6a 5 LM833 150pF 47k 36k 24k .001 .001 +12V 8 2 LG6 15k D F 1kHz S1b E 1 IC5b G 4 S2 B S1a 1k LEVEL VR5 10k 10kHz 5 6 -12V IC8b 7 VR4 10k D1 1N4004 16VAC D2 PLUG1N4004 PACK 470 25VW 7812 7812 GND OUT 100 16VW +12V 0.1 100 GND 16VW IN 62  Silicon Chip 7912 7912 OUT 7812 7912 I GO GIO A 0.1 0.1 LED1  K -12V 27k +12V 2.2k 0.1 0V 470 25VW OUTPUT 560W 560W S1c 10kHz IN 1 1kHz 100Hz 1kHz 8 IC7a 3 LM833 4 -12V 100Hz A +12V 2 -12V 10kHz OSCILLATOR 100Hz +12V 150pF 47k 36k 24k .001 .001 4 10kHz 8 1 IC6b 3 LG5 3 .0015 47k 36k 24k 4 8 2 1 IC4b 3 LG3 3 A -12V 1 4 2 1 D 8 15k 8 IC8a 3 LM833 4 100Hz OSCILLATOR LG2 IC1b 2 0.1 0.1 4 LG1 15k 1 IC2b 3 +12V .015 .015 47k 36k 24k A D3 OA91 D4 OA91 K LOW-DISTORTION 3-SPOT OSCILLATOR 10k 100uA 10k Use light duty hook-up wire for the front panel connections & bind the leads with cable ties to keep the layout tidy. The PC board is secured to the base of the case using machine screws & nuts, with additional nuts used as spacers. ▲ Note that the final circuit uses two lamps in series in­ stead of just one lamp. This has been done to further reduce the initial distortion of the oscillator sections. VR1 sets the gain of IC1a and is adjusted to provide a 2V output with the level control at maximum during the setting-up procedure. The remaining section of the circuit consists of three op amps connected as a 6th-order Butterworth low-pass filter. It’s made up of three cascaded second-order filters which gives an ultimate slope of 36dB/octave above the cut-off frequency. This topology is known as a multiple feedback (MFB) filter. The cutoff frequency of the circuit Fig.2 (left): the circuit uses three similar Wien bridge oscillator & filter sections to generate three spot frequencies at 100Hz, 1kHz & 10kHz. IC8b amplifies & buffers the selected frequency, while D3, D4 & their associated parts provide drive to an optional 100µA level meter. is below the oscillator frequency; ie, around 75Hz for the 100Hz oscillator. Thus, the second and higher harmonics will be heavily attenuated with respect to the fundamental. As a result, we end up with a circuit which has fast settling time and very low distortion. The output from the filter stage appears at pin 1 of IC8a and is fed to S1a which is one pole of a 3-pole 3-position rotary switch. From there, the selected signal is fed via level control VR5 to op amp IC8b. This functions as a unity gain buffer stage and drives the output socket via a 560Ω current limiting resis­tor. This resistor ensures that IC8b is not damaged if the output is shorted out. IC8b also drives an optional output signal metering circuit via VR4 and a 27kΩ resistor. The metering circuitry consists of a pair of germanium diodes (D3 & D4) connected in a bridge arrange­ment with two 10kΩ resistors. Trimpot VR4 allows the meter to be adjusted to produce a full-scale read- ing when the level control is set to maximum. As indicated previously, the 1kHz and 10kHz oscillator/filter stages function in exactly the same manner as the 100Hz stage. There is one anomaly, however – the 10kHz oscil­lator is based on a TL072 dual op amp, whereas the other two oscilla­tors use LM833 devices. The reason we’ve used a TL072 op amp for the 10kHz oscilla­tor is that we found that the LM833 produced some very high frequency bursts in parts of the 10kHz waveform. By replacing it with an op amp with a lower transition frequency (Ft), this problem is eliminated. The LM833 devices are a little cheaper than the TL072 and perform flawlessly at the lower frequencies. Power supply Power for the circuit is derived from a 16VAC plugpack connected via on/ off switch S2. This eliminates the need for a mains transformer inside the case and the attendant hum and distortion problems that this would create. The AC voltage from the plugpack is halfwave rectified by D1 and D2, filtered December 1994  63 OUTPUT SOCKET 36k 24k 0.1 0.1 15k IC1 LM833 IC2 LM833 0.1 TOMETER .015 36k 47k 1 47k LG2 1 .015 36k 24k 0.1 15k 4 D4 IC8 LM833 1 1 LG1 27k VR4 D3 VR5 VR1 1k 560  15k 68 47k .015 0.1 24k 1 15k 10k 47k 36k 0.1 0.1 24k 10k .0015 .01 LG4 .01 6 2.2k 24k .0015 7 100uF 47k LG3 1 24k 15k 3 IC4 LM833 .01 1 .01 .01 IC3 LM833 VR2 36k S1 2 15k 68  5 0.1 100uF 0.1 4 150pF LG6 36k 47k 1 .001 470uF 470uF 1 150pF 150pF 3 36k LG5 IC7 LM833 .0015 47k 1 LED1 7912 2 IC6 LM833 47k VR3 15k S2 IC5 TLO72 36k 5 7812 .0015 2x.001 24k 7 24k 15k 6 15k 68  47k 36k .001 D1 D2 24k Fig.3: install the parts on the board as shown here, taking care to ensure that all polarised parts are correctly oriented. Note particularly that IC5 is a TL072 device; the remaining ICs are all LM833 types. Be sure to mount the 7912 3-terminal regulator adjacent to the edge of the board. and regulated by two 78-series regulators to produce ±12V rails to power the op amps. 64  Silicon Chip LED 1 and its associated 2.2kΩ current limiting resistor provide power on/off indication. PLUG-PACK SOCKET To further ensure that the output signal is as clean as possible, the two unwanted oscillator sections are shut The light globes are installed by plugging them into 2-way pin headers derived from a Molex pin strip. They should be left until last. down to eliminate crosstalk. This is achieved by switching the supply rails to the oscillator stages using switches S1b and S1c. When a particular frequency is selected, these two switch poles select the ±15V supply rails for that oscillator and switch out the other two. As a result, only one oscillator section is powered up at any one time and this completely eliminates cross-coupling bet­ween oscillator stages. Construction Most of the parts for the 3-Spot Sinewave Oscillator are installed on a PC board coded 01110941 and measuring 158 x 100mm. Before you begin construction, check the board carefully against the published pattern for possible etching defects. In the vast majority of cases the board will be perfectly OK but it’s always a good idea to make sure. Fig.3 shows where the parts go on the PC board. Begin by installing PC stakes at the external wiring points, then install the wire links and resistors. It’s a good idea to check each resistor value on your DMM as it is installed, as some of the colours can be diffi­cult to decipher. Once the resistors are in, install the capacitors and the trimpots. Take care with the electrolytic capacitors – they must be inserted with the correct polarity. The light globes (LG1-LG6) are all mounted using 2-way pin headers (derived from a Molex pin strip) and these may be in­stalled now. Do not plug the globes in yet though, as they are easily damaged. The board assembly can now be Fig.4: this is the full-size etching pattern for the PC board. completed by installing the ICs, regulators and diodes. Note that the ICs are all oriented in the same direction and be sure to use a TL072 for IC5. The two regulators are mounted with their leads bent at 90° so that their metal tabs sit flat against the board surface. Make sure that the LM7912 regulator is adjacent to the edge of the board. Although the level meter is optional, its asso­ciated driver circuitry should be installed regardless as to whether you intend using a level meter or not. That’s because this circuit is used later during the adjustment procedure, either with the optional meter or with a multimeter in its place. Final assembly A plastic zippy case measuring 195 x 113 x 60mm is used to house the circuitry. The first step involves mounting the PC board – it’s secured to the base using 6mm standoffs and machine screws and nuts. You can use the board as a template for marking out its mounting holes. This done, attach the front panel label to the lid and use this as a template for drilling the holes for the front-panel controls and the LED. Additional holes December 1994  65 will also have to be drilled at either end of the case to accommodate the plugpack socket and the RCA output socket. Note that it’s best to drill all holes to 3mm and then enlarge them as necessary using a tapered reamer. As supplied, switch S1 will be a 3-pole 4-position type. It must be converted to a 3-position type by lifting the locking ring at the front of the switch bush and rotating it anticlock­wise one 66  Silicon Chip Test & adjustment Fig.5: this full-size artwork can be used as a drilling template for the front panel. POWER FREQUENCY (kHz) 10 0.1 1 LOW-DISTORTION 3-SPOT SINEWAVE OSCILLATOR LEVEL the switch connec­tions and light-duty speaker cable for the connections to the pot (VR5), output socket and LED. Take care to ensure that the LED is wired with the correct polarity. The assembly can now be completed by plugging the six light globes into their 2-way pin headers and fitting four rubber feet to the base of the case. position. Check that the switch now has three positions before mounting it in place, along with the other items of hard­ware. Note that the rotary switch must be oriented so that the point­er on the knob aligns with the 0.1kHz position when the switch is set fully anticlockwise. The wiring between the PC board and the external hardware items is run using light-duty hook-up wire for To test the unit, you will need to monitor the output using either an oscilloscope, a frequency counter or an audio amplifi­ er. Initially, set all trimpots in the oscillator stages to midrange, then apply power and check that the ±12V rails from the 3-terminal regulators are correct. Switch off immediately if you encounter an incorrect reading here and correct the fault before proceeding further. If you have an oscilloscope, check that a sinewave trace appears when each range is selected and that its frequency is in the ballpark. Alternatively, you can measure the frequency di­rectly if you have a frequency counter or simply listen for a tone if you are feeding the output into an audio amplifier. Assuming that the circuit is working correctly, VR1-VR3 can now be adjusted to provide the correct levels. The procedure is as follows: (1). Select the 100Hz range, set the Level control (VR5) to maximum and connect a multimeter set to a low AC voltage range across the output (ie, across the RCA output socket). (2). Adjust VR1 for a 2VAC reading on the multimeter. (3). If you have installed the optional 100µA level meter, adjust VR4 so that this meter reads full-scale when the output level is at 2VAC. This done, select the 1kHz range and adjust VR2 for a full-scale reading. Finally, select the 10kHz range and adjust VR3 for a full-scale reading. (4). If you are not using a level meter, ignore step 3, set VR4 to midrange and connect the multimeter across the meter termi­nals. Select a low DC voltage range, check that the level control is still at maximum and note the reading on the multimeter. Finally, select the 1kHz and 10kHz ranges in turn and adjust VR2 and VR3 respectively to give the same reading. That completes the adjustment procedure. Your Low-Distor­tion 3-Spot SC Oscillator is now ready for use.