Fullscreen HTML5Med Res (??MB)HTML4

Build this low-cost sinewave oscillator This small PC board allows you to build a low distortion sinewave oscillator using only junkbox bits. It runs off a 12V plug pack, gives a low-impedance output signal of up to 6V p-p and costs peanuts to build! By DARREN YATES Sinewaves are as fundamental to electronics as resistors and ICs, but unless you're willing to pay big bucks for the generators currently available, professional low distortion sinewave gear is out of reach for most of us. However, it's only on very rare occasions that you need the wide frequency selection from DC to daylight that these expensive boxes provide. For example, when you see an operational amplifier quoted in some databook, they'll quote a distortion figure at some particular frequency, usually lkHz. Most power amplifiers that you buy or see described in magazines, usually provide distortion measurements at selected frequencies. Often, these include lO0Hz, lkHz and lOkHz. To fill in this present gap, we've designed this project for low distortion (.015% or better), low cost (about $15-$20), and the ability to run from This is the 10kHz version of the sinewave oscillator but versions for other frequencies look exactly the same (only a few component values are changed). Take care with component orientation when installing the parts on the board. 54 SILICON CHIP a standard 12VDC plug pack. We've also given you the choice of three very common frequencies: lO0Hz, lkHz or lOkHz. Circuit theory So, how do we go about making low distortion sinewaves? Well, in the end, there are two ways of doing this. You can either start out by making the best oscillator money can buy or you can start with a so-so one and improve the signal coming out of it. We chose the second option because it was easier to do, and a lot cheaper into the bargain! Block diagram Fig.1 shows how it is done. First of all, we start off with a Wien bridge oscillator, which is one of the oldest circuits around, and then we take the output signal from that and run it through a fairly brutal low-pass filter. This removes a large amount of the unwanted multiples of the fundamental frequency or "harmonics" which make up what we call "distortion". It does this by increasingly attenuating higher frequencies but allowing the frequency of interest to pass through. These higher frequencies are knocked off at the rate of 24dB/octave. This means that if we started with a lkHz signal, then the 2kHz harmonic present at the output will be about 24dB below or about 1116th the amplitude of the lkHz signal. The 4kHz harmonic would be 48dB below or 11250th the amplitude of the lkHz signal, and so on. The result is a dramatic improvement in distortion. For example, if we start with a sinewave that has about 0.5% distortion, we would end up with a sinewave that has only .01 % distortion after filtering - and improvement of 50 times! The circuit The circuit diagram is shown in Fig.2. It only requires an LF347N quad -- WIEN BRIDGE OSCILLATOR 4TH ORDER BUTTERWORTH LOW-PASS FILTER LOW DISTORTION i----- SINEWAVE OUTPUT Fig.1: block diagram of the sinewave oscillator. It consists of a modified Wien bridge oscillator driving a 4th order Butterworth low-pass filter. The low pass filter atlenuates harmonics above the wanted frequency at the rate of 24dB/octave & this drastically reduces the distortion. op amp , a couple of signal diodes and a few passive components. You will probably already have some of or all of these components lying around in your spare parts bin or junkbox. ICla & IClb form the Wien bridge oscillator while IClc & ICld form the 4th order low-pass Butterworth filt er. This 4th order filter actually consists of two 2nd order filt er sections connected together. Butterworth filters are easy to calculate and have the advantage of having a flat respons e across the passband. You will notice that there are several components on the circuit that have no specific values. If you look at Table 1, these components have different values, depending on the frequency you want. When assembling the unit, you simply go to the frequency you want and read off the corresponding component value. Looking at the circuit, ICla and IClb form an unusual Wien bridge in that resistor RZ is not returned to ground as in conventional designs, but forms part of the unity-gain inverting amplifier formed by IClb. This provides gain compensation and helps stablise the output amplitude. Diodes Dl and DZ also do this job but because they are non-linear in their response, the more they interfere with the signal, the more distortion they produce. Most designs use a thermistor or small 12V light globes for this job. We chose the diodes because suitable thermistors can be expensive and hard to get while lamps require extra current and , because of their slow repsonse, take some time to reach their final resistance. This results in a sinewave which has a fairly long settling time, particularly at low frequencies. The diodes speed up this process and because of the high value of resistor R6 (470kQ to 1.ZMQ) in series, they only have minimal affect on signal distortion. The gain of ICla is set by resistors R4, R5 & R7. Resistor R7 sets the gain of ICla just enough for the oscillator to start. If we have too much gain, the oscillator starts OK but introduces heaps of distortion; if we don't have enough gain , then it won't start at all! The Wien bridge itself is formed by components Rl, RZ, Cl & CZ. The frequency of the sinewave pro duced is: F = 1 /( 21tR1C1) . The sinewave produced at the output of ICla [pin 1) will have a total harmonic distortion of about 0.5% to 1 % - which is certainly nothing to write home about. Low-pass filter The signal from pin 1 ofICla is DCcoupled to the first stage of the filter formed by ICld and its associated components. This section has a 3dB cutoff frequency set to the frequency of interest - whether it be 100Hz, lkHz or 10kHz - by selecting the correct components from Table 1. This leaves us with a problem, though. To get maximum effect from the filters, we need to set their cutoff frequency at our frequency of inter- 470 U R2 OUTPUT LINK SEE TEXT R3 100k .,. +12V 01 220k 220k-:- 2x1N914 10 16VW I 100k .,.. 02 16VW! +12V 0.1 0.1 u - - -- -,__--e-_ __ _ _,__~ov ":' SPOT FREQUENCY SINEWAVE GENERATOR Fig.2: the final circuit is based on a single LF347 quad op amp package. ICla & IClb form the Wien bridge oscillator, while IClc & ICld together make up the 4th order Butterworth filter. Note that some of the resistor & capacitor values are selected to give the desired frequency. FEBRUARY1991 55 your application. If you wish, you can replace the wire link at the output with a 22µF 16VW electrolytic capacitor to provide DC isolation. You can also reduce the output signal amplitude by changing the 4700 and 10okn output resistors with a potential divider of your own. For example, with two lkQ resistors , the output would be reduced to half. Power supply est. But this results in about 3dB attenuation of the signal. By the time it has gone through both sections, we would then get 6dB attenuation. In other words, the signal is reduced to half its original amplitude. To overcome this, we give each section a gain of roughly 3dB (or 1.42), so in the end, the filters have unity gain at the frequency we want. By the time the signal comes out of ICld, its distortion is of the order of .05% to 0.1 % - an improvement, but we can still go better than that! In giving the filters extra gain, we are also amplifying the harmonics, but since they are still being attenuated at the rate of 24dB/octave, the relative amplitude of the wanted frequency to those we don't want remains unchanged. In other words, it doesn't make the distortion worse. The final signal is taken from the output of IClc (pin 8) where the distortion has now dropped to around .015% for the 100Hz and lkHz versions, and to about .005% for the 10kHz version. We have set the output impedance of the circuit at 4 70Q with the resistor at the output but you can increase this to 600Q or any other value to suit Table 1: Component Values A1}- 100Hz 1kHz 10kHz Resistors 56 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 15kQ 100kQ 10kn 470kQ 330Q 18kQ 27kQ 560kQ 18kQ 27kQ 560kQ 1.2MQ 4.7kQ 1.8kQ 3.3kQ 47kQ 1.8kQ 3.3kQ 47kQ 820kQ 470Q 470Q 470Q 5.6kQ 1.5kQ 2.7kQ 5.6kQ Capacitors C1 C2 C3 0.1µF 0.1µF 0.1µF .0015µF .0015µF 0.1µF .001 5µF .0015µF .015µF SILICON CHIP Power is provided by a 12V DC plugpack which drives a 7812 3-pin 12V regulator. Even though this regulator requires a couple of volts of headroom to operate correctly, typical 12V DC plugpacks provide about 16-17V DC when lightly loaded and and so will work perfectly well with this circuit. Extra decoupling and filtering of the power supply is provided by the 100µF and 470µF electrolytics, as well as O. lµF greencaps. Construction All components for the oscillator are mounted on a PC board which is coded SC04102911 and measures 104 x 57mm. This can also be housed in a standard zippy box, measuring 130 x 68 x 41mm. As there are no controls to mount on the front panel, it is a simple case of drilling two holes, say for an RCA socket for the output signal and a DC socket for the power supply. Before you start assembly, check the board carefully for breaks or shorts in the tracks. If there are any, it's best to correct them now. Once you're happy that everything is OK, take a look at the wiring diagram. This shows you where each component fits into place. Begin by installing the wire links and the resistors. Some of the colour bars on the resistors may be difficult to distinguish, in which case, use your multimeter to make sure of the correct value. Again, make sure that you have the correct resistors and capacitors for the selected frequency from Table 1. Now install the diodes. It's best to do this now while the flat components only are on the board, otherwise they become difficult to put in. It doesn't matter which way round you put the two diodes in, as long as they face in opposite directions. We suggest that you put them in as shown on the wiring diagram. PARTS LIST 1 PC board, code SC04102911, 105 x 57mm 1 12VOC plug pack 4 PC stakes Semiconductors 1 LF347N quad FET-input op amp (IC1) 1 7812 +12V regulator 2 1N914 signal diodes (01, 02) 0 SC04102911 Fig.4: compare your PC board against this full-size artwork before installing any of the parts. Next, solder in the PC stakes. You may need to hammer these in, depending on the type of pins you get, or you can enlarge the holes using an appropriate drill. The capacitors can now be installed. Make sure you get the polarity of the electrolytics correct, particularly those in the power supply, otherwise they could quite easily pop. All that should be left is the two ICs. Solder in the 7812 regulator first and then the LF34 7. Testing Now check over the board again and compare it to the wiring diagram. When you're sure the board is correct, you can hook up the power sup- CAPACITOR CODES 0 0 0 0 Value IEC Code 100n 0.1µF .015µF 15n .0015µF 1n5 EIA Code 104 153 152 ply. When you do so, put your multimeter, switched to a DC milliamps range, in series with the power supply and the circuit. The current drain sho uld be no more than about 20mA. If you get more than this, then it is possible you have a short circuit somewhere. If you have a CRO, monitor the output and check that you get a stable sinewave at the frequency you selected. If you don't have a CRO, just connect it up to an audio amplifier (turn the volume control down first). If you hear a tone when you turn the volume up, it's a good bet that the circuit is working correctly. If you don 't get any signal, first check that there is 12V at the output of the regulator. If that's OK, check that it appears across pins 4 and 11 of IC1. If the voltage is there, try touching both sides of the diodes with your finger. If the signal appears and then dies away when you remove your finger, then it is probably due to the fact that IC1a doesn't have enough gain to keep oscillating. To fix this , Capacitors 1 470µF 25VW PC electrolytic 1 100µF 25VW PC electrolytic 3 10µF 25VW PC electrolytic 4 0.1 µF metallised polyester (greencaps) 4 .0015µF metallised polyester Resistors (0.25W, 5%) 1 1.2MQ 1 4.7kQ 2 220kQ 2 3.3kQ 10 100kQ 2 1.8kQ 2 47kQ 1 470Q Miscellaneous Solder, hookup wire etc Note: This parts list is for the 1kHz version . Other versions will require different resistor & capacitor values - see Table 1. change R7 to the next highest standard value; ie, if it was 330Q, make it 390Q instead. Once the circuit is working, you may like to house it in a zippy box to keep the dust and bugs away from it. For those who may have access to the necessary equations, you may like to try to work out other frequencies as you need them. The LF347N should be capable of producing a clean sinewave at well above 20kHz. SC RESISTOR CODES 0 0 0 0 0 0 0 0 0 No Value 4-Band Code (5%) 5-Band Code (1%) 1 2 10 2 1 2 2 1.2MQ 220kQ 100kQ 47kQ 4.7kQ 3.3kQ 1.8kQ 470Q brown red green gold red red yellow gold brown black yellow gold yellow violet orange gold yellow violet red gold orang~ orange red gold brown grey red gold yellow violet brown gold brown red black yellow brown red red black orange brown brown black black orange brown yellow violet black red brown yellow violet black brown brown orange orange black brown brown brown grey black brown brown yellow violet black black brown 1 FEBRUARY 1991 57