APPLICATION BULLETIN ® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132 BUILD A THREE PHASE SINE WAVE GENERATOR WITH THE UAF42 By Johnnie Molina (602) 746-7592 Figure 1 shows how a three phase sine wave oscillator can be built using one UAF42 state variable filter along with some resistors and diodes. Three output nodes are available; highpass out, bandpass out and lowpass out. The signal at the bandpass and lowpass out nodes are 90° and 180° out of phase, respectively, with the highpass out node. An on-chip auxiliary op amp is available for use as a buffer or gain stage. f OSC = Where, (1) 1 2πRC R = RF1 = RF2 C = C1 = C2 = 1000pF The max fOSC obtainable using the UAF42 state variable filter is 100kHz. Distortion becomes a factor though for frequencies above 10kHz. For low frequencies of oscillation (fOSC < 100Hz), the use of external capacitors is recommended. They should be placed in parallel with the internally supplied C1 and C2 capacitors. This will reduce the SETTING THE FREQUENCY OF OSCILLATION The frequency of oscillation is set with resistors RF1 and RF2 using Equation 1. V+ R1 1N914 A sin(ωt) A sin(ωt + 90°) Highpass Out R2 CEXT(1) RF1 Bandpass Out RF2 RFB Lowpass Out CEXT(1) 5 13 12 A sin(ωt + 180°) 8 14 7 1 50kΩ UAF42 C2 C1 50kΩ 1000pF Auxiliary Amp 1000pF 6 50kΩ 50kΩ 2 3 8 7 9 V– 10 V+ 11 4 R4 1N914 R3 NOTE: (1) CEXT is required for fOSC ≤ 100Hz—see text. V– FIGURE 1. Three Phase Quadrature Oscillator. © 1994 Burr-Brown Corporation AB-096 1 Printed in U.S.A. July, 1994 value of frequency setting resistors RF1 and RF2 which can exceed tens of megaohms for low frequency oscillator designs. An NPO ceramic or mica capacitor is recommended. The value used for C in Equation 1 should be the sum of both the external and on-chip 0.5% 1000pF capacitor. Example: To design a 1kHz, 1.4V peak oscillator, use Equation 1 to calculate the value of frequency setting resistors RF1 and RF2. R F1 = R F 2 = = 159. 2kΩ 2 π 1kHz 10 −9 Use Equation 2 to determine values for signal magnitude setting resistors R1, R2 and R3, R4. SETTING THE SIGNAL MAGNITUDE Resistors R1, R2, R3 and R4 should be selected using Equation 2 to set the desired signal amplitude. R1 R 3 V O + V SUPPLY = = –1 R2 R4 V O – 0.15 (2) ) Assuming VSUPPLY = 15V, R1 R 3 = = 12.1 R2 R4 Setting R1 and R3 equal to 12.1kΩ and R2 and R4 equal to 1kΩ provides the proper resistor ratios. Note that these resistors act as loads to the internal op amp. The maximum load current for the UAF42 is 10mA. Table I indicates that RFB should be a 10MΩ resistor. START UP Resistor RFB provides a positive feedback path from the bandpass out node to the summing amplifier input. This provides the necessary “start up” required to begin oscillation. Suggested values are shown in Table I below. ≥ 1kHz ( R1 R 3 V O + V CC = = –1 R 2 R 4 V O – 0.15 Actual signal amplitude may vary somewhat from the designed for value. This is due to the non-ideal characteristics of the diodes and op amps. Some gain adjustment on the R1/R2 and R 3/R4 ratios, or the auxiliary op amp gain stage (if used), may be required. fOSC 1 RFB 10MΩ 10Hz to 1kHz 5MΩ < 10Hz 750kΩ TABLE I. Note that resistor R FB influences the signal magnitude since it introduces positive feedback in the first integrator/ summing amplifier loop. Using smaller values than those suggested above may increase the signal amplitude and introduce distortion. The time required to begin oscillation is proportional to the frequency of oscillation. Low frequency designs can be started quickly by switching in a 1kΩ resistor in parallel with RFB at turn on and then switching it out. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. 2