BASIC SQUARE WAVE-TRIANGULAR WAVE OSCILLATOR

Transcription

1 BAS SQUAE WAVE-TANGULA WAVE OSLLATO. ircuit description NTEGATO SHMTT TGGE E E E B O by O +Vsat -Vsat UT N N Vsup Vref LT The above oscillator is basically a switched integrator that outputs a triangular wave whose slopes are controlled by the charging current of, more specifically: V o LT VSAT VSAT 0.5 T E E E E UT ( + ) T ( UT LT)( + ) We can see on the above waveforms that if the capacitor charging current is doubled, the frequency of oscillation also doubles because the capacitor is charged twice as fast which results in a slope dv o /dt twice as large. Note here that the capacitor is charged linearly and not exponentially because the charging current is constant. f the +ve and ve saturation voltages are the same, only then are the +ve and ve charging currents the same which results in a 50% duty cycle square wave. UT and LT are the trigger voltages of the Schmitt trigger.. Sauriol ev. /3/003 age

2 øöçèæ V O 0.5T 0.5T UT m + m - LT +V sat V O -V sat V + V + V E O E + E V ref. Analysis example V - V E O E + E NTEGATO SHMTT TGGE 0k 0k 0 n 0k 30k O 0. u O +Vsat -Vsat UT 0k 00k +V Vref LT Assuming the op amps are L347 with the following specs: Specs for V sup ±5V minimum typical maximum O/ Voltage swing, L 0K ±V ±5V --- / voltage range ±V, -V --- Determine the two O/ waveforms showing voltages and range of W and SW of square wave. Assume ±V supply voltages and typical op amp parameters. Analysis of trigger points of O: LT V k 30 k V+ +V V UT +6.67V k V+ +V 30 k V. Sauriol ev. /3/003 age

3 V O T T 33.3 µ s 66.6 µ s V O 6.66 ( 0.833) 0n µ s 33.3 µ s 0.5 0k 0k 3.75 khz 7.5 khz NOTE: f the +ve and ve saturation voltages are not exactly equal, then the integrator capacitor charge and discharge currents will be different and the duty cycle will not be exactly 50%. To remedy this, one should use rail-to rail op amps with matching +ve and ve supply voltages provided by a dual tracking regulator. 3. Design example Design an oscillator that meets the following specifications: Supply Voltages: ±5V Op amps: O9 DUAL rail-to-rail / and O/ requency range: khz to 5 khz Triangular wave: ±V Square wave: ±5V with fixed 50% duty cycle and 0V with 0% to 00% adjustable duty cycle. Sauriol ev. /3/003 age 3

4 Schmitt Trigger Design. No pull-up resistor required + V V 5 ( 5) O. O.5 E K and K UT LT ( ) 30 E 3. alculation of V E V E V + 0V at triggering Therefore the ve / of A has to be grounded. LT -V 0.67 k V+ 0V 30k B k 30k 8.57k 8. k ntegrator Design Let VO 40 kv / s 8 kv / s ( ) 00µ 500µ max, therefore 0. 5 n 40 kv / s 8 kv / s Let + 50k E E E k + E E VO n n 5V par.8k 5.6k 8.7k par 4.94k 43k ( 40k 8k) Let closest pot value above is 50K, therefore ( + ) 7.6k 8k Standard pots available: k, k, 5k, 0k, 0k, 50k, 00k, 00k, 500k Level Detector Design E E ave B E 5.6K A3 is powered with only to obtain 0V and levels at the O/, therefore we cannot apply the triangular wave directly to the / of A3. The two resistors will attenuate the triang. wave by 50% and will also pull it up to +.5V average. 3 provides a little D hysteresis to prevent O/ chattering of A3. Let the hysteresis be 0 mv.. Sauriol ev. /3/003 age 4

5 To be safe and ensure that 0% to 00% range is covered, let V 3 - range from.v to 3.8V v -.V min +.V - V- 3.8V max max V V /.85 /3.66 V - min V - max Assuming mid-range setting of pot for balancing / resistance of A3 (for min O/ offset voltage), we have N 3 3.3K ( K ) ave (3.3K ) + + V V k 3 O 3 O k (430k ) UT LT 0m N. Sauriol ev. /3/003 age 5

6 inal ircuit 43K 50K 5.6K n 0K 30K EQUENY khzto5khz 8K A O9 8.K A O9 50% DUTY YLE 430K +V 3.3K 3.3K -V TANGULA WAVE OUTUT 0K 3.3K K A3 O9 390 DUTY YLE SNESHAE 0. u O9 A4.5K N448 SNEWAVE OUTUT THD < % The sineshaper uses the diodes non-linearity to distort the triangular wave into a crude sinewave that usually has less than % THD (total harmonic distortion) which is typical for a general purpose lab function generator but not good enough for testing audio equipment where < 0.0% THD would be required for the sinewave test signal. To adjust the amplitude of the output waveforms, we could have a S4T (single pole, quadruple throw) switch that selects one of the four O/ waveforms and then use a potentiometer to control the final amplitude of the selected waveform.. Sauriol ev. /3/003 age 6