CHAPTER 14

1. CHAPTER 14 Nonsinusoidal Oscillators

2. Objectives Describe and Analyze: • Operation of the 555 IC • Inverter oscillators • Schmitt oscillators • Wave-shaping • Sawtooth oscillators • Troubleshooting

3. Introduction • There are other ways to make an oscillator besides phase-shifters and resonators. • The term astable covers a group of oscillator circuits, many based on hysteresis in one form or another. It also covers chips designed for the purpose, such as the 555. • The old term “multivibrator” is also used to name these circuits. It goes back to vacuum tube days when they actually used electromechanical vibrators in circuits.

4. Square-Wave Oscillators Square wave from a “free-running” 555 circuit.

5. The “Internals” of a 555 Frequency set by RA, RB, and C.

6. Functions of the 555 • The 555 is still popular after all these years because it is easy to use. It performs two functions: • Square-wave oscillator (astable) • One-shot (monostable) • Strictly speaking, a square-wave has a 50% duty cycle. But unless the duty cycle is low, astables are called square-wave oscillators even if it’s not 50%. • A one-shot produces a fixed-width output pulse every time it is “triggered” by a rising or falling edge at its input.

7. 555 Oscillator fOSC = 1.44 / [(RA + 2RB) C]

8. 555 One-Shot t = 1.1RC

9. Inverter Oscillator fOSC depends on the number of inverters (must be odd).

10. A Calculation • For the circuit of the previous slide, find the frequency range if each inverter has a delay of 10 ns  1 ns. Period T = delay  2  # of inverters, so TLONG = 11 ns  2  3 = 66 ns and TSHORT = 9 ns  2  3 = 54 ns So fLO = 1 / 66 ns  15.2 MHz and fHI = 1 / 54 ns  18.5 MHz

11. Crystal-Controlled <insert figure 14-15 here> Commonly used for microprocessor clock.

12. Hysteresis Oscillator Schmitt trigger circuit on an op-amp.

13. Example Calculation • For the circuit of the previous slide: • Let R1 = R2 = R3 = 10 k. Let C1 = .01 μF • Find the frequency of oscillation. • [Hint: it takes about 1.1 time constants to get 67% voltage on capacitor.] • The 2:1 divider formed by R2 & R3 keeps the (+) input at Vout/ 2. C1 has to charge up to Vout/ 2 to flip the compara-tor. But it starts from –Vout/ 2, which is equivalent to charging from 0 to 2V / 3 with V applied. So, 1.1R1C1 = 110 μs, but it takes two “flips” for one cycle. So f = 1 / 220 μs  4.5 kHz.

14. Square to Triangle Integrating a square wave makes a triangle wave.

15. Triangle to Sine With enough diodes, the signal is very close to a sine.

16. Sawtooth Oscillator Also called a “ramp generator”, it can be used to generate the horizontal sweep in a CRT circuit.

17. A Relaxation Oscillator Shockley diode converts integrator into a “relaxation” oscillator, so called because the diode periodically relieves the capacitor’s “tension” (voltage)

18. Sample Calculation • For the circuit of the previous slide, let the input resistor Ri = 100 k, the feedback capacitor C = 0.1 F, and let Vin = –1 Volt. Calculate the frequency if the Shockley diode “fires” at 10 Volts. • Iin = 1V / 100 k = 10 A, and charging a capacitor with a constant current means the voltage ramps up linearly at a rate of V/ t = I/C. So t = (C/I) V. • The period T = (0.1 F / 10 A)  10 Volts = 0.1 sec. • So f = 1 /T = 10 Hertz.

19. Troubleshooting • As always, check all DC voltages. • Typically, these oscillators either work or they do not; they do not tend to drift. • Frequencies are not precise (except for crystal stabilized) so oscilloscope measurements are OK. • Though not often used, if an aluminum electrolytic is the timing capacitor, it is a suspect. • If a potentiometer is used to adjust an RC time constant, check if it has been “tweaked”. • Look for physical damage to components.