CHAPTER 14 Nonsinusoidal Oscillators Objectives Describe and Analyze: • Operation of the 555 IC • Inverter oscillators • Schmitt oscillators • Wave-shaping • Sawtooth oscillators • Troubleshooting 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. Square-Wave Oscillators Square wave from a “free-running” 555 circuit. The “Internals” of a 555 Frequency set by R A , R B , and C. 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. 555 Oscillator f OSC = 1.44 / [(R A + 2R B )  C] 555 One-Shot t = 1.1RC Inverter Oscillator f OSC depends on the number of inverters (must be odd). 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 T LONG = 11 ns  2  3 = 66 ns and T SHORT = 9 ns  2  3 = 54 ns So f LO = 1 / 66 ns  15.2 MHz and f HI = 1 / 54 ns  18.5 MHz [...]... 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 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 . CHAPTER 14 Nonsinusoidal Oscillators Objectives Describe and Analyze: • Operation of the 555 IC • Inverter oscillators • Schmitt oscillators • Wave-shaping • Sawtooth oscillators. vacuum tube days when they actually used electromechanical vibrators in circuits. Square-Wave Oscillators Square wave from a “free-running” 555 circuit. The “Internals” of a 555. 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