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Astable Multivibrators Oscillator Basics Technician Series ©Paul Godin Created February 2007 Modified March 2015 Definitions ◊ Astable ◊ No stable state ◊ Produces alternate high/low states ◊ Astable Multivibrators are also known as: ◊ Clocks ◊ Oscillators astable 1.2 Uses of Astables ◊ Provide edges for edge-triggered devices ◊ ◊ ◊ ◊ ◊ ◊ flip-flops counters shift registers Digital to Analog / Analog to Digital converters microprocessors communications, etc… ◊ Can provide sound for certain applications ◊ practical audible sound in the 100Hz to 5kHz range ◊ exercise caution when applying square waves to speakers astable 1.3 Period and Duty Cycle ◊ Review Duty cycle describes the ratio of the time in the high state versus the overall period of the pulse. T tH tL tH D.C. 100% tH tL astable 1.4 Does Duty Cycle Matter? Review ◊ To an edge-triggered device, does the duty cycle affect its operation? ◊ If a 10% D.C. clock is applied to the following circuit, what is the output D.C.? astable 1.5 Square waves and speakers ◊ Cautions: ! ◊ The average power for a square wave is higher than for a sine wave with the same peak voltage. Speaker coil damage may result. ◊ A speaker is an electro-mechanical device. It is physically unable to produce the instantaneous motion of a square wave. Damage to the cone and physical structure may result. ◊ Speakers have a low impedance and likely represents a greater load than the driving circuit is capable of handling. Damage to the driving circuit may result. astable 1.6 Speaker Interfaces ◊ ◊ ◊ ◊ Use cheap speakers! Keep the output voltages low. Use an output device that can handle the load. Filter the output square waves ◊ Use an RC circuit in series. ◊ Use an audio transformer. Discussion in class astable 1.7 Schmitt-Triggered Oscillators astable 1.8 Oscillator Circuits ◊ Describe the output for the following device: astable 1.9 Oscillator Parameters ◊ In ◊ ◊ ◊ the previous oscillator circuit: What determines the output frequency? What is the waveform of the output? What determines the duty cycle? ◊ How can we slow the process down? In-Class Discussion astable 1.10 Controlling the Simple Oscillator ◊ The output frequency of the oscillator can be adjusted by adding an RC to the circuit: astable 1.11 Schmitt Oscillator ◊ The separation between Vt+ and Vt- can be used to create an oscillating circuit. ◊ An RC network is used to control the oscillation rate by controlling the charge and discharge time of the capacitor voltage. ◊ Easy oscillator to build. Used where precise or accurate frequency isn’t necessary. ◊ displays ◊ visual effects astable 1.12 Simple Oscillator Output Vc: Charge/Discharge Cycle Charge Time Discharge Time Oscillator Animation astable 1.13 Schmitt Trigger Oscillator Control Schmitt Triggered Oscillators may be controlled by the use of RC circuits. To achieve a specific frequency, the values of R and C may be calculated. astable 1.14 The Simple Schmitt Oscillator ◊ Advantages: ◊ Easy to build ◊ Fair range of frequency ◊ Small footprint ◊ Disadvantages: ◊ Unstable, as the frequency will vary with temperature variations. ◊ Difficult to predict values due to the range of Vt+ and Vt- between different gates, even within the same IC package. astable 1.15 Crystal Oscillators astable 1.16 Crystal Oscillators ◊ Crystal Oscillators are commonly used in conjunction with microprocessors, communications circuits and other frequencysensitive devices because of their: ◊ ◊ ◊ ◊ reliability stability accuracy ease of use Symbol astable 1.17 Crystal Oscillators ◊ A crystal oscillator is constructed from a piece of quartz crystal that is cut and shaped to the appropriate size. ◊ A property called piezoelectricity happens with quartz crystals. ◊ ◊ If pressure is applied, it creates voltage If voltage is applied, it physically vibrates ◊ When a voltage is applied to the crystal, it vibrates at a very specific frequency. ◊ Crystal oscillators commonly require small capacitors to aid with the back-and-forth voltage, and require a source of current. astable 1.18 Crystal Oscillator Circuits There are many different configurations for crystal oscillators. Following are some examples of basic circuits: astable 1.19 Crystal Oscillator Circuits There are many other ways to create a stable oscillation with crystals. astable 1.20 Crystal Oscillator Circuits astable 1.21 Crystal Oscillators ◊ Crystal Oscillators are often packaged in an ovalshaped metallic “can”. ◊ Those with 2 leads require external circuitry; those with 4 leads typically already possess the internal circuitry required to produce the oscillation (voltage and ground needs to be applied). astable 1.22 Operation of the Simple Oscillator 1- Logic 0 read by input of inverter. 4-Output becomes logic 0. 5- The capacitor discharges to VT-. 1 0 0 1 2-Output becomes logic 1. 3- Capacitor voltage increases to VT+. The gate senses a logic 1 input. Animated astable 1.23 END ©Paul R. Godin prgodin°@ gmail.com astable 1.24