1. Astable Multivibrators ©Paul Godin Created February 2007 Modified March 2015 Oscillator Basics Technician Series

2. Definitions ◊Astable ◊No stable state ◊Produces alternate high/low states ◊Astable Multivibrators are also known as: ◊Clocks ◊Oscillators astable 1.2

3. 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

4. Period and Duty Cycle ◊Duty cycle describes the ratio of the time in the high state versus the overall period of the pulse. T tHtH tLtL Review astable 1.4

5. Does Duty Cycle Matter? ◊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.? Review astable 1.5

6. 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

7. 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

8. Schmitt-Triggered Oscillators astable 1.8

9. Oscillator Circuits ◊Describe the output for the following device: astable 1.9

10. 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

11. Controlling the Simple Oscillator ◊The output frequency of the oscillator can be adjusted by adding an RC to the circuit: astable 1.11

12. 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

13. Simple Oscillator Output Vc: Charge/Discharge Cycle Discharge Time Charge Time Oscillator Animation astable 1.13

14. 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

15. 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

16. Crystal Oscillators astable 1.16

17. Crystal Oscillators ◊Crystal Oscillators are commonly used in conjunction with microprocessors, communications circuits and other frequency- sensitive devices because of their: ◊reliability ◊stability ◊accuracy ◊ease of use Symbol astable 1.17

18. 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

19. Crystal Oscillator Circuits There are many different configurations for crystal oscillators. Following are some examples of basic circuits: astable 1.19

20. Crystal Oscillator Circuits There are many other ways to create a stable oscillation with crystals. astable 1.20

21. Crystal Oscillator Circuits astable 1.21

22. Crystal Oscillators ◊Crystal Oscillators are often packaged in an oval- shaped 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

23. Operation of the Simple Oscillator 0 1- Logic 0 read by input of inverter. 1 2-Output becomes logic 1. 5- The capacitor discharges to VT-. 4-Output becomes logic 0. 0 3- Capacitor voltage increases to VT+. The gate senses a logic 1 input. 1 Animated astable 1.23

24. ©Paul R. Godin prgodin ° @ gmail.com END astable 1.24