Experiment 8: Diodes (continued) Project 4: Optical Communications Link

1. Experiment 8: Diodes (continued)Project 4: Optical Communications Link

2. Agenda • Brief Review: Diodes • Zener Diodes • Project 4: Optical Communication Link • Why optics? • Understanding Modulation • Initial Design of optical link • Transmitter • Receiver • PSpice Model • Your final design

3. What you will know • What a Zener diode is used for • How a signal is modulated to carry information • How what you’ve learned to this point in this course can be used for the optical link • What is expected in Project 4 • What the PSpice model is representing • What the simulated output tells you

4. Introduction to Diodes • A diode can be considered to be an electrical one-way valve. • They are made from a large variety of materials including silicon, germanium, gallium arsenide, silicon carbide …

5. Introduction to Diodes • In effect, diodes act like a flapper valve • Note: this is the simplest possible model of a diode

6. Introduction to Diodes Only positive current flows

7. Diode i-v Characteristic Curves • What is a i-v characteristic curve? • i-v curve of an ideal diode • i-v curve of a real diode

8. i-v characteristic of a real diode • Real diode is close to ideal Ideal Diode

9. Diode Circuits • Rectifiers • Voltage Limiters (Clippers)

10. A Half Wave Rectifier Since the diode only allows current in one direction, only the positive half of the voltage is preserved.

11. Smoothing Capacitors • Filtering can be performed by adding a capacitor across the load resistor • This RC combination is a low pass filter • It smoothes out the output to make it more like DC

12. A Full Wave Rectifier The rectifier we have just seen is called a half-wave rectifier since it only uses half of the sinusoidal voltage. A full wave rectifier uses both the negative and positive voltages.

13. 1.4V (2 diodes) A Full Wave Rectifier Note: Since a small voltage drop (around 0.7V) now occurs over two diodes in each direction, the voltage drop from a full wave rectifier is 1.4V.

14. Full Wave Rectifier With Smoothing Capacitor holds charge

15. Voltage Limitation • In many applications, we need to protect our circuits so that large voltages are not applied to their inputs • We can keep voltages below 0.7V by placing two diodes across the load

16. Voltage Limitation

17. Zener Diodes • Introduction • i-v curve for a Zener diode • Zener diode voltage regulation

18. Zener Diodes • Up to this point, we have not taken full advantage of the reverse biased part of the diode characteristic.

19. Zener Diodes • For the 1N4148 diode, the breakdown voltage is very large. If we can build a different type of diode with this voltage in a useful range (a few volts to a few hundred volts), we can use such devices to regulate voltages. This type of diode is called a Zener diode because of how the device is made. • Zener diodes are rated according to where they break down. A diode with a Zener voltage (VZ) of 5V, will have a breakdown voltage of -5V.

20. i-v characteristic of Zener diodes Knee Current • For a real Zener diode, a finite current (called the knee current) is required to get into the region of voltage regulation • Just like regular diodes, Zener diodes have a small reverse saturation current in the reverse bias region and a forward bias threshold voltage of about 0.7V

21. Zener Diodes Circuits • Although Zener diodes break down at negative voltages, Zener voltages are given as positive and Zener diodes are typically placed in circuits pointing away from ground. • The voltage in this circuit at point B will • hold at VZ when the Zener diode is in the breakdown region. • hold at -0.7 when the Zener diode is forward biased • be equal to the source voltage when the Zener diode is off (in the reverse bias region).

22. Zener Diodes • Note the voltage limitation for both positive and negative source voltages

23. Wall Warts

24. Transformer Rectifier • Adding a full wave rectifier to the transformer makes a low voltage DC power supply, like the wall warts used on most of the electronics we buy these days.(In reality, VAC is 120Vrms => 170Vpeak)

25. Transformer Rectifier Filtered Unfiltered

26. Zener Diode Voltage Regulation Note stable voltage

27. Diodes and Light • Light Emitting Diodes (LEDs) • Photodiodes and Phototransistors

28. Light Emitting Diodes • The Light-Emitting Diode (LED) is a semiconductor pn junction diode that emits visible light or near-infrared radiation when forward biased. • Visible LEDs emit relatively narrow bands of green, yellow, orange, or red light. Infrared LEDs emit in one of several bands just beyond red light.

29. Photodiodes and Phototransistors • Photodiodes are designed to detect photons and can be used in circuits to sense light. • Phototransistors are photodiodes with some internal amplification. Note: Reverse current flows through the photodiode when it is sensing light. If photons excite carriers in a reverse-biased pn junction, a very small current proportional to the light intensity flows. The sensitivity depends on the wavelength of light.

30. Phototransistor Light Sensitivity The current through a phototransistor is directly proportional to the intensity of the incident light.

31. Project 4: Optical Communication Link 1. Optical Communications 2. Initial Design 3. PSpice Model 4. Final Design 5. Project Report

32. Why use optics? Advantages of optical communication(over Radio Frequency) • Wider bandwidth • Larger capacity • Lower power consumption • More compact equipment • Greater security against eavesdropping • Immunity from interference • More directed energy http://www.andor.com/image_lib/lores/introduction/introduction%20(light)/intlight%201%20small.jpg http://spie.org/x8857.xml

33. 1. Optical Communications

34. “Lighting the way to a revolution”http://news.bbc.co.uk/1/hi/sci/tech/4671788.stm • The exponential increase of sharing information is largely due to optical communication technology • A few revolutionary technologies based on or effected by optical communication • Internet (ex. Ethernet LAN based on Infrared Technology) • Cell phones • Satellite communication • Others? 1966 Dr. Kao and George Hockham: fiber optics to carry information with light

35. Transmitting an audio signal using light In free space (air) Transmitter Circuit audio signal Receiver Circuit

36. Modulation • Modulation is a way to encode an electromagnetic signal so that it can be transmitted and received. • A carrier signal (constant) is changed by the transmitter in some way based on the information to be sent. • The receiver then recreates the signal by looking at how the carrier was changed.

37. Modulation Modulating Input signal Carrier signal Output (modulated carrier) depends on the type of modulation used

38. Modulation Types • General • Frequency Modulation • Amplitude Modulation • Pulse • Pulse Width Modulation • Pulse Position Modulation • Pulse Frequency Modulation

39. Amplitude Modulation Frequency of carrier remains constant. Input signal alters amplitude of carrier. Higher input voltage means higher carrier amplitude. http://cnyack.homestead.com/files/modulation/modam.htm

40. Frequency Modulation Amplitude of carrier remains constant. Input signal alters frequency of carrier. Higher input voltage means higher carrier frequency. http://cnyack.homestead.com/files/modulation/modfm.htm

41. Toff Ton Pulse Modulation • Remember duty cycle definition and equation • Carrier has a constant variable • Pulse Width Modulation - Period is constant • Pulse Position Modulation - Pulse width is constant • Pulse Frequency Modulation - Duty cycle is constant • Input modulates carrier and effects other two variables

42. Period of carrier remains constant. Input signal alters duty cycle and pulse width of carrier. Higher input voltage means pulses with longer pulse widths and higher duty cycles. Pulse Width Modulation http://cnyack.homestead.com/files/modulation/modpwm.htm

43. Pulse width of carrier remains constant. Input signal alters period and duty cycle of carrier. Higher input voltage means pulses with longer periods and lower duty cycles. Pulse Position Modulation http://cnyack.homestead.com/files/modulation/modppm.htm

44. Pulse Frequency Modulation Duty cycle of carrier remains constant. Input signal alters pulse width and period of carrier. Higher input voltage means pulses with longer pulse widths and longer periods.

45. 2. Initial Design transmitter receiver • The initial design for this project is a circuit consisting of a transmitter and a receiver. • The circuit is divided into functional blocks. • Transmitter: Block A-B and Block B-C • Transmission: Block C-D • Receiver: Block D-E, Block E-F, Block F-G, and Block G-H • You will need to examine each block of the circuit.

46. 555 Timer Similar to astable multivibrator configuration: Pin five input alters frequency of pulses Transmitter Circuit RRC with variable resistor: Changes sampling frequency (of carrier signal)

47. Transmitter Circuit:Input and Modulated Output Output signal: Light modulation from LED Input signal: function generator or audio

48. Special Capacitors DC Blocking Capacitor (High Pass Filter) Keeps DC offset from 555 Timer from interfering with input Bypass Capacitor (Low Pass Filter)

49. Sample Input and Output • When input is higher, pulses are longer • When input is lower, pulses are shorter

50. Your signal is what? The type of modulation this circuit creates is most closely categorized as pulse frequency modulation. But the pulse width is also modulated and we will use that feature.