Electronic Instrumentation Experiment 8: Diodes (continued) Project 4: Optical Communications Link
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
What you will know Half wave and full wave rectifier circuit Voltage limiter circuit 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
Introduction to Diodes ANODE D1 DIODE CATHODE 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
Introduction to Diodes In effect, diodes act like a flapper valve Note: this is the simplest possible model of a diode
Introduction to Diodes D1 VAMPL = 10V FREQ = 1k V1 V D1N4002 V R1 1k Only positive current flows 0 10V 5V 0V -5V -10V 0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms V(D1:1) V(D1:2) Time
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
When a voltage source is less than 0.7V it looks like an open circuit When a voltage source is greater than 0.7V it looks like a 0.7V dc voltage source i-v characteristic of a real diode Real diode is close to ideal Ideal Diode Real diode
Diode Circuits Rectifiers Voltage Limiters (Clippers)
A Half Wave Rectifier Since the diode only allows current in one direction, only the positive half of the voltage is preserved.
Smoothing Capacitors Filtering can be performed by adding a capacitor across the load resistor D1 D1N4148 V2 R1 1k C1 47uF 0 This RC combination is a low pass filter It smoothes out the output to make it more like DC
A Full Wave Rectifier The rectifier we have just seen is called a halfwave rectifier since it only uses half of the sinusoidal voltage. A full wave rectifier uses both the negative and positive voltages.
A Full Wave Rectifier R3 50 V D1N4148 D5 D6 VOFF = 0 VAMPL = 10 FREQ = 1k V2 R4 10k V- V+ D7 D8 D1N4148 0 Note the path of current when source is positive. What diodes does the current pass through when the source voltage is negative? In what direction does the current travel through the load resistor?
A Full Wave Rectifier 1.4V (2 diodes) 10V 5V 0V -5V -10V 110.0ms 110.5ms 111.0ms 111.5ms 112.0ms 112.5ms 113.0ms V(D5:2) V(R4:2,D7:1) Time 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.
Full Wave Rectifier With Smoothing R1 50 D1N4148 D1 D3 VOFF = 0 VAMPL = 10 FREQ = 1k V1 R2 10k C1 D4 0.1uF D1N4148 D2 0 Capacitor holds charge 10V 5V 0V -5V -10V 110.0ms 110.5ms 111.0ms 111.5ms 112.0ms 112.5ms 113.0ms V(R1:2) V(R2:2,D1:1) V(R4:2,D7:1) Time
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 A R1 1k B V1 D1 D1N4148 D2 D1N4148 0
Voltage Limitation R1 VOFF = 0 VAMPL = 10 FREQ = 1k V3 1k V D1 D1N4148 V D2 D1N4148 10V 0 5V (1.2420m,718.277m) 0V -5V -10V 0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms V(R1:1) V(R1:2) Time
In Class Problem: Diode Limiter Circuits Redraw what the circuit looks like for and give the value of Vout: +2V -2V 0V Replace the diodes that are on with voltage sources and diodes that are off with open circuits. Indicate the voltage value of Vout. REMEMBER note direction of diodes!
Part II: Diode Limiter Circuits Vin= +2V Vin= -2V Vin= 0V
Part II: Diode Limiter Circuits Sketch the output at Vout on the graph of Vin 0.7V -1.4V
Zener Diodes Introduction i-v curve for a Zener diode Zener diode voltage regulation
Zener Diodes Up to this point, we have not taken full advantage of the reverse biased part of the diode characteristic. Ideal Zener Diode I -V Z V
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 (V Z ) of 5V, will have a breakdown voltage of -5V.
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
A R1 B Zener Diodes Circuits 1V V1 1k D1 D1N7 0 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 V Z 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).
Zener Diodes R1 VOFF = 0 VAMPL = 10 FREQ = 1k V1 V 1k 0 V D1 D1N750 Note the voltage limitation for both positive and negative source voltages 10V 5V 0V -5V -10V 0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms V(R1:1) V(D1:2) Time
Wall Warts
Transformer Rectifier R1 TX1 VOFF = 0 VAMPL = 120 FREQ = 60 V1 5 V D1 D1N4148 D4 D3 D1N4148 D2 C2 33uF V R2 1k D1N4148 D1N4148 0 0 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)
Transformer Rectifier 120V 80V Filtered 40V -0V -40V Unfiltered -80V -120V 10.000s 10.005s 10.010s 10.015s 10.020s 10.025s 10.030s 10.035s 10.040s 10.045s 10.050s V(R1:2) V(R3:2) V(D2:2) V(R4:1) Time
Diodes and Light Light Emitting Diodes (LEDs) Photodiodes and Phototransistors
Light Emitting Diodes The Light-Emitting Diode (LED) is a semiconductor pn junction diode that emits visible light or nearinfrared 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.
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 reversebiased pn junction, a very small current proportional to the light intensity flows. The sensitivity depends on the wavelength of light.
Phototransistor Light Sensitivity The current through a phototransistor is directly proportional to the intensity of the incident light.
Electronic Instrumentation Project 4: Optical Communication Link 1. Optical Communications 2. Initial Design 3. PSpice Model 4. Final Design 5. Project Report
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 http://www.andor.com/image_lib/lores/introduct ion/introduction%20(light)/intlight%201%20sm all.jpg http://spie.org/x8857.xml
1. Optical Communications
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
Transmitting an audio signal using light In free space (air) Transmitter Circuit Receiver Circuit
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.
Modulation 8.0V 7.0V 6.0V 5.0V 4.0V 3.0V Modulating Input signal 2.0V 1.0V 0V -1.0V -2.0V -3.0V Carrier signal -4.0V 0s V(R2:1) V(R1:1) Time 4.0ms Output (modulated carrier) depends on the type of modulation used
Modulation Types General Frequency Modulation Amplitude Modulation Pulse Pulse Width Modulation Pulse Position Modulation Pulse Frequency Modulation
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
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
Pulse Modulation Remember duty cycle definition and equation T on T off Duty _ Cycle Ton T T Ton Toff Duty _ Cycle Pulse_ width Period 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
Pulse Width Modulation 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. http://cnyack.homestead.com/files/modulation/modpwm.htm
Pulse Position Modulation 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. http://cnyack.homestead.com/files/modulation/modppm.htm
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.
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.
RRC with variable resistor: Changes sampling frequency (of carrier signal) Transmitter Circuit 5V V1 Rpot 100k C8 330u R3 1k.001uF C2 R2 0 27k 4.7uF C3 Function_Gen_1 2 4 5 6 7 VCC 8 GND 1 X1 TRIGGER RESET OUTPUT CONTROL THRESHOLD DISCHARGE 555D R19 100ohms LED 3 D1 555 Timer Similar to astable multivibrator configuration: Pin five input alters frequency of pulses
Transmitter Circuit: Input and Modulated Output Rpot 100k 5V V1 C8 330u R3 1k.001uF C2 R2 27k 4.7uF C3 Function_Gen_1 2 4 5 6 7 VCC 8 GND 1 X1 TRIGGER RESET OUTPUT CONTROL THRESHOLD DISCHARGE 3 555D R19 100ohms LED D1 Input signal: function generator or audio 0 Output signal: Light modulation from LED
Special Capacitors 5V V1 Rpot 100k C8 330u R3 1k.001uF C2 R2 27k 4.7uF C3 Function_Gen_1 2 4 5 6 7 VCC 8 GND 1 X1 TRIGGER RESET OUTPUT CONTROL THRESHOLD DISCHARGE 3 555D R19 100ohms LED DC Blocking Capacitor (High Pass Filter) Keeps DC offset from 555 Timer from interfering with input D1 Bypass Capacitor (Low Pass Filter) 0
Sample Input and Output When input is higher, pulses are longer When input is lower, pulses are shorter
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.
Sampling Frequency The pot (used as a variable resistor) controls your sampling frequency Input frequency in audible range max range (20-20kHz) representative range (500-4kHz) Sampling frequency should be between 8kHz and 48kHz to reconstruct sound Input amplitude should not exceed 2Vp-p Function generator can provide 1.2Vp-p
Receiver Circuit 56k Add a 100 Ohm resistor in series with the speaker to avoid failures.
Receive Light Signal 56k Add a 100 Ohm resistor in series with the speaker to avoid failures.
Inverting Amplifier (Pre-Amp) 56k Add a 100 Ohm resistor in series with the speaker to avoid failures.
Audio Amplifier 56k Add a 100 Ohm resistor in series with the speaker to avoid failures.
Audio Amplifier Details increases gain 10X (not needed) 386 audio amplifier high pass filter volume Add a 100 Ohm resistor in series with the speaker to avoid failures. low pass filter
Special Capacitors 56k Not needed Bypass Capacitor DC Blocking Capacitor Add a 100 Ohm resistor in series with the speaker to avoid failures.
3. PSpice Model You will compare the performance of your circuit to a PSpice model. The PSpice for the initial design will be given to you. You will use the PSpice to help you make decisions about how to create your final design.
Comparing Output of Blocks Take pictures of the signal on each side of the circuit block. A on channel 1 and B on channel 2 B on channel 1 and C on channel 2 Take all measurements relative to ground Does the block behave as expected? How does it compare to the PSpice output?
10V 5V 0V Comparing Output of Blocks wide-angle view Shows overall shape and size of input and output -5V 8.0ms 8.4ms 8.8ms 9.2ms 9.6ms 10.0ms V(R1:1) V(L1:2) Time 1.0V 0V -1.0V 8.301ms V(R1:1) 8.400ms 8.500ms 8.600ms 8.700ms V(L1:2)/10 Time 8.799ms close-up view Output divided by 10 Shows sampling frequency Shows shape of samples
4. Final Design The signal is reconstructed well enough by the initial design that it will be audible. In order to improve the quality of the signal, you will add an integrator, which will more exactly reconstruct it. Types of integrators passive integrator (low pass filter) active integrator (op amp integrator circuit) You will then improve the signal further with a smoothing capacitor.
Vin Passive Integration E R1 C1 Vout 500mV 250mV V f out C 0 1 V RC 1 2 RC in dt 0V 1.0Hz 10KHz 100MHz V(R1:2) Frequency Integration works only at high frequencies f >>fc. Unfortunately, your amplitude will also decrease.
Active Integration E F 500mV 250mV 0V 1.0Hz 10KHz 100MHz V(R1:2) Frequency V out f 1 R C C i V in 1 2 R dt f C Integration works at f >>fc Your gain goes from -R f /R i to -1/R i C The amplitude of your signal will decrease or increase depending on components
Input at A vs. Output at H 10V 5V 0V -5V 8.0ms 8.4ms 8.8ms 9.2ms 9.6ms 10.0ms V(R1:1) V(L1:2) Before addition Time of integrator 4.0V 2.0V 0V -2.0V 8.0ms 8.4ms 8.8ms 9.2ms 9.6ms 10.0ms V(V4:+) V(L1:2) Time After addition of integrator
Effect of Smoothing Capacitor D1 V D1N4148 V VOFF = 0 VAMPL = 5v FREQ = 1k V1 R1 1k C1 5u 0 Recall what the smoothing capacitor did to the output of the half wave rectifier.
Input at A vs. Output at H 4.0V 2.0V 0V -2.0V 8.0ms 8.4ms 8.8ms 9.2ms 9.6ms 10.0ms V(V4:+) V(L1:2) Before smoothing Time capacitor 2.0V 0V -2.0V 8.0ms 8.4ms 8.8ms 9.2ms 9.6ms 10.0ms V(V4:+) V(L1:2) -v(l1:2) After smoothing Time capacitor
Project Packet Initial Data with Function Generator PSpice Mobile Studio plots from circuit Brief Comparison Block Description For Blocks: A-B, A-C, A-D, A-E, A-F, A-G Overall System: A-H Initial Data with Audio Mobile Studio plots from circuit For E-F and A-H
Project Packet Final Data (integrator only) with Function Generator PSpice Mobile Studio plots from circuit Brief Comparison For E-F and A-H Final Data (integrator and smoothing) PSpice only PSpice Compare to without smoothing For E-F and A-H
Project Packet Final Data with Integrator (and possibly Smoothing) with Audio Mobile Studio plots from circuit For E-F and A-H Extra Credit Mobile Studio picture of A-H with input from function generator and integrated, smoothed output. Indicate values of components and where used.
Work in teams Put the transmitter on one protoboard and the receiver on a second. One pair do the transmitter circuit This is the easier circuit, so maybe also start the PSpice simulation. The other pair build the receiver circuit One report for the entire team Report is closer to an experiment report than a project report See details in handout.