Digital Communications 2

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Edition 1 34935-10

Basic Electricity and Electronics

by

Instructor’s Guide

Digital Communications 2

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FIRST EDITION

First Printing, June 2002

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form by any means, electronic, mechanical, photocopied, recorded, or otherwise, without prior written permission from Lab-Volt Systems, Inc.

Information in this document is subject to change without notice and does not represent a commitment on the part of Lab-Volt Systems, Inc. The Lab-Volt Information Technology software and other materials described in this document are furnished under a license agreement or a nondisclosure agreement. The software may be used or copied only in accordance with the terms of the agreement.

Lab-Volt and F.A.C.E.T.® logos are trademarks of Lab-Volt Systems, Inc.

All other trademarks are the property of their respective owners. Other trademarks and trade names may be used in this document to refer to either the entity claiming the marks and names or their products. Lab-Volt System, Inc. disclaims any proprietary interest in trademarks and trade names other than its own.

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Lab-Volt License Agreement

By using the software in this package, you are agreeing to become bound by the terms of this License Agreement, Limited Warranty, and Disclaimer.

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Attention: Program Development

Phone: (732) 938-2000 or (800) LAB-VOLT Fax: (732) 774-8573

Technical Support: (800) 522-4436

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FOREWORD

This instructor's guide provides a unit-by-unit outline of the principal points made in the FACET curriculum. For each unit, instructors are given a unit objective, a brief description of the material covered in each unit and a list of important points to emphasize. Review question answers, unit test answers, faults and circuit modifications (CM) are provided in the appendices.

SAFETY

Safety is everyone's responsibility. All must cooperate to create the safest possible working environment. Students must be reminded of the potential for harm, given common sense safety rules, and instructed to follow the electrical safety rules.

Any environment can be hazardous when it is unfamiliar. The F.A.C.E.T.

computer-based laboratory may be a new environment to some students. Instruct students in the proper use of the F.A.C.E.T. equipment and explain what

behavior is expected of them in this laboratory. It is up to the instructor to provide the necessary introduction to the learning environment and the equipment. This task will prevent injury to both student and equipment.

The voltage and current used in the F.A.C.E.T. Computer-Based Laboratory are, in themselves, harmless to the normal, healthy person. However, an electrical shock coming as a surprise will be uncomfortable and may cause a reaction that could create injury. The students should be made aware of the following

electrical safety rules.

1. Turn off the power before working on a circuit.

2. Always confirm that the circuit is wired correctly before turning on the power. If required, have your instructor check your circuit wiring.

3. Perform the experiments as you are instructed: do not deviate from the documentation.

4. Never touch "live" wires with your bare hands or with tools. 5. Always hold test leads by their insulated areas.

6. Be aware that some components can become very hot during operation. (However, this is not a normal condition for your F.A.C.E.T. course

equipment.) Always allow time for the components to cool before proceeding to touch or remove them from the circuit.

7. Do not work without supervision. Be sure someone is nearby to shut off the power and provide first aid in case of an accident.

8. Remove power cords by the plug, not by pulling on the cord. Check for cracked or broken insulation on the cord.

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INSTALLATION INSTRUCTIONS

Installing Courseware

Courseware is to be installed using the Configurator of Tech-Lab. For more detailed information on installing Courseware refer to the Tech-Lab and

GradePoint 2020 Installation Guide Courseware installation section. The manual number is 34288-E0.

Installing Resources

Resources are to be installed using the Configurator of Tech-Lab. For more detailed information on installing and linking Resources to the courseware refer to the Tech-Lab and GradePoint 2020 Installation Guide Resource installation section. The manual number is 34288-E0.

Installing Applications

Install all applications per the manufacturer’s recommended settings. Refer to the manufacturer documentation for assistance.

Applications are to be linked to the courseware using the Configurator of Tech-Lab. For more detailed information on installing Applications refer to the Tech-Lab and GradePoint 2020 Installation Guide Application installation section. The manual number is 34288-E0.

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1-5

NEW TERMS AND WORDS

asynchronous - relating to events that occur without a regular or predictable time relationship to a specified event; operating at independent frequencies. baseband - the band of frequencies associated with an original signal. BER (bit error rate) - the number of incorrect data bits received in a digital transmission in a specified period of time.

broadband - relating to the transmission of signals over a frequency range that is produced by a modulated-carrier system.

data block - a group of data bits transmitted as a unit.

modem - a signal conversion device that contains both a modulator and demodulator.

on-off keying (OOK) - a form of ASK modulation in which the smaller amplitude is 0V.

protocols - rules of communication system operation that must be followed. synchronous - relating to events that occur at the same time or that depend on the occurrence of a common timing signal; operating at the same frequency or at a frequency derived from the system.

start bit - a bit that precedes data bits in a transmission and signals the beginning of the transmission.

stop bit - a bit that follows data bits in a transmission and signals the end of the transmission. It also provides space between transmissions.

EQUIPMENT REQUIRED F.A.C.E.T. base unit

DIGITAL COMMUNICATIONS 2 circuit board Power supply, 15 Vdc (2 required)*

Oscilloscope, dual trace

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1-7

• recover NRZ from MAN using the synchronous decoder • observe frequency changes on an FSK signal

• observe PSK, ASK, and OOK signals

• distort signals using noise from the channel simulator • demonstrate BER operation

• recover data using the synchronous detector • recover data using the asynchronous detector • demonstrate modem operation

CMs AVAILABLE

CM10 - Selects the FSK phase comparator filter in the SYNC DETECTOR PLL circuit.

FAULTS AVAILABLE None required

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1-9

EXERCISE 1-2 Communications System Model

EXERCISE OBJECTIVE

Demonstrate the operation of a communication system stage by stage.

DISCUSSION

• The DIGITAL COMMUNICATIONS 2 circuit board has circuit blocks that per-form the functions of all the elements of the comunications model.

• The ENCODER block genetates signals that are encoded with digital data. • In the MODULATORS circuit block, the encoded signals modulate a carrier

signal.

• The CHANNEL SIMULATOR circuit block simulates the transmission medium over which the modulated signal is carried.

• Demodulation is accomplished using the detectors.

• The MAN SYNC DECODER block recovers the NRZ and clock signals. CMs AVAILABLE

None required

FAULTS AVAILABLE None required

PROCEDURE

The 30 procedure steps in this exercise include the following:

• produce ASK by using the MAN output to the ENCODER block to amplitude-modulate a carrier

• transmit the ASK signal through the CHANNEL SIMULATOR and introduce noise

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1-10

• recover the Manchester signal from the ASK using the SYNC DETECTOR

circuit block

• decode the recovered Manchester signal into NRZ and clock signals using the

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2-3

Special line coding schemes are used with AMI signals to provide proper clock recovery when long strings of 0's are transmitted.

In a Binary 8-Zero Substitution scheme (B8ZS), strings of 8 zeros are detected at the transmitter and substituted with code patterns containing 1's. At the receiver, these substituted code patterns are used for clock recovery and then changed back to the correct code of 8 zeroes.

NEW TERMS AND WORDS

baseband data transmission - when a signal is transmitted without modification; no carrier signal.

baud - a unit of signaling speed equal to the reciprocal of the shortest element duration in seconds.

data - a group of characters (1's and 0's) that make up a message or information. data speed (bps) - the rate at which bits or binary digits are transmitted.

decode - to produce clear information from previous encoded data. encoded - to express information in terms of a code.

frequency spectrum - the distribution of the amplitude (energy) of a signal as a function of frequency.

line coding - encoding techniques used in data transmissions.

polar signals - signals possessing positive or negative polarity with respect to circuit common (zero).

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2-5

CMs AVAILABLE

CM2 - Data pattern = 10101010; sent at 300 bps

CM1 & CM2 - Data pattern = 01000100; sent at 1200 bps

FAULTS AVAILABLE None required.

PROCEDURE

The 62 procedure steps in this exercise include the following: NRZ LINE CODING

• measuring the time period of the CLK • determining the length of the data word

• determining the value of the bits contained in the data word • measuring the baud rate

• determining the transmission speed RZ LINE CODING

• determining the value of the bits contained in the data word • determining the baud rate

• determining the transmission speed MANCHESTER LINE CODING

• determining the value of the bits contained in the data word

• observe the clocking information on the MANCHESTER coded signal • determining the baud rate

• determining the transmission speed • relate baud rate to signal bandwidth LINE LEVEL CODING

• compare polar and unipolar NRZ signals • measure the dc content of a polar NRZ signal

• measure the dc content of a polar Manchester signal

(24)

2-9

EXERCISE 2-2 Decoding

EXERCISE OBJECTIVE

Describe three common methods used to decode RZ and Manchester signals into NRZ signals.

DISCUSSION

• Sometimes data and clock signals are sent over separate lines.

• RZ and Manchester coded data can be decoded using a D-type flip-flop. • An XOR gate can be used to decode Manchester coded data.

CMs AVAILABLE

CM2 - Data pattern = 10101010; sent at 300 bps

PROCEDURE

The 51 procedure steps in this exercise include the following: DECODING RZ

• decode RZ using a D-type flip-flop • compare RZ to decoded NRZ

• observe the limited clock information available from an RZ signal • compare the decoded NRZ with the NRZ before it was coded as RZ DECODE MANCHESTER.

• decode Manchester using an XOR gate.

• observe decoding spikes (glitches) that can occur.

• compare the decoded NRZ with the NRZ before it was Manchester coded.

• lock the PLL

• recover the CLK from the Manchester. • decode Manchester to NRZ.

(25)

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3-3

If the channel's passband is wide enough, two carrier signals can be used to provide full-duplex operation. The BELL 103 standard defines a full-duplex 300 baud FSK modem using two audio carrier signals. The station originating the call transmits 1070 Hz for a logic low and 1270 Hz for a logic high. The station

answering the phone transmits a 2025 Hz for a logic low and 2225 Hz for a logic high. Each change in the baseband signal generates one change in the 300 baud BELL 103 FSK carrier frequency.

FSK demodulators fall into two basic categories, synchronous and asynch-ronous. Asynchronous demodulators filter the carrier signal before using an envelope detector to recover the baseband signal. Synchronous demodula-tors synchronize a reference signal with the carrier signal to detect changes in carrier frequency and recover the baseband signal.

NEW TERMS AND WORDS

analog channels - communication pathways intended for analog message signals. These channels are characterized by limited bandwidth and poor low frequency response.

analog multiplexer (MUX) - a circuit that will pass one of several analog sig-nals selected by the control signal(s).

analog switches - electronically controlled switches intended for use with ana-log signals.

asynchronous - relating to events that occur without a regular or predictable time relationship. Operating at independent frequencies.

(28)

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3-5

EXERCISE 3-1 FSK Signal Generation

EXERCISE OBJECTIVE

Describe the relationship between FSK and the baseband digital modulating signal. Describe how an analog multiplexer can be used as an FSK modulator. Describe the frequency spectrum of an FSK signal.

DISCUSSION

• FSK is a simple low-cost modulation technique.

• The FSK modulator can be acoustically coupled or direct coupled. • A VCO can be used as an FSK modulator.

• The circuit board uses an analog MUX as an FSK modulator. CMs AVAILABLE

CM6 - Changes the phase of the HI TONE carrier signal.

CM12 - Removes the HI TONE carrier from thr FSK MODULATOR.

FAULTS AVAILABLE None Required.

PROCEDURE

The 39 procedure steps in this exercise include the following: • determine the baud rate of the NRZ modulating signal • determine the baud rate of the FSK MODULATOR output

• compare the amplitude of the FSK when the modulating signal is high and low

• compare the FSK phase before and after the frequency changes • compare the FSK frequency for an NRZ high and low

• observe the details of how a MUX is used to generate FSK

• describe the frequency spectrum of an FSK signal as the spectrum of two OOK signals

• observe FSK switching discontinuities and note their effect on signal bandwidth

(30)

3-7

EXERCISE 3-2 FSK Asynchronous Detection

EXERCISE OBJECTIVE

Recover the baseband NRZ signal from the FSK signal, demonstrate how a filter can convert an FSK signal into changes that represent the baseband signal, and demonstrate the operation of an asynchronous envelope detector.

DISCUSSION

• The FSK demodulator recovers the baseband digital signal by detecting the frequency changes in the FSK carrier signal.

• An FSK signal consists of two on-off keying (OOK) signals.

• A bandpass filter can be used to pass one of the OOK carrier signals while attenuating the other.

• The filter output will change in amplitude as the FSK changes frequency. • The amplitude changes are detected by an asynchronous detector. CMs AVAILABLE

CM7 - Changes the CHANNEL bandwidth to 1600 Hz

CM16 - Changes the center frequency of the ASYNC DETECTOR BANDPASS filter from 3000 Hz to 1500 Hz.

(31)

3-8

PROCEDURE

• review the relationship between the modulating NRZ and modulated FSK signals

• observe the amplitude changes in the bandpass filter output

• determine that the FWR block output is a full wave rectified ASK signal • observe the data present in the low-pass filter output

• adjust the voltage comparator reference voltage to restore the logic levels of the recovered coded data

• reduce the bandwidth of the channel simulator and observe that the NRZ is more difficult to recover

(32)

3-9

EXERCISE 3-3 FSK Synchronous Detection

EXERCISE OBJECTIVE

Recover a digitial signal from an FSK signal using a syncronous detector,

demonstrate how a phase-locked loop can be used to detect the baseband digital signal, and describe the operation of a phase-locked loop configured as a

(33)

3-10

DISCUSSION

• The FSK demodulator recovers the baseband digital signal by detecting the frequency changes in the FSK carrier signal.

CMs AVAILABLE

CM6 - Changes the phase of the HI TONE carrier signal. CM7 - Changes the CHANNEL bandwidth to 1600 Hz

CM10 - Selects the FSK phase comparator filter in the SYNC DETECTOR PLL circuit.

FAULTS AVAILABLE None Required.

PROCEDURE

• observe that the PLL output is not synchronous with the FSK signal when the phase comparator has no input

• observe that the phase comparator output keeps the VCO output synchronous with the FSK signal

• observe the XOR function used by the phase comparator

• determine that the level of the VCO input (phase comparator output) represents the frequency of the FSK signal

• measure the frequency of the VCO when the NRZ is high and low • observe that the VCO input represents the state of the NRZ signal • determine that the VCO input is low-pass filtered before the NRZ logic

levels are restored using a voltage comparator

• determine that a synchronous detection is less sensitive to amplitude variations than asynchronous detection

(34)

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4-2

The regenerated carrier signal is then combined with the PSK signal in a product detector. The product detector output is low-pass filtered, and the resulting pulses are shaped by a voltage comparator to recover the original digital intelligence signal from the received signal.

NEW TERMS AND WORDS

There are no new terms and words in this unit. SCHEMATIC, CMs AND FAULTS

SCHEMATIC SWITCH NUMBER CM NUMBER FAULT NUMBER CIRCUIT CHANGE WHEN ACTIVE

S6 CM6 changes the phase of the HI

TONE carrier signal

S11 CM11 selects the ASK/PSK phase

comparator fitler in the SYNC DETECTOR PLL circuit

S22 F2 opens the output of the buffer

amplifier in the ASK/PSK MODULATOR circuit Refer to the schematic at the end of this volume.

EXERCISE 4-1 PSK Signal Generation

EXERCISE OBJECTIVE

Explain and demonstrate how PSK signal generation is accomplished on the circuit board.

DISCUSSION

• The original signal is shifted from 0 and +5V logic levels to -5V and +5V polar logic levels.

• The polar digital signal is then multiplied with a carrier signal in a balanced modulator.

• Multiplying by a positive voltage produces a 0° phase shift. • Multiplying by a negative voltage produces a 180° phase shift. • PSK can be used with any type of encoding.

(37)

4-3

CMs AVAILABLE

CM6 - Changes the phase of the HI TONE carrier signal.

PROCEDURE

• configure the MODULATORS block to produce a PSK modulated signal • use the BAL pot to control polar signal dc offset

• determine that the modulator output is a PSK signal that represents the digital input

• measure the phase of the PSK signal when a logic high is input to the modulator

• observe a discontinuous PSK output when the phase between the carrier and the baseband data signal is changed

(38)

4-5

EXERCISE 4-2 Synchronous Detection

EXERCISE OBJECTIVE

Explain and demonstrate synchronous detection of a PSK signal.

DISCUSSION

• The carrier synchronizer regenerates a carrier from the received PSK signal. • The PLL VCO OUT frequency will also be twice the PSK signal frequency. • The doubler is a full wave rectifier and bandpass filter which removes the

intelligence from the PSK and doubles its frequency.

• The final stage divides the VCO OUT frequency by 2 and shifts it by 90° to produce the regenerated carrier.

• The regenerated carrier is mixed with the PSK to demodulate the signal. • The low-pass filter and voltage comparator perform the final shaping of the

pulses.

CMs AVAILABLE

CM11 - Selectes the ASK/PSK phase comparator filter in the SYNC DETECT- OR PLL circuit.

PROCEDURE

(39)

4-6

• observe the output of the rectifier on the DOUBLER block

• observe the output of the bandpass filter on the DOUBLER block

• determine that the PLL VCO has the same frequency as the DOUBLER output

• observe the output of the PHASE SHIFTER

• adjust the balance of the mixer to obtain the intelligence information • adjust the VOLT COMP reference to recover the baseband digital signal • note that the ASYNC DETECTOR can not demodulate the PSK signal

(40)

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5-2

An electronic SPST switch can be used to illustrate a simpler type of ASK

modulation. The carrier is connected (switch closed) to the output for a binary 1.

The carrier is disconnected (switch open) from the output for a binary 0. This special technique of amplitude modulation is called on-off keying (OOK). The abrupt on and off changes between signaling elements requires an increased channel bandwidth over standard ASK.

ASK demodulators (detectors) can be either asynchronous or synchronous. An asynchronous detector does not use a reference carrier to recover the ASK amplitude changes. A synchronous detector recovers the ASK amplitude changes using a reference carrier that agrees in frequency and phase with the original ASK carrier signal.

NEW TERMS AND WORDS

scaling summer - an inverting operational amplifier that allows scaling of each input before addition.

on-off keying (OOK) - a form of ASK in which the carrier is turned on to trans-mit a binary 1 and off to transtrans-mit a binary 0.

(43)

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5-4

• ASK modulation can be used with other types of encoding such as RZ and Manchester.

CMs AVAILABLE

CM1 & CM2 - Data pattern = 01000100; sent at 1200 bps.

PROCEDURE

• generate an ASK modulated signal from an NRZ encoded signal • observe inputs and outputs with an oscilloscope

(45)

5-6

EXERCISE 5-2 ASK Signal Detection

EXERCISE OBJECTIVE

Explain and demonstrate how ASK detection is accomplished on your circuit board.

DISCUSSION

• Detection, or demodulation, is the process of recovering the transmitted digital intelligence from a modulated signal.

• The amplitude changes in the ASK input signal are detected to recover the original NRZ signal.

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6-5

Noise can affect either the amplitude or phase of a signal. Phase noise is due to inherent delays in circuits and components. Amplitude noise can be meas-ured by determining the bit error rate (BER).

There are several ways to measure the BER. This block diagram shows the method used by the BER COUNTER block on your circuit board.

The transmitted and received data are compared bit by bit. If the bits do not match, an error pulse is generated. The errors are totalized in a counter over a fixed time period generated by a one-shot. A display indicates how many err-ors occurred within the time interval.

NEW TERMS AND WORDS

bandpass noise - noise that occurs over a band of frequencies, but not out-side those frequencies.

bit error rate (BER) - the number of incorrect bits received in reference to the total number of bits transmitted.

external noise - noise originating outside a communications system that can enter the system via the channel.

internal noise - noise originating inside a circuit or component in a transmitter or receiver.

low-pass noise - noise that ranges from dc to a certain cutoff frequency. noise - random, undesirable electrical energy that can interfere with the transmitted message in a communications system.

shot noise - random noise introduced by current flow in a semiconductor junction.

signal-to-noise ratio - the ratio of signal amplitude to noise amplitude. thermal noise - internal noise generated by thermal agitation of atoms.

white noise - a type of noise that has the same amount of energy over a wide range of frequencies.

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6-7

• An XOR gate is used because an error can be defined as a condition where the received data is not the same as the transmitted data.

• Each time you press and release the RESET pushbutton, the control circuit simultaneously resets the counter and triggers a 106 ms one-shot. In your circuit, 106 ms is the time required for 128 data bits.

• Error pulses from the XOR gate are totalized by the counter only during the 106 ms window.

• If you press and release RESET a second time, only the number of error pulses in the second interval is displayed.

CMs AVAILABLE

CM7 - Changes the channel bandwidth to 1600 Hz. CM15 - Changes the channel noise frequency.

PROCEDURE

The 22 procedure steps in this exercise include the following: • apply a sine wave signal to the channel

• observe the effects of channel band-limiting on the recovered signal • observe inputs and outputs with an oscilloscope

• add noise to the signal and observe the effects on the recovered signal • observe the effects of changes in the noise bandwidth

• convert your peak-to-peak measurements to rms values • calculate the signal-to-noise ratio (SNR) in decibels

• determine the bit error count of an NRZ signal applied to the channel • calculate the bit error rate (BER) from the bit error count

(54)

6-9

EXERCISE 6-2 The Effects of Noise on ASK/PSK

EXERCISE OBJECTIVE

Explain and demonstrate the effects of noise on ASK- and PSK-modulated signals.

DISCUSSION

• Noise can cause errors in digital transmission by causing logic levels to be read incorrectly.

• Zero volts is much smaller than the amplitude of a normal ASK signal, so noise is often not sufficient to affect the recovered digital signal.

• A low zero-state amplitude condition results in better noise immunity. • Because the PSK signal has a constant amplitude, it is not as sensitive to

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6-12

EXERCISE 6-3 The Effects of Noise on FSK

EXERCISE OBJECTIVE

(57)

6-13

DISCUSSION

• Noise can change the amplitude of an FSK signal, which also affects the amplitude at the output of the bandpass filter.

• Amplitude changes are passed on to the envelope detector which results in errors in the recovered digital signal.

• FSK signals can be detected either synchronously or asynchronously.

• Synchronous detection provides better amplitude noise response because the phase comparator senses phase changes independent of the signal amplitude. • In the case of asynchronous detection, noise response is improved by the

bandpass filter. Any noise frequencies outside the passband are attenuated by the filter.

• Noise frequencies above the high cutoff frequency and below the low cutoff frequency will be rejected by the channel.

• Telephone circuits are designed to pass a bandwidth limited to about 300 to 3000 Hz, which includes the high and low FSK carrier frequencies. Any noise frequencies outside the passband are attenuated.

CMs AVAILABLE

CM10 - Selects the FSK phase comparator filter in the sync detector PLL circuit.

PROCEDURE

• observe the effects of noise on an FSK-modulated, synchronously-detected signal

• observe the effects of noise on an asynchronously-detected FSK signal • observe inputs and outputs with an oscilloscope

• compare the noise response of the sync and async detectors with an FSK signal applied

• demonstrate that the sync detector has better noise immunity than the async detector

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Digital Communications 2

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Basic Electricity and Electronics

Instructor’s Guide

Lab-Volt License Agreement

Limited Warranty and Disclaimer

FOREWORD

SAFETY

INSTALLATION INSTRUCTIONS

TABLE OF CONTENTS

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