Understanding Amplitude Demodulation (AM Demodulation) 2

June 12, 2017 | Author: Anonymous | Category: Documents
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Unit N. 5: Understanding amplitude demodulation (AM demodulation). Understanding the principle of amplitude demodulation...

Description

Unit N.5: Understanding amplitude demodulation (AM demodulation)

Understanding the principle of amplitude demodulation (AM demodulation)

Objectives:

etecting the AM demodulation

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!valuating the "ualit# of detected signal in AM demodulation

Objectives:

etecting the AM demodulation

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!valuating the "ualit# of detected signal in AM demodulation

Measuring and adjusting an demodulated signals

Minimum level of communication techni"ues understanding

Medium level of electronics components and devices understanding

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Understanding the amplitude modulation mechanism

%igh level of health and safet# ris&s understanding

'ommunication s#stems theoretical manual  *55M+,

 *55A- or stabilied suppl# source

-,M% double trace oscilloscope

Operative instruments: !rror: $eference source not found

set of connection cables

Where we start from

In previous experiment (Exp3) we where dealing with modulating a carrier with data signal (audio) through AM method !o" we put the foundation of radio sender

#n the other side" at receiver side" the $asic idea is to receive such modulated signal" and to remove the carrier for efficient use of audio signal %oth parts represent the dream of all radio amateurs around the world &he main parameters that descri$e the demodulation process are: Am ' audio amplitude Ac ' carrier amplitude f m ' audio freuenc f c ' carrier freuenc ' the spectrum of the audio signal at the output of demodulating device *or the practical evaluation" we would li+e to remem$er ou the fastest formula for modulation index calculus

E ' AM demodulation with analogue multiplier ,!% ' full carrier 

 -ow" we are familiar with experimenting with ,e .oren/o $oards .et0s $uild up the demodulating process !o" in order to experiment with demodulation" we need modulated signal" so previous experiments are welcomed and reuired We are also familiar with connecting electronic modules &he experimental circuits are shown next &he $loc+ diagram connection is shown next 1ou will pa attention to the sender0s" and receiver0s side &here is not place and time to tal+ a$out the area $etween dashed lines

As ou understand" it is the simplest experiment for demodulating AM signal

.et0s run it and see what we get on the oscilloscope &here is the place for a little indication: most of oscilloscopes have a facilit to ad2ust displaing parameters" let0s sa' an auto mode of measuring In order to save time during acuiring data" tr to use such facilit We did it" and we got next signal shapes & a+e a loo+ to measuring scales" and to shapes of the signals If ou have mood for that" test the result with our 4 spea+ers lease measure signals" anal/e the wor+ of demodulator" $ring comment related" and ma+e an o$servations related to the o$tained audio signal Well" now let0s go dipper into received signal ro$a$l" when ou tested the audio signal with 4 spea+ers" ou found some unpleasant sounds 1ou have right5 If we ma+e a /oom of the received signal (the output of module E)" we will see a picture li+e next one !ee the detail A5

It is the time to anal/e what is here &here are some methods to do these &he easiest wa is to use a spectral anal/er that will anal/e all signals that composes such signal Most of modern oscilloscopes have at list a **& math function *ind it on our scope" run it and anal/e the result If ou are not ver familiar with scope functions" p lease ta+e out the measuring ca$le for original audio signal" in order to $e sure ou will anal/e the E module output M ees tell me that the audio signal result is #6 (the module E output) &he **& function tells me that the signal is composed $ a 78 +9/ signal (8 d% ;rms)" and some harmonics  -ow" depending $ next processing signals" some time this simple processing stage is enough" or not

What +ind of measure do ou propose in order to ensure the ualit of demodulated signal>

E ' !!% ' detector 

 -ow" we +now what does mean AM" ,!%" !!%  -ow" it is the time to understand how to manipulate signals for energ saving in transmission process #ne method is $ased on adapting the modulation tpe in accordance with transmitted data *or instance" there are a lot of applications that uses !!% modulation !o" now we are tring to understand how to use successfull the !!% modulation % using ,e .oren/o experimental $oard ,. [email protected] we have modulated signal with ,!%' full carrier In order to experiment using !!% detector" we have two side$ands" and it is necessar to remove on side of the modulated signal  -ext figure shows the positionsB of signal for $etter understanding how to remove one AM side$and *or our experiment we use a 78th order low pass filter' module C to remove one side of AM signal After that we can use a !!% detector

Well" remem$er" we want to recuperate the audio signal that we have used in modulation  process at the highest ualit &hat0s wh we have to compare extracted audio signal with the original one &he connection diagram of modules who are involved in this experiment is shown in upper figure &he proposed experimental panel loo+s li+e next figure:

.et0s run the experiment *rom the $eginning" I would li+e to prevent ou that we need to test results almost at $orders freuencies of audio generator' D +9/" and ?8 +9/ &he reason of this is to test the ad2ustment wor+ of 78th order low pass filter

*irst of all we exercise removing side$ands % switching $etween !7 and ,!% from module % and testing the results on the output of module C" understand how to use 78th order low pass filter lease exercise this" $ ad2usting audio generator freuenc Ad2ust also the level of carrier in order to wor+ with fine oscillations through the electronic module

After ta+ing some precautions" we can pla with audio generator freuenc" and module % switch % ad2usting also the output voltage of the filter" we can anal/e its effect over the modulated signal

 -ow" we are sure that we have well !!% modulated signal It is the time to run the !!% detection experiment

A particular detail: on this picture ou will see that the output of module 9 is connected to the input of module , the second measurement channel is connected to the output of general purpose amplifier I am +indl as+ing ou to 2ustif this action

lease remem$er that we need to chec+ some experimental situations: the level of audio signal the level of the carrier the $order freuencies of the audio generator the $order freuencies of the carrier the output level of the filter  &he list of the experimental situation is not close" $ut it is recommended to chec+ at list these" in order to $e sure a$out !!% transmission process When ou acuire the signals (the output of module %" and the output module ,)" after  proper ad2ustments of the +no$s" ou will get some concluding figures Are ou satisfied> I do5 &he audio signal" at $oth $orders of tone generator is well recuperated

lease exercise with our experiment" in order to g et nice shapes of the signal lease also do some **& processing of the output signal of the !!% detector in order to  $e sure that the signal is accurate



What +ind of extra measure do ou propose in order to ensure the ualit of demodulated signal>

F ' !!% receiver with AC4

We are now familiar with AM" ,!%" !!% We should remem$er that" the gain control is a method for ad2usting the receiver sensitivit for the $est reception of signals of widel varing amplitudes &he complex form of automatic gain control (AC4)" o r instantaneous automatic gain control (IAC4) is used during normal operation

% anal/ing the electronic circuit of AM modulator with analogue multiplier" ou will o$serve that the output has two soc+ets ;am" ;7am &he $elong to a voltage divider (7=78)" which is used in our experiment for simulating low level of R* signal &he experimenting $oard offer the possi$ilit to experiment also AC4 for low level of R* signal We are continuing the experimentation from previous experiment" $ adding a A4C facilit from module F

&he electronic module is $uilt around integrate circuit .M73G88 ,ual #perational &ransconductance Amplifiers with .ineari/ing ,iodes and %uffers *or our purpose is used onl one operational amplifier &he diagram is extremel simple" with few external components &he pin input 7 is used for controlling the gain for %IA! amplifier *or the purpose of the experiment" we are using the varia$le ,4 voltage Remem$er  please' use the proper colour for the ca$le5 &he $loc+ diagram of the improvement of experimental !!% receiver is shown in next figure &he module is integrated in previous experiment diagram" with onl minor changes (the AM modulated signal is collected from ;7am" instead of ;am) Again" the issue is to recuperate the audio signal that we have used in modulation process at the highest ualit &hat0s wh we have to compare extracted audio signal with the original one &he next figure shows the proposed experimental panel As in previous experiment" we exercise removing side$ands

% switching $etween !7 and ,!% from module % and testing the results on the output of module C" understand how to use 78th order low pass filter lease exercise this" $ ad2usting audio generator freuenc Ad2ust also the level of carrier in order to wor+ with fine oscillations through the electronic module

Again" we should remem$er that we need to chec+ some experimental situations: the level of audio signal the level of the carrier the $order freuencies of the audio generator the $order freuencies of the carrier the output level of the filter  Also in this case" the list of the experimental situation is not close" $ut it is recommended to chec+ at list these" in order to $e sure a$out !!% transmission process When ou acuire the signals (the output of module %" and the output module ,)" after proper ad2ustments of the +n o$s" ou will get some concluding figures Again" we are satisfied with results &he audio signal" at $oth $orders of tone generator is well recuperated

lease exercise with our experiment" in order to g et nice shapes of the signal lease also do some **& processing of the output signal of the !!% detector in order to  $e sure that the signal is accurate

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