Experiment 8. AM Transmitter. Prelab

Transcription

1 Experimen 8 AM Transmier Prelab In his experimen you will use wha you learned in your previous lab sessions o make an AM ransmier circui. You will consruc a chopper modulaor o generae AM. P1) Inroducion One o he simples schemes or wireless communicaion is Ampliude Modulaion (AM). Modulaion is necessary or a number o reasons: The aenuaion o he channel, i.e. air, is high or low requency voice signals, bu he aenuaion is signiicanly lower or higher requencies. I he voice signal is ransmied as is, i.e. baseband ransmission, only one channel can be broadcas a a ime. Oen i is needed o have many channels ransmiing a he same ime, e.g. dieren radio and TV saions need o broadcas a he same ime. Jus hink o i or a momen ha he radio was broadcased in baseband, you couldn even sleep a nigh! The idea behind AM is very simple: since high requencies can propagae arher hrough air, we make a high requency sinusoid (he carrier) carry he low requency signal ha we wan o ransmi. In AM, he signal o be ransmied, rides on he ampliude o he carrier, hus orming he envelope o he modulaed signal (Fig. P1). Mahemaically his can be represened as: s() = ( 1+ msbaseband () ) cos( c) where s() is he ransmied signal, sbaseband () is he original baseband signal ha we wan o ransmi., c is he carrier requency and m is reerred o as he modulaion index. The modulaion index deines how large he modulaing signal will be compared o he carrier. Envelope Figure P1 Ampliude Modulaion (AM). 1

2 P2) Frequency Domain Represenaion In order o beer undersand how ampliude modulaion works, le s analyze he modulaion in requency domain. Basically, muliplying he inpu signal by he carrier signal resuls in he requency conen o he inpu signal o be moved around he carrier requency (Fig. P2). As explained beore, he original baseband signal may no be able o propagae well hrough he channel, bu requencies near carrier requency will. By muliplying he signal wih he carrier, is requency conen moves o around he carrier requency and can hus propagae hrough he channel eicienly. Assume here is a requency band, say 20 imes he bandwidh o he original signal, ha can propagae hrough he channel eicienly. This requency band can be divided beween 10 dieren saions ransmiing wih AM. This can be done by assigning each o hem a unique carrier signal and suicien spacing beween he carrier requencies (Fig P3). This procedure is reerred o as muliplexing. c c Figure P2 le The original baseband inpu signal requency conen, middle The requency conen o he resul o muliplying he inpu and carrier signal, righ The AM ransmied signal, noe ha he consan carrier erm appears as a dela uncion. Saion 1 Saion 2 Saion 3 Saion 4 c1 c2 c3 c4 Figure P3 le The original baseband inpu signal requency conen o our saions, righ Muliplexing he our saions using dieren carrier requencies. The spacing beween carriers is chosen such ha he requency conens won overlap. 2

3 g() P3) AM Transmier Circui There are dieren ways o perorm ampliude modulaion. In his experimen you will use a chopper circui o ge he ampliude modulaed signal. In his secion you will learn how he circui uncions. Assume g() = 1+ m (), where () is he inpu modulaing signal. I g() is chopped and hen bandpass ilered a he carrier requency, he resul will be an AM signal (Fig. P4). To see how his works assume S () o be he swiching uncion: 1, cos( ωc) 0 S () = 0, cos( ωc) < 0 Then chopping he signal will be equivalen o muliplying i wih S. () Hence, he chopped signal will be: gchopped () = g() S() Since S () is an square wave wih uni ampliude, i will have a Fourier series expansion as ollows: S () = cos( ωc) cos( 3ωc) 2 + π 3π + gchopped () AM oupu (a) 2πω c (b) Figure P4 (a) Signal g(), (b) Signal g() chopped, (c) Filered chopped signal, as can be seen, i is an AM signal. Noe ha he modulaion index is greaer han 1 here. (c) 3

4 so, g() 2g() 2g() gchopped () = cos( ωc) cos( 3ωc) 2 + π 3π + As you can see he second erm is he desired AM signal, which can be ilered ou using a bandpass iler. In requency domain he chopped signal will look like igure P5. G() G chopped () c 3 c Figure P5 le Frequency conen o signal g(), righ Frequency conen o Signal g() chopped. This chopper modulaor can be realized wih a diode bridge (Fig. P6). Assume he diodes are ideal and ry o explain how he bridge uncions as a volage conrolled swich. The conrolling volage is v1(). g chopped () Figure P6 Chopper circui employing diode bridge. v 1 () Figure P7 The swiching signal v 1 (). 4

5 1) The Transmier Circui The AM ransmier circui consiss o 3 major pars: 1. The local oscillaor 2. The chopper circui 3. The ank circui and oupu sage To build he circui proceed as ollows: 1. Connec he oscillaor circui as in he oscillaor design experimen. 2. Tes your circui and make sure i s working properly. 3. Connec he circui in igure Tes he operaion o he circui by eeding a 1KHz sine wave rom he signal generaor. Also eed a 10KHz sine wave wih 8Vpp o he oscillaor inpu o he circui. 5. Tes i he circui works properly by connecing he oupu o he bridge o he oscilloscope and check i you ge he chopped signal. 6. Hook-up he inal sage and he ank circui o he oupu o he bridge. 7. Connec collecor volage o he oscilloscope and view he oupu signal. Make sure your circui is working as i should. Oupu Figure 1 The chopper circui. The Oscillaor shown in he circui is he oupu sage o he oscillaor circui you have buil. The oupu o his sage is ed ino he inal sage and he ank circui. 5

6 Figure 1 The inal sage o he ransmier circui. The ank circui ilers ou he desired componen and he oupu is ed ino he anenna or ransmission. 6