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Digital Signal Processing Lab Paper AM –Transmitter

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					                   THE GEROGE WASHINGTON UNIVERSITY




                  AM –Transmitter
        Real-Time Digital Signal Processing –ECE 294

                        Tamir B Suliman
                           5/13/2009




Professor: Milos Doroslovacki
Abstract- One of the simplest modulation schemes is Amplitude Modulation, which is normally
just abbreviated as AM. For several decades commercial AM radio broadcasts is used on many
consumer radios in most of the countries around the world. Additionally, AM provides an easily
understood modulation scheme that can be thought as the starting point for many of today’s
more complicated modulation schemes.


   I.      Introduction

Amplitude modulation (AM) is a technique used in electronic communication, most commonly
for transmitting information via a radio carrier wave. A.M works by varying the strength of the
transmitted signal in relation to the information being sent. For example, changes in the signal
strength can be used to reflect the sounds to be reproduced by a speaker, or to specify the light
intensity of television pixels. (Contrast this with frequency modulation, also commonly used for
sound transmissions, in which the frequency is varied; and phase modulation, often used in
remote controls, in which the phase is varied.

   II.     Forms of Amplitude Modulation

In radio communication what is modulated is a continuous wave radio signal (carrier wave)
produced by a radio transmitter. In its basic form, amplitude modulation produces a signal with
power concentrated at the carrier frequency and in two adjacent
sidebands. Each sideband is equal in bandwidth to that of the                     The challenge
modulating signal and is a mirror image of the other. Amplitude
                                                                          Implement         the     AM
modulation that results in two sidebands and a carrier is often called
                                                                          transmitter     using    DSK
double sideband amplitude modulation (DSB-AM). Amplitude
                                                                          C6713 Kit in the AM
modulation is inefficient in terms of power usage and much of it is
                                                                          transmitter module. Using
wasted. At least two-thirds of the power is concentrated in the carrier
                                                                          different message signals,
signal, which carries no useful information (beyond the fact that a
                                                                          such as a square wave, a
signal is present); the remaining power is split between two identical
sidebands, though only one of these is needed since they contain
                                                                          periodic triangular function
identical information.

To increase transmitter efficiency, the carrier can be removed
                                                    (suppressed)
                                                   from the AM
                                                  signal.    This
                                                  produces       a
                                                   reduced-carrier
                                                  transmission or
                                                  double-sideband      suppressed-carrier
                                                  (DSBSC) signal. A suppressed-carrier
                                                  amplitude modulation scheme is three
                                                  times    more    power-efficient  than

                                                                                               2
            Figure 1 Modulated Signal Shape
traditional DSB-A.M. In AM, the carrier itself does not fluctuate in amplitude. Instead, the
modulating data appears in the form of signal components at frequencies slightly higher and
lower than that of the carrier. These components are called sidebands. The lower sideband (LSB)
appears at frequencies below the carrier frequency; the upper sideband (USB) appears at
frequencies above the carrier frequency. The LSB and USB are essentially "mirror images" of
each other in a graph of signal amplitude versus frequency, as shown in the illustration. The
sideband power accounts for the variations in the overall amplitude of the signal.




                                     Figure 2 AM Modulation Process




   III.    AM Theory & Transmission
Amplitude modulation (AM) is a technique used in electronic communication, most commonly
for transmitting information via a radio carrier wave. AM works by varying the strength of the
transmitted signal in relation to the information being sent.
For example, changes in the signal strength can be used to reflect the sounds to be reproduced by
a speaker, or to specify the light intensity of television pixels. (Contrast this with frequency
modulation, also commonly used for sound transmissions, in which the frequency is varied; and
phase modulation, often used in remote controls, in which the phase is varied)
There are three types of amplitude modulation: conventional amplitude modulation (AM) (also
known as double sideband (DSB) with carrier), single sideband (SSB), and double sideband
suppressed carrier (DSB-SC).




                                                                                               3
   (1) Conventional AM:




    is the amplitude of the message

      is the message

Ac is the amplitude of the carrier

   is the carrier frequency




   (2) DSB- SC:

Double-sideband suppressed-carrier transmission (DSB-SC) transmission has the functionalities
of:

(a) Frequencies produced by amplitude modulation are symmetrically spaced above and below
    the carrier frequency.

(b) The carrier level is reduced to the lowest practical level, ideally completely suppressed.




                                     Figure 3 DSB-SC AM Modulation


                                                                                                 4
In the double-sideband suppressed-carrier transmission (DSB-SC) modulation, unlike AM, the
wave carrier is not transmitted; thus, a great percentage of power that is dedicated to it is
distributed between the sidebands which imply an increase of the cover in DSB-SC, compared to
AM, for the same power used.



DSB-SC transmission is a special case of Double-sideband reduced carrier transmission. This is
used for RDS (Radio Data System) because it is difficult to decouple.

   (3) Single Side Band (SSB):

Single-sideband modulation (SSB) is a refinement of amplitude modulation that more efficiently
uses electrical power and bandwidth. Amplitude modulation produces a modulated output signal
that has twice the bandwidth of the original baseband signal. Single-sideband modulation avoids
this bandwidth doubling, and the power wasted on a carrier, at the cost of somewhat increased
device complexity.

 One method of producing an SSB signal is to remove one of the sidebands via filtering, leaving
only either the upper sideband (USB) or less commonly the lower sideband (LSB). Most
often, the carrier is reduced or removed entirely (suppressed), being referred to in full as single
sideband suppressed carrier (SSBSC). Assuming both sidebands are symmetric, which is the
case for a normal AM signal, no information is lost in the process.

An alternate method of generation known as a Hartley modulator, named after R. V. L. Hartley,
uses phasing to suppress the unwanted sideband.

To generate an SSB signal with this method, two versions of the original signal are generated,
mutually 90° out of phase. Each one of these signals is then mixed with carrier waves that are
also 90° out of phase with each other. By either adding or subtracting the resulting signals, a
lower or upper sideband signal results. A benefit of this approach is to allow an analytical
expression for SSB signals, which can be used to understand effects such as synchronous
detection of SSB.

Shifting the baseband signal 90° out of phase cannot be done simply by delaying it, as it contains
a large range of frequencies. This method, utilizing the Hilbert transform to phase shift the
baseband audio, can be done at low cost with digital circuitry.




            is the Hilbert transform of the message


                                                                                                 5
   IV.    The DSK implementation in C

Things we have to consider when we start the implementations the DSP in a real time must
processes the data from the ADC in real-time therefore we cannot wait for all the samples to be
received prior to beginning the algorithmic process.
The program is broken into different sections as the diagram below shows:




                                                                •Read Message from the
                                                                 ADC
                                                                •Calcualte the fc
                   •Bias Level                                  •Calculate the AM signal
    Declaration    •Carrier Frequency
                                                  PROCESS        value
                                                                •Scale the AM signal for
                                                                 the DAC
                                                                •Write the AM signal to
                                                                 DAC




If we directly implemented the AM generation equation below without considering the required
scaling for the DAC we will likely to exceed allowable range so we assume the ADC range is for
input data so code data channel Left can range from -32768 to +32767.Thus the bias must be
32768 to prevent the combined bias + code data channel Left to be negative.




The Scale factor 0.5 is needed to prevent the AM value from exceeding the allowable range for
the DAC.



  CodecData.Channel[LEFT] = (float) 0.5*(bias +CodecData.Channel[LEFT])*sinf(phase);




                                                                                                6
PC                                Oscilloscope




                          Line Out


 Line IN                                     DSK Board




              USB




     Figure 4 DSK C6713 Connections for AM




                                                         7
   V.      The single tone message example for the SSB

There exists a simple method to and the expression of a SSB signal when the message is a single
tone waveform that does not require computing any Hilbert transforms. Let us write the message.
The corresponding DSB-SC signal based on a carrier frequency fc:




Modifying the code in the equation above to get the SSB is by:



CodecData.Channel[LEFT] = (float) 0.5*(bias +(.5*CodecData.Channel[LEFT]))*sinf(phase);




                                                                                             8
   I.      Conclusion
The SSB effective power output is greater than in normal AM (the carrier and redundant
sideband account for well over half of the power output of an AM transmitter).SSB generation
by the filter method at other frequencies can be expensive. Transmission of signals with a small
"guard band" requires very good (and therefore expensive) filters, which increases the cost of a
SSB transmission system. This is why other methods are also used to generate SSB signals.
In DSB-SC the transmitted power is more as the name suggest it is double side band with
suppress carrier when we multiply a information signal which is low in frequency band with the
high frequency carrier the resultant signal is carrier wave + two signals(original) shifted in the
high frequency band. Thus as only one signal is necessary to transmit (the other one is carrying
the same information). Hence we go for SSB again suppress carrier means carrier is not
transmitted as it does not carry any kind of information.




                                                                                                9
References:
[1] S K Hasnain and Pervez Akhter, Digital Signal Processing (Theory and worked examples)-
Jan 2006.

[2] Thad B. Welch, Cameron H. G. Wright, Michael G. Real-time digital signal processing from
MATLAB to C with the TMS320C6x DSK

[3] Texas Instruments Inc., TMS320C6713 DSK User’s Guide, 2005.

[4] http://en.wikipedia.org/wiki/Double-sideband_suppressed-carrier_transmission

[5] http://ecow.engr.wisc.edu/cgi-bin/get/ece/601/vanveen/labmanual/ssb.pdf




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DOCUMENT INFO
Description: One of the simplest modulation schemes is Amplitude Modulation, which is normally just abbreviated as AM. For several decades commercial AM radio broadcasts is used on many consumer radios in most of the countries around the world. Additionally, AM provides an easily understood modulation scheme that can be thought as the starting point for many of today’s more complicated modulation schemes.