The geroge washington university

AM –Transmitter

Real-Time Digital Signal Processing –ECE 294

Tamir B Suliman

5/13/2009

The challenge

Implement the AM transmitter using DSK C6713 Kit in the AM transmitter module. Using different message signals, such as a square wave, a periodic triangular function

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 modulating signal and is a mirror image of the other. Amplitude modulation that results in two sidebands and a carrier is often called double sideband amplitude modulation (DSB-AM). Amplitude modulation is inefficient in terms of power usage and much of it is wasted. At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information (beyond the fact that a signal is present); the remaining power is split between two identical sidebands, though only one of these is needed since they contain 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

2Figure 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.

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).

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Figure 2 AM Modulation Process

(1) Conventional AM:

S (t )=Ac [1+ Am M (t ) ]cos (2 π f c t)

Am is the amplitude of the message

M (t) is the message

Ac is the amplitude of the carrier

f c 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.

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Figure 3 DSB-SC AM Modulation

M (t ) is the Hilbert transform of the message

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.

S ( t )=Ac Am M (t )cos (2 π f c t)

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.

S ( t )=( Ac Am)

2M (t ) cos(2π f c t)+

( Ac Am)2

M (t ) sin 2 π f c t

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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:

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 codecdata channel Left can range from -32768 to +32767.Thus the bias must be 32768 to prevent the combined bias + codecdata channel Left to be negative.

S (t )=Ac [B+ Am M (t ) ]cos (2 π f c t )

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

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Bias LevelCarrier FrequencyDeclaration

Read Message from the ADCCalcualte the fcCalculate the AM signal valueScale the AM signal for the DACWrite the AM signal to DAC

PROCESS

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

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USB

OscilloscopePC

DSK BoardLine IN

Line Out

Figure 4 DSK C6713 Connections for AM

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:

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

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.

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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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