<< Chapter < Page Chapter >> Page >

There are two basic approaches to an AGC. The traditional approach uses analog circuitry to adjust the gain before the sampling. The more modern approach uses the output of the sampler to adjust thegain. The advantage of the analog method is that the two blocks (the gain and the sampling) are separate and do not interact.The advantage of the digital adjustment is that less additional hardware is required since the DSP processing is already presentfor other tasks.

The goal of the AGC is to maintain the dynamic range of the signal by attenuating it when it is too large (as in (a)) and by increasing it when it is too small (as in (b)).
The goal of the AGC is to maintain the dynamic range of the signal by attenuating it when it is too large (as in (a))and by increasing it when it is too small (as in (b)).

A simple digital system for AGC gain adjustment is shown in [link] . The input r ( t ) is multiplied by the gain a to give the normalized signal s ( t ) . This is then sampled to give the output s [ k ] . The assessment block measures s [ k ] and determines whether a must be increased or decreased.

An automatic gain control adjusts the gain parameter a so that the average energy at the output remains (roughly) fixed, despite fluctuations in the average received energy.
An automatic gain control adjusts the gain parameter a so that the average energy at the output remains (roughly) fixed, despitefluctuations in the average received energy.

The goal is to choose a so that the power (or average energy) of s ( t ) is approximately equal to some specified s 2 . Since

a 2 avg { r 2 ( t ) } t = k T avg { s 2 ( k T ) } avg { s 2 [ k ] } ,

it would be ideal to choose

a 2 s 2 avg { r 2 ( k T ) } ,

because this would imply that avg { s 2 ( k T ) } s 2 . The averaging operation (in this case a movingaverage over a block of data of size N ) is defined by

avg { x [ k ] } = 1 N i = k - N + 1 k x [ i ]

Unfortunately, neither the analog input r ( t ) nor its power are directly available to the assessment blockin the DSP portion of the receiver, so it is not possible to directly implement [link] .

Is there an adaptive element that can accomplish this task? As suggested in the beginning of "Iteration and Optimization" , there are three steps to the creation of a viableoptimization approach: setting a goal, choosing a solution method, and testing. As in any real life engineering task, a propermathematical statement of the goal can be tricky, and this section proposes two (slightly different) possibilitiesfor the AGC. By comparing the resulting algorithms (essentially, alternativeforms for the AGC design), it may be possible to trade off among various design considerations.

One sensible goal is to try to minimize a simple function of the difference between the power of the sampled signal s [ k ] and the desired power s 2 . For instance, the averaged squared error in the powers of s and s ,

J L S ( a ) = avg 1 4 ( s 2 [ k ] - s 2 ) 2 = 1 4 avg { ( a 2 r 2 ( k T ) - s 2 ) 2 } ,

penalizes values of a which cause s 2 [ k ] to deviate from s 2 . This formally mimics the parabolic form of the objective [link] in the polynomial minimization example of the previous section.Applying the steepest descent strategy yields

a [ k + 1 ] = a [ k ] - μ d J L S ( a ) d a a = a [ k ] ,

which is the same as [link] , except that the name of the parameter has changed from x to a . To find the exact form of [link] requires the derivative of J L S ( a ) with respect to the unknown parameter a . This can be approximated by swapping the derivative and the averaging operations to give

d J L S ( a ) d a = 1 4 d avg { ( a 2 r 2 ( k T ) - s 2 ) 2 } d a 1 4 avg d ( a 2 r 2 ( k T ) - s 2 ) 2 d a = avg { ( a 2 r 2 ( k T ) - s 2 ) a r 2 ( k T ) } .

Questions & Answers

anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Good
Berger describes sociologists as concerned with
Mueller Reply
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Software receiver design. OpenStax CNX. Aug 13, 2013 Download for free at http://cnx.org/content/col11510/1.3
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Software receiver design' conversation and receive update notifications?

Ask