<< Chapter < Page Chapter >> Page >

Digital amplitude modulation

Two ELEC 241 students disagree about a homework problem. The issue concerns the discrete-time signal s n 2 f 0 n , where the signal s n has no special characteristics and the modulation frequency f 0 is known. Sammy says that he can recover s n from its amplitude-modulated version by the same approach used in analog communications. Samanthasays that approach won't work.

  1. What is the spectrum of the modulated signal?
  2. Who is correct? Why?
  3. The teaching assistant does not want to take sides. He tells them that if s n 2 f 0 n and s n 2 f 0 n were both available, s n can be recovered. What does he have in mind?


One way for someone to keep people from receiving an AM transmission is to transmit noise at the same carrierfrequency. Thus, if the carrier frequency is f c so that the transmitted signal is A T 1 m t 2 f c t the jammer would transmit A J n t 2 f c t φ . The noise n t has a constant power density spectrum over the bandwidth of the message m t . The channel adds white noise of spectral height N 0 2 .

  1. What would be the output of a traditional AM receiver tuned to the carrier frequency f c ?
  2. RU Electronics proposes to counteract jamming by using a different modulation scheme. The scheme'stransmitted signal has the form A T 1 m t c t where c t is a periodic carrier signal (period 1 f c ) having the indicated waveform . What is the spectrum of the transmitted signal with the proposed scheme?Assume the message bandwidth W is much less than the fundamental carrier frequency f c .
  3. The jammer, unaware of the change, is transmitting with a carrier frequency of f c , while the receiver tunes a standard AM receiver to aharmonic of the carrier frequency. What is the signal-to-noise ratio of the receiver tuned to theharmonic having the largest power that does not contain the jammer?

Secret comunications

A system for hiding AM transmissions has the transmitter randomly switching between two carrier frequencies f 1 and f 2 . "Random switching" means that one carrier frequency is used for some period of time, switches tothe other for some other period of time, back to the first, etc. The receiver knows what the carrierfrequencies are but not when carrier frequency switches occur. Consequently, the receiver must be designed toreceive the transmissions regardless of which carrier frequency is used. Assume the message signal hasbandwidth W . The channel adds white noise of spectral height N 0 2 .

  1. How different should the carrier frequencies be sothat the message could be received?
  2. What receiver would you design?
  3. What signal-to-noise ratio for the demodulated signal does your receiver yield?

Am stereo

Stereophonic radio transmits two signals simultaneously that correspond to what comes out of the left and rightspeakers of the receiving radio. While FM stereo is commonplace, AM stereo is not, but is much simpler tounderstand and analyze. An amazing aspect of AM stereo is that both signals are transmitted within the samebandwidth as used to transmit just one. Assume the left and right signals are bandlimited to W Hz. x t A 1 m l t 2 f c t A m r t 2 f c t

  1. Find the Fourier transform of x t . What is the transmission bandwidth and how does it compare with that of standard AM?
  2. Let us use a coherent demodulator as the receiver, shown in [link] . Show that this receiver indeed works: It produces the left and right signalsseparately.
  3. Assume the channel adds white noise to the transmitted signal. Find the signal-to-noise ratioof each signal.

Questions & Answers

anyone know any internet site where one can find nanotechnology papers?
Damian Reply
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.
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
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
characteristics of micro business
for teaching engĺish at school how nano technology help us
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
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
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.
what is the actual application of fullerenes nowadays?
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.
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.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
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.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
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
Answers please
Nikki Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Fundamentals of electrical engineering i. OpenStax CNX. Aug 06, 2008 Download for free at http://legacy.cnx.org/content/col10040/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Fundamentals of electrical engineering i' conversation and receive update notifications?