# Doubt clarification

Discussion in 'Wireless & RF Design' started by saraswathi.95, Jan 9, 2014.

1. ### saraswathi.95 Thread Starter Member

Apr 11, 2013
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i am beginner to this subject,please help me to understand how the rf waves overcome all other signals present in the atmosphere and reach receiver,in so many attempts in search of this topic i have read that 2 signals having different frequencies do not interfere,but how is this possible?give me any real time analysis.

2. ### MikeML AAC Fanatic!

Oct 2, 2009
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Throw a rock into the bay (ocean). It makes waves (ripples) that spread outward. Notice that these rock-induced wave coexist with the waves that are arriving from wind-induced events far out in the ocean... At some distance from where the rock fell, both sets of waves with be superimposed on the water at the same time...

The physical principle is superposition.

3. ### saraswathi.95 Thread Starter Member

Apr 11, 2013
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thank you,sir i have understood that one ,but what about the case when you throw so many stones randomly

4. ### MikeML AAC Fanatic!

Oct 2, 2009
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If superposition works for two signals, why would it not work for a million signals?

5. ### saraswathi.95 Thread Starter Member

Apr 11, 2013
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can you give me any explanation for why 2 signals having different frequencies do not interfere

6. ### WBahn Moderator

Mar 31, 2012
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You just said that you understood that one.

It would help if you described what you meant by "interfere".

7. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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The Superposition Theorem is a wonderful thing! In a LINEAR system (which is what empty space is) every possible frequency maintains its identity. You can use the selectivity of a receiver to discriminate between any number of simultaneous radio waves.

Eric

8. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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Superposition is what also allows you to transmit audio containing thousands of different frequencies over a single pair of wires. As long as the wire is linear (follows Ohm's Law precisely) there will be no interaction between any of the possible frequencies. This is because at any point in time there can be one and only one voltage across a pair of wires!

Eric

9. ### saraswathi.95 Thread Starter Member

Apr 11, 2013
40
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thank you for reply sir,but what my doubt is that if receiver selects its own signal from so many available signals then will it not be badly affected by so many million signals in the environment,although modulation assigns different frequency to it.

Apr 5, 2008
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11. ### saraswathi.95 Thread Starter Member

Apr 11, 2013
40
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Thanks,i have one more general doubt
our atmosphere will have so many atom present in it ,will these em waves in the atmosphere do not disturb them from their stable state so that they acquire sufficient ionization potential,and they in-turn disturb the propagating waves strength.

12. ### WBahn Moderator

Mar 31, 2012
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The wire doesn't have to be purely resistive (i.e., obey Ohm's Law) in order to be linear. It can have inductance and capacitance, such as a transmission line, and still be linear.

Also, even if it is non-linear, there can still only be one and only one voltage across a pair of wires.

13. ### WBahn Moderator

Mar 31, 2012
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If everything -- the transmitter, the environment, the receiver, and everything else -- were perfectly linear then it there would be not problem at all. But nothing is perfectly linear and so there is always some interference and the more signals there are the more interference there will be. This is why strong signals at one frequency can mask weak frequencies at other frequencies. If nothing else, the strong signals can drive the input stage of a receiver non-linear (such as into saturation) causing it to be insensitive to any other signal, even one at the frequency it is tuned for.

14. ### saraswathi.95 Thread Starter Member

Apr 11, 2013
40
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I do have one general doubt,when em waves travel in the atmosphere which contains so many atoms and they are stable,some of these molecules may absorb the energy from the waves and do they in turn won't create any electric or magnetic field around them?(i,e. are those force lines of travelling waves are not disturbed?)

15. ### WBahn Moderator

Mar 31, 2012
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You seem to be asking the same question over and over.

Consider light, which is just an electromagnetic wave.

Imagine a mirror set on a stage in front of an audience of several hundred people. Each person looks at the mirror and discribes what they see. Each person sees something different, don't they? Perhaps you are sitting low on the right side and so you see the face of the usher that is standing high up on the left side. I, on the other hand, am sitting way off to the left and so I see the fire extinguisher that is on the wall near the right-rear exit. The lightwaves making up the images that each person sees are all striking the mirror all at the same time and yet you don't see any part of the fire extinguisher and I don't see any part of the usher.

The same is true when you are standing outside. Lots of people are looking through the same volume of space and seeing vastly different things without what one person sees interfering with what anyone else sees, because the light waves pass through each other. Yes, at any given point, the add as they meet, but then they "break back up" and go on their separate ways just as though they had never met.

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16. ### saraswathi.95 Thread Starter Member

Apr 11, 2013
40
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hello,
Can we apply TDM(time division multiplexing) to digital signals modulated using ask,fsk,psk?
if it is so how is it possible to apply TDM to continuous signals obtained by those modulation techniques?

Last edited: Jan 19, 2014
17. ### WBahn Moderator

Mar 31, 2012
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Where are these questions coming from? An assignment?

18. ### saraswathi.95 Thread Starter Member

Apr 11, 2013
40
0
No,its a general doubt
please try to help me as quickly as possible.

Last edited: Jan 19, 2014
19. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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It does. It would be a strange universe if it didn't!

20. ### saraswathi.95 Thread Starter Member

Apr 11, 2013
40
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According to maxwell's equations a time varying magnetic field produces electric field and vice-verse,consider an antenna ,when we say that it radiates energy in tem mode ,should we consider that an electric force line introduced in to the free space generates a magnetic field around that flux line or should we consider it as an another flux line from antenna which is in phase and perpendicular to electric flux lines.