L10 : Modulation - Physics Communication Systems, Physics, Class 12 Video Lecture

hello friends this video on
communication systems back 10 is brought
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from exam so what is modulation you
already spoke about it right but still
let us have a quick review so that we
can get back to the topic once again
what is modulation the word modulation
means modifying like you would have
often heard people say try to modulate
your voice
that means try to modify your voice
right so the word modulate means to
modify so what does modulation it is the
superposition of information contained
in low frequency message signal on a
high frequency wave right so let us
quickly recap the same example of letter
so what happens is you want to send the
letter
so letter is the message signal now how
will that letter travel from Delhi to
Bangalore
so it needs something which can carry it
so what is going to carry it something
which has a higher frequency the letter
has a lower frequency I mean if the
letter is carried by you you have a
lower frequency you will just walk or
you with max at at the max you run but
even that'll take a lot of time but if
you have something which can take the
carrier that later and travel really
fast that will be great right so that is
nothing but the high frequency wave or
the courier wave so the courier wave
actually carries the low frequency
signal and takes it to the receiver
where does it occur
it occurs either transmit your end so
the same letter
how will it reach from the transmitter
to the receiver so this is your
transmitter and this is your receiver
and what is your channel channel is let
us suppose 3 so rail can be a channel
flat can be a channel so what is your
care courier wave is that train so the
train actually
is the letter now since the train has a
higher speed so the letter also travels
with the speed of the Train
so how was modulation helpful in this
case modulation was helpful because
during the process of modulation we
could take out the benefit of a high
frequency wave right that was the
advantage of this process of modulation
so let us see why do we need modulation
that isn't a very important question why
do we actually need an additional wave
called a courier wave by doing at all
you need a high frequency wave now when
I talk about the example of the letter
you very clearly understand why we
needed the Train because it saves a lot
of time now let us try to understand it
in terms of signal transmission why do
we at all need this process of
modulation for transmission of signals
now when I talk about the transmission
of signals there are a various factors
which are involved or which can affect
the transmission of signals so let us
look at the major factors which affect
the signal transmission over long
distances so once we know know the
factors which affect the signal
transmission then we can see that if
there is any need of having a high
frequency wave correct okay so with this
that approach let us try to understand
the different factors which affect the
transmission of signals over long
distance the first factor is size of
antenna so here you get a new term that
is antenna which we have not talked
before but answer is something which all
of you are aware of like you would have
had a dish antenna for your television
or you would have seen an antenna in
your car for the radio or you would have
seen an antenna in the in the normal
radio right so antenna is something
which is a very common thing and all of
us are aware of it effective power
radiated by antenna okay that is another
fact
mixing up of signals from different
transmitters so these are three
important factors which affect the
transmission of signals over long
distance now just telling these three
things doesn't make much sense to you
right you don't understand how how does
the size of an antenna matter or how
does the power radiated by an antenna
matter at all for that let us start
discussing each of these factors in
detail one by one so for that let us
first try to understand what is an
antenna then we will see how the size of
antenna affects signal transmission so
what is an antenna it is an electronic
device that converts electric power into
radio waves and vice versa as I
mentioned before as well when you look
at the structure of a communication
system how does it look like so when you
look at the basic structure of a
communication system how is it we had a
source Silurian then we had a
transmitter and then we had the channel
and then here we had the receiver right
and then from the receiver it went to
the destination correct so where do you
think there is a need to convert
electric power or electric signals into
radio waves or radio waves into electric
signals so what is happening from the
source comes the message signal the
transmitter will comfort whether it is
an electrical or non electrical signal
it will convert it into electrical
signals which are capable of
transmission now those electrical
signals how will they transmit through
the medium they need to be get converted
into a radio waves that is
electromagnetic waves so here we have an
antenna so what will this antenna do
this antenna will take the electric
signals which are produced by the
transmitted
and convert them into radio waves and
those radio waves will then propagate
through the medium now how will the
receiver receive those radio waves
because the receiver is capable of
understanding only the electric signals
so here again we have another antenna so
this is the receiving antenna so what
will this antenna do this will convert
the radio waves into electric power so
these electric signals will be
interpreted by the receiver right so
what does the purpose of antenna
actually helps in converting the
electric signals into radio waves and
radio waves into electric segments right
so it helps in radiating energy as
electromagnetic waves through air or
through free space so it sends radio
waves to distant sites and it also
receives radio waves from sources for
example here if you look at this antenna
this is a transmitting antenna because
it sends the radio waves to distant
places and this is a receiving antenna
because it receives the radio waves
coming from distant places so that is
basically an antenna is read it is also
termed as aerial this is another name
for antenna you would have seen this
kind of antenna on your rooftops
sometimes during for televisions and all
they use these kind of antennas as well
how I put these these mostly you have
the dish antennas which look somewhat
like this Fred
so now you understand the purpose
Panthera what does an antenna basically
do right so now the question is how does
the size of an antenna affect the signal
transmission or how does it affect the
communication system so that is the
question now a very important concept to
understand here is that the size of an
antenna should always be comparable to
the wavelength of the signal let us note
it down the size of ant
should be comparable to the wavelength
of the signal now you might ask why how
is the wavelength of the signal related
to the antenna so in the previous slide
what will we see what is the purpose of
antenna its purpose is to sense the
signal and then convert the signal into
electromagnetic waves right now how do
you think the antenna is going to sense
the signal with the help of the
wavelength of the signal because
wavelength of a signal actually makes
you understand that there is a signal
spread so if the size of the antenna is
not if it is very less than the
wavelength of the signal what would
happen or if it is very large as
compared to the wavelength of the signal
what would happen in either of these
cases the antenna will not be able to
detect the presence of the signal or it
will not be able to catch the signal
just take a small example let us suppose
that you have a small insect or you
later suppose you have a small cat so
this is a cat just imagine that this is
a cat okay now you want to trap this cat
so how do you do how can you do that you
take a net right to trap this cat now
let us suppose if you take a net which
has very big holes like that
what does a net a net will basically
have it is like a too many threads just
connected together so they'll have holes
in between right now if you take a new
net which has very big holes will you be
able to catch this cat know why because
the cat because now just imagine that
this is the size of your antenna and the
size
of the stat is the wavelength of the
signal let us suppose if this is the
wavelength and this is your antenna size
now if the size of the antenna is very
big that means the holes in the net is
very big what will happen when you try
to patch the fat it will just pass
through this these holes so you will not
be able to catch it right now similarly
if you want to catch the same cat and
for that you take a very small net now
what will happen when you try to catch
this cat the cat is bigger than the net
so the cat will not fit inside the next
record so you will not be able to catch
the tag I mean if you really want to
catch this cat the size of the net the
spacing of the holes in the net should
be comparable to the science of the cat
right so you should use the appropriate
size of the net so similarly if the
antenna has to catch a signal it will
catch the signal with its wavelength so
the size of the antenna should also be
comparable to the wavelength of a signal
right so that the antenna can properly
sense the time variation of a signal
because the signal is nothing it is it
will keep on changing with time so the
antennas should sense that time
variation of them signal right so now we
will see how the size of antenna affect
the signal transmission we all know that
lambda that is wavelength is related to
frequency by this relation that is free
lambda or wavelength is equal to the
speed of light divided by the frequency
right now let us suppose if we have a
signal if we have a signal with
frequency around 20 kilo Hertz let us
suppose if this is a signal which we
have and we need an ant
to catch this signal so what would be
the size of that antenna so the size of
that antenna will be comparable to the
wavelength of the signal so if we are
able to find out the wavelength of the
signal then we will be able to say how
much should be the size of the antenna
so let us try to find out the wavelength
of the signal so wavelength would be
equal to C divided by nu so C is 3 into
10 to the power 8 meter per second and
nu is 20 kilo Hertz that is 20 into 10
to the power 3 Hertz so this comes out
to be 1 point 4 into 10 to the power 4
meters which is nothing but 15
kilometres so that means this is the
wavelength of the signal so what does
that mean that means that the size of
the antenna should be somewhere around
15 kilometers so do you think that it is
feasible to construct an antenna whose
size is 15 kilometers you understand
right how long is 15 kilometers it's a
huge distance so it is not feasible to
construct an antenna of science 15
kilometers right so constructing this is
not feasible so what does that mean that
means that the size of an antenna also
puts an restriction it puts a
restriction to the transmission of
signals so that means if I say that I
cannot construct an antenna of just any
sizes so that is say by saying that it
is not possible to transmit signals of
any wavelength right that means the size
of an antenna is also putting a
condition or a limitation to the
wavelength of signal which can be
transmitted so what do we see we see
that wavelength is inversely
proportional to frequency now in order
that transmission happens if we want
that signal transmission should happen
successfully the antenna size should be
possible limits now the antenna size
will be within possible limits only when
the value of lambda is less because
antenna size is directly proportional to
lambda so antenna size is directly
proportional to lambda now my antenna
size should be less so for that this
lambda should also be less now if I want
lambda to be less the value of frequency
should be more that means frequency
should be more like so what is our
conclusion conclusion is that we want
high frequency signals because if the
signals have high frequency they we have
lesser wavelength and if they have
lesser wavelength the size of the
antenna will be smaller right so the
first factor which affects the
transmission of signal C is that high
frequency signals are desirable right so
we got one vote in favor of modulation
because what does modulation it
increases the frequency of the message
signal by combining it with a high
frequency wave that is the carrier wave
so one vote for modulation so we
understand one reason why modulation is
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