Understanding FM Receiver Basics : GATE Communications
Summary
TLDRThe video covers the fundamentals of an FM receiver, explaining its components and functionality compared to an AM receiver. It emphasizes the use of a superheterodyne receiver for both AM and FM, highlighting the differences in demodulation and additional blocks like amplitude limiters. Concepts such as pre-emphasis and de-emphasis are discussed to enhance audio quality by managing signal-to-noise ratios. The video also explains the fidelity of FM receivers, the selection of intermediate frequency (IF), and noise reduction mechanisms, providing an understanding of the signal processing in FM broadcast systems.
Takeaways
- 📡 The FM receiver operates in the frequency range of 88 MHz to 108 MHz, and it uses a superheterodyne receiver like AM, with the main difference being in the demodulation process and additional blocks.
- 📶 The receiving antenna is the first block, designed to capture signals across the full FM broadcasting range of 88 MHz to 108 MHz.
- 🎛️ Multi-stage RF amplifiers are used to amplify all possible signals without the issue of image frequencies, as the image frequency is kept outside the tuning range due to a carefully selected intermediate frequency (IF).
- 🔄 The mixer block, working with a local oscillator, converts frequencies using down-conversion (FL - FS = IF). The local oscillator frequency (FL) is always greater than the signal frequency (FS).
- 📈 The IF amplifier stage is crucial for signal selection, designed with a fixed IF value of 10.7 MHz and a bandwidth of 200 kHz, ensuring proper signal selection and tuning.
- 🔊 An amplitude limiter (double-ended clipper) is used to reduce noise by clipping amplitude variations, as FM signals do not carry information in the amplitude component.
- 📡 FM demodulation is done using a phase-locked loop (PLL), which is followed by an audio and power amplification process to enhance the signal for loudspeakers.
- ⚙️ Pre-emphasis at the transmitter and de-emphasis at the receiver are used to improve the signal-to-noise ratio, particularly for higher frequency components.
- 📊 Pre-emphasis artificially boosts higher frequency components at the transmitter to improve their signal-to-noise ratio before modulation, while de-emphasis reverses this process at the receiver after demodulation.
- 🎶 Fidelity is the ability of a radio receiver to reproduce all input frequencies accurately, and it is enhanced in FM receivers by using pre-emphasis and de-emphasis to manage the signal-to-noise ratio.
Q & A
What is the frequency range for FM broadcasting?
-The frequency range for FM broadcasting is from 88 MHz to 108 MHz.
What type of receiver is used in both AM and FM systems?
-Both AM and FM systems use a superheterodyne receiver.
What is the purpose of a multi-stage RF amplifier in an FM receiver?
-The multi-stage RF amplifier is used to amplify each received signal to ensure they are strong enough for further processing.
Why is there no issue with image frequencies in an FM receiver?
-Image frequencies are not an issue in FM receivers because the intermediate frequency (IF) value is selected in such a way that image frequencies fall outside the tuning range.
What is the typical intermediate frequency (IF) used in FM receivers?
-The typical intermediate frequency (IF) used in FM receivers is 10.7 MHz.
What is the role of an amplitude limiter in an FM receiver?
-An amplitude limiter removes amplitude variations caused by noise, as FM signals do not carry information in their amplitude.
What is the significance of pre-emphasis and de-emphasis in FM transmission?
-Pre-emphasis boosts high-frequency components before transmission to improve the signal-to-noise ratio. De-emphasis reduces these boosted frequencies at the receiver to restore the original signal.
What is the purpose of the phase-locked loop (PLL) in FM demodulation?
-The phase-locked loop (PLL) in FM demodulation is used to lock onto the frequency of the incoming FM signal and demodulate it to retrieve the original audio signal.
What is the fidelity of a radio receiver?
-Fidelity is the capability of a radio receiver to reproduce all input frequencies accurately at the output.
Why is pre-emphasis needed in FM systems, but not in AM systems?
-Pre-emphasis is needed in FM systems to improve the signal-to-noise ratio for higher frequencies, as noise power increases with frequency. In AM systems, the audio signal range is limited to lower frequencies, making pre-emphasis unnecessary.
Outlines
📡 Introduction to FM Receiver Basics
The video begins by introducing FM receivers, comparing them to AM receivers, and discussing the key components. A superheterodyne receiver is used for both AM and FM, but the main difference lies in the demodulation process and the blocks involved. The FM broadcasting range is from 88 to 108 MHz. The receiver's design ensures that it covers this range. Notably, FM receivers do not face issues with image frequencies due to the wide gap between desired and image frequencies, ensuring clean signal reception.
📶 RF Amplifiers and Image Frequency Management
This section dives into the function of the RF amplifiers in FM receivers, which are used to amplify signals within the FM range. The discussion includes how the gap between desired and image frequencies (21.4 MHz) prevents interference. Since the image frequencies fall outside the tuning range (88-108 MHz), FM receivers avoid issues commonly seen in AM receivers. The mixer and local oscillator work together to convert frequencies, ensuring accurate signal reception and minimizing interference.
🎚️ Pre-Emphasis and Noise Reduction
The importance of pre-emphasis and noise reduction in FM receivers is explained. The amplitude limiter, a double-ended clipper circuit, helps mitigate noise by eliminating variations in the signal’s amplitude, which does not contain information in FM. After noise reduction, phase-locked loops (PLLs) are used for demodulation. Pre-emphasis is introduced at the transmitter to enhance the higher frequency components and improve the signal-to-noise ratio. The corresponding de-emphasis process occurs at the receiver to restore the original signal.
🎛️ Fidelity and Signal-to-Noise Ratio in FM Transmission
This section elaborates on fidelity, the ability of a receiver to reproduce all input frequencies at the output. The discussion highlights how signal-to-noise ratio (SNR) affects signal reconstruction. As frequency increases, noise becomes more prominent, reducing the SNR. After a certain point (F1), it becomes impossible to fully reconstruct the signal. To combat this, pre-emphasis is applied to boost high-frequency components at the transmitter, while de-emphasis at the receiver helps balance the signal.
🔊 The Role of Pre-Emphasis and De-Emphasis
The video further explores pre-emphasis and de-emphasis. Pre-emphasis artificially boosts higher frequency components before modulation at the transmitter, while de-emphasis reduces these boosted components after demodulation at the receiver. This process helps maintain a high signal-to-noise ratio, ensuring the fidelity of the FM transmission. The specific circuitry used includes phase lead compensators and low-pass filters, which help maintain the frequency characteristics needed for accurate signal recovery.
Mindmap
Keywords
💡FM Receiver
💡Superheterodyne Receiver
💡Image Frequency
💡Intermediate Frequency (IF)
💡Amplitude Limiter
💡Phase-Locked Loop (PLL)
💡Pre-Emphasis
💡De-Emphasis
💡Fidelity
💡Signal-to-Noise Ratio (SNR)
Highlights
Introduction to FM receivers and their comparison with AM receivers.
FM receivers also use superheterodyne receivers, just like AM receivers, but with additional blocks like demodulators.
FM broadcasting range is from 88 MHz to 108 MHz, and the receiver must be designed to operate in this range.
In FM receivers, there is no problem with image frequencies due to the selection of IF (Intermediate Frequency).
RF amplifiers are used to amplify all possible signals after the receiving antenna.
The mixer block performs down-conversion by subtracting the local oscillator frequency from the desired frequency.
Intermediate frequency (IF) for FM receivers is typically 10.7 MHz.
Amplitude limiter is used in FM receivers to clip off noise-related amplitude variations, as amplitude doesn't carry information in FM signals.
Phase-Locked Loop (PLL) is used as the demodulator in FM receivers.
De-emphasis and pre-emphasis are applied to improve the signal-to-noise ratio for higher frequency components in FM signals.
Pre-emphasis boosts higher frequencies before transmission to combat noise interference, and de-emphasis counteracts this boost after reception.
The quality of FM sound is better than AM due to the inclusion of pre-emphasis and de-emphasis techniques.
Fidelity is the ability of a radio receiver to reproduce all the input frequencies accurately.
The calculation of image frequency is based on the tuning range and intermediate frequency, ensuring image frequency falls outside the range of 88 MHz to 108 MHz.
Practical IF values for FM receivers are around 10.7 MHz, which help maintain image frequencies outside the tuning range.
Transcripts
hello student in this video we will be
learning about fm receiver
[Music]
now in this topic
we are going to be discussing about the
fm receiver
like even for fm also what we are using
super heterodyne receiver only for am
also superhead today receiver but what
is the main what's the main drawbar
main differences demodulators and little
bit more blocks will come in the fm
receiver whereas in am receiver
demodulator is envelope detector here
the difference will be there of course
pll circuit we are using
so any receiver the starting block will
be the
receiving antenna
now whenever we are designing this
receiving antenna for fm
it should be able to
it should be able to receive all the
possible signals
where to where
the fm broadcasting range
fm broadcasting range is from
88 mega to
anson up to 108 mega
this is the total frequency range so
whenever we are designing fm receiver
ah
fm receiver that should be able to
work in the range of 88 to 108 because
there is a total fm range
after that after the receiving antenna
what is the next block we are having we
are having
multi-stage
rf amplifier
here this multi-stage rf amplifiers are
to amplify each and every possible
signal
and one more important thing is students
here in fm receiver there is no problem
of image frequencies
why there is no problem of image
frequencies are
here c
what is the gap between design and image
frequency
what is the gap between desired and
image frequency
desired frequencies
fs
and image frequencies fsi the gap is 2
if
now here if l we have selected in such a
manner that
images are coming outside the ranges
let's see
2 into 10.4 10.7 because i have value
10.7 2 into 10.7 means 21.2
so the gap between fs fsa is 21.4
if i take fs value starting value 88
then where is the image comes 88 plus
21.4 so one not 9.4 but our tuning range
only up to or not eight
even if you take one not eight
minus 21.4 it's 87 something will come
86 point something will come so
ief value we are selected in such a
manner that the images are coming
outside the tuning ranges so there is no
problem of image frequency here
so this rf amplifiers to amplify the
signals all possible signals
after that as usual same process we are
having a mixer block
mixers are used for conversion of
frequencies
another one will comes from the local
oscillator
that is fl
always will maintain
always will maintain
f l value greater than f fs
for better capacitor tuning what is that
point i will explain you later
always in the mixer down conversion down
conversion means
f l minus f s
which is equal to i f
are you getting my point or not
what is this if values are here 10.7
mega h
by using this f l what is happening sir
tuning will be done
means whatever maybe the channel if you
want to listen we are changing fl value
are you getting my point we are changing
fl value corresponding to that all
channels we are bringing to 10.7 not
kilo 10.7 mega h
so we are bringing to a particular value
after that what we are having
we are having if amplifier
where in the if amplifier selection will
be there
this is acting as a tuned section
here the selection process will be there
where this is exactly designed
here the selection
where it is exactly designed this is
exactly designed at
10.7
fr val is 10.7 mega h
this is gain gain versus frequency
characteristics it will be like that
and what is the bandwidth we are
maintaining sir
bandwidth is 200 k
of course in that one 20 will be the
guard band these are the fixed
characteristics
so no problem of selection we already
know
after the selection generally in the am
we are going for demodulator but here
demodulators we are having a limiter
circuit of course called
amplitude limiter
double ended clipper circuit we are
using
what is this amplitude limiter is
generally noise will affect the
amplitude
so if any variations in the amplitude of
fm signal the total will be clipped off
even if you clip off also no problem
because in fm signal there is no
information the amplitude of the signal
so any variations amplitude we can flip
off
so most of the noise will be gone here
next we are giving to the demodulator
what is the demodulator we are using for
fm phase lock will loop
after the pll
next we are before going to the audio
and power amplifiers
here is another block about this i will
explain you that is called d emphasis
and the transmitter side another thing
also will do that is called pre-emphasis
about dm passes and pre-emphasis i will
explain you wait
after this we are increasing the audio
levels common emitter
configuration we are using as well as to
increase the power levels power
amplifier i already told class b power
amplifier
audio amplifier means voltage amplifier
current amplifier
so obviously power will increase
next will be the loudspeaker
which will convert this electrical
signal into physical phenomenon
this is the
block diagram of a fm receiver
when compared to am receiver what is
additional blocks are amplitude limiter
and the dm processor
because of this amplitude limiter most
of the noise will be gone so figure off
merit of a fm is very very high
i already given generally 37.50
in am only one by three maximum itself
that is 0.33
and values also i shown
i given the previously so what is the dm
processor and what is the preamp
processor
try to understand
this concept of pre-emphasis and dmv
scheme see
in fm radio from where to where we are
transmitting the signal in audio signal
50 to
15 kilo
in am maximum itself is 5 kilo
in am these concepts are not needed but
in fm need why because practically if
you see
practically if you see
the power spectral density psd of audio
signal
as well as
if you see the
psd of noise signal
psd means power spectral density power
spectral density power spectral
instruments just you can feel that power
versus frequency graph power versus
frequency graph
so if i draw for them
if i draw for them
frequency versus gain
frequencies is power per
second so if i draw the frequency versus
power
of audio signal as well as noise signal
also
what is its graph is actually
as a frequency value increasing the
strength of the components will be
decreases
this is the signal power
but in coming the noise as the frequency
increases
noise power also increases nice for
denoting with n
let's take some value
for example f1 here also
the same value for example f1
by seeing the graph itself
by seeing the graph itself you can
understand
up to
up to f1
signal to noise ratio is greater than
one only
but after
f1 signal to noise ratio is less than
one you can see the graphs are you
understand my point
so up to f1 signal to noise ratio
greater than one means
we can reconstruct our signal back
but after f1
the signal to noise ratio if you observe
there is less than one less than one
means after this we are unable to
reconstruct the frequency
components are you getting my point
so here there is a another word also
called that is called
fidelity students
there is another one also called that is
a fidelity
what is the meaning of fidelity is it is
a capability of a radio receiver it is a
capability of a radio receiver to
produce all the input frequencies and
the output of a receiver also
but what is happening here is students
because of what is the stefan value will
tell you practically because of this
this is a practicality because of the
here up to f1 s by n value greater than
one greater than one means we can
reconstruct our signal
because signal strength is more but
after f1 we cannot because signal
strength is less just like
if my y strength is more when compared
to all other external disturbances in
your room then you can able to listen
what i am saying
understanding or not is a different
issue but you will definitely listen
what i am saying
like
like
here also if you are if the signal
strength means my voice
if that is more you can able to listen
if ectional disturbance is more you
cannot listen what i am saying
like if signal to noise ratio more
means greater than one or far greater
than one we can reconsider we can
reconstruct the signal less than one
means we cannot
so here in fm it is suffering from the
fidelity fidelity is saying that you
need to reconstruct all frequency
components what we are transmitting at
the input section of course at the input
section we are transmitting up to 15
kilo
are you getting my point around f1
valley is around some three kilohertz
some 23 800 something is their practical
value
so but after 3000 signal to noise ratio
is decreasing we are unable to make it
so that's why what we are doing is we
are going for a pre-emphasis at the
transmitter and we are going for a
corresponding de-emphasis at the
receiver what is the pre-emphasis are
pre-emphasis means to
reconstruct these higher frequency
components are to improve the signal to
noise ratio of this higher frequency
components to improve the signal to
noise ratio of this higher frequency
components we are artificially boosting
the higher frequency components that
artificial boosting of higher frequency
components itself is called students
pre-emphasis
are you getting my point
artificial boosting of this higher
frequency components to improve signal
to noise ratio is itself is called
pre-emphasis
where we are doing pre-emphasis
pre-emphasis will be done at the
transmitter sites are before modulations
are
before modulation we are doing the
pre-emphasis
are you getting my point practical
values from 2800 edges are 2.8 kilo
around three kilohertz you can say that
so what we are doing these we are
artificially boosting
this will be done pre-emphasis will be
done
preemphasis will be done by sir at the
transmitter
transmitter
before modulation cell before modulation
we are doing the pre-emphasis
what is the circuit we are using for
pre-emphasis i pass filter or you can
say lead compensator phase lead
compensator we are using sir
if you want the circuit diagram of that
how that phase lead compensator
this is a circuit diagram of a phase
lead compensator
how that uh diagram how the
characteristics will be there it's a
characteristics if you take
k inverses of frequency characteristics
predict the gain versus frequency
characteristics
up to what is the desired frequency it
is constant later its slope is increases
this value i will maintain as a f1
up to there it will maintain a constant
gain of 1 that is a phase rate
compensator
now what is the dm processor
dm process will be done at the receiver
after demodulations are
after demodulation what is the circuit
we are using for dm processes will be
the low pass filter
so what is the dm vs characteristics if
i write
gain versus frequency characteristics of
a low pass filter
or dm circuit we here also up to f1 will
maintain constant gain after the slope
is decreases
here the slope
of the decreasing one here the slope of
this increasing one will maintain both
are same if we are maintaining both are
same means exactly we can get back our
original signal
so here three terms i discussed
one is fidelity
another is called pre-emphasis dm
process fidelity means it is a
capability of a radio receiver to
produce all input frequencies at the
output of the receiver also
fm is suffering because of this signal
to noise ratio is less so what we are
doing before the modulation itself in
the transmitter we are artificially
boosting and we are increasing the
signal to noise ratio then obviously we
can reconstruct them at the receiver so
how much we are boosting that itself is
called pre-emphasis the same amount of
boosting will be decreases that is
called emphasis after the demodulation
this is developed by the dolby company
this pre-emphasis and d emphasis because
of that the audio signal or the music
quality will be excellent
because we are able to produce all the
frequency components
see the next problem on the fm receiver
a super hydrogen fm receiver operates
in the frequency range 88 to 108 mega
the i f and the low capacitor
frequencies are related f i f less than
f l
we require that
image frequency fall outside the range
88 to 100 means outside the tuning range
first question
the minimum required f i f is you given
some options second question the range
of variation the local oscillator is a
simple one i am not given the options
anyhow
what is saying that you need to
calculate f i f in such a manner that
the images has to come outside the
tuning range
means what is the ranges of is operating
from
operating from weight to versa 88 omega
2
one not eight mega
88 to one not eight mega
now follow
for example if i take a fs value is 88
for example
where is the image has to come
outside the tuning range means outside
the 108
or if i take this value is f x it has to
come before 88
what is the image frequency formulas are
f s plus
2 i f value greater than or equal to 1 0
8 mega
then fs value 88 comes to say 20 so
2 if greater than or equal to 20
megahertz then if value greater than or
equal to 10 mega
so then at least its value minimum value
should be 10 mega
that is awesome sometimes
here the logic one more time c
88 to one not 80 is a practical fm range
so what is the practical value of if
10.7 if sometimes in any of the option
10.7 mega is there choose that
if 10.7 is not there actually from the
mathematically calculation 10 mega if
both are there also this is the first
preference this is the second frequency
because 88 to 1080 is a practical range
given any other ranges are given
calculate from the mathematics concept
only
here the question is
we need to get the image outside the
tuning range
tuning from 88 to 108
clear then it should be greater than
that if 10.7 is going to conserve
because this is a practical range any
other radius for example 48 to 58 58 to
60 like different different is given
then you can go with the normal
mathematically calculations only don't
bother about that
clear
yeah see the next problem
the range of variation in the local
oscillator frequency
local oscillator frequency means what's
a f l value what is the formula for f l
f fl is fs plus if
fs is varying from 88 to 108 plus ififl
we got 10
so it will be
98 omega to one not eight mega if
practical range is given ninety eight
point seven two one not eight point
seven
that is about the local oscillator means
f l value got the clarity
yeah
see the next problem it is a session
reason type of question what is a
session given
preemphasis is used to amplify high
frequencies is
after f1 to improve the signal to noise
ratio we are artificially boosting that
itself is called pre-emphasis so
excession is a correct only
and what is the emphasis the emphasis
also we are using for same high
frequencies the same proportion to
decrement
but see the reason the emphasis circuit
is easy to de-emphasis low frequency
components no for the same high
frequency components we'll use
so reason is wrong then there is no
discussion for corrective reason or not
so a session is correct prison is wrong
so directly options
pre-emphasis and de-emphasis both we are
using for high frequency components only
you
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