Understanding FM Receiver Basics : GATE Communications

Summary

The 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

00:00

📡 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.

05:00

📶 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.

10:01

🎚️ 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.

15:02

🎛️ 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.

20:02

🔊 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.

Keywords

💡FM Receiver

An FM (Frequency Modulation) receiver is an electronic device used to demodulate and process frequency-modulated signals. In the video, the FM receiver is compared with the AM receiver, highlighting that both use superheterodyne architecture. However, the FM receiver contains additional blocks such as an amplitude limiter and a phase-locked loop (PLL) demodulator, making it more complex.

💡Superheterodyne Receiver

A superheterodyne receiver is a common type of radio receiver that converts a high-frequency signal to a lower intermediate frequency (IF) to simplify processing. Both FM and AM receivers in the video use this architecture, with a key difference in the demodulation process. The video also discusses the role of the intermediate frequency in avoiding image frequency issues.

💡Image Frequency

Image frequency is an undesired frequency that can interfere with the signal being received. In the video, it is explained that FM receivers are designed to avoid image frequencies by setting the intermediate frequency (IF) to a value that ensures image signals fall outside the tuning range. This design element eliminates the problem of image frequencies.

💡Intermediate Frequency (IF)

The intermediate frequency (IF) is a key frequency used in superheterodyne receivers to convert incoming signals to a fixed lower frequency for easier processing. In FM receivers, the IF is set to 10.7 MHz. The video describes how the choice of IF helps in frequency selection and tuning while avoiding image frequency interference.

💡Amplitude Limiter

An amplitude limiter is a circuit used in FM receivers to clip any variations in the amplitude of the signal, which could be caused by noise. Since the information in an FM signal is encoded in the frequency, not the amplitude, the limiter helps reduce noise without affecting the signal. This block is unique to FM receivers and improves signal quality.

💡Phase-Locked Loop (PLL)

A phase-locked loop (PLL) is a control system used in FM demodulation to synchronize the frequency of an oscillator with the incoming signal. In the video, PLL is explained as the key component for demodulating FM signals, allowing the receiver to extract the audio signal from the frequency-modulated carrier wave.

💡Pre-Emphasis

Pre-emphasis is a technique used in FM transmission to boost the amplitude of higher frequency components of the audio signal before modulation. This compensates for the fact that noise tends to affect higher frequencies more. The video explains how pre-emphasis improves signal-to-noise ratio and ensures that the higher frequencies can be accurately reproduced at the receiver.

💡De-Emphasis

De-emphasis is the inverse process of pre-emphasis, applied at the receiver after demodulation. It restores the original balance of audio frequencies by reducing the artificially boosted high frequencies. The video describes how the combination of pre-emphasis and de-emphasis ensures high fidelity audio transmission in FM systems.

💡Fidelity

Fidelity refers to the accuracy with which a receiver can reproduce the original audio signal. In FM systems, fidelity is a critical measure of performance. The video discusses how FM receivers, aided by pre-emphasis and de-emphasis, maintain high fidelity by improving the signal-to-noise ratio, particularly for higher frequency components.

💡Signal-to-Noise Ratio (SNR)

Signal-to-noise ratio (SNR) is the measure of signal strength relative to background noise. In the video, SNR is emphasized as a crucial factor in determining the quality of the received signal. The discussion highlights how FM systems use pre-emphasis to improve the SNR for higher frequency components, thereby enhancing overall signal quality.

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