Transcript fm transmission

FM TRANSMISSION
P.J. PARDESHI
Asst. Professor
MITCOE
What is frequency modulation?
When the frequency of carrier wave is changed in
accordance with the message signal, The process is called
frequency modulation.
In FM the carrier amplitude remain constant the carrier
frequency varies
It is a type of Angle modulation
Why Frequency modulation is called nonlinearmodulation?
Frequency Modulation Waveform
• In fact, someone needed to carry part of the walkie talkie on their
back because it was so large.
• Around the same time Al Gross was working on his model, Donald
Hings was working on his version of the walkie talkie which he
called a “packset”. Hings invention was used during War World
Two starting in 1942 and was very important in the war effort.
• After World War II the use of the hand held radio spread
throughout different public sectors. With more compact
designs police and fireman began to rely more on the
devices for communication.
• Later walkie talkie use moved from public to private
sectors and to everyday use for the average person or
even toys for children.
All India Radio
• Officially known since 1956 as Ākāshvāṇī, is the national public
radio broadcaster of India.
• All India Radio is one of the largest radio networks in the
world and is headquartered at the Akashvani Bhavan in New
Delhi.
• AIR’s home service comprises 414 stations today located
across the country, reaching nearly 92% of the country’s area
and 99.19 % of the total population.
• AIR originates programming in 23 languages and 146
dialects.
FM Broadcasting in India
The Regulations
Regulates AIR
Regulates Private Radio Broadcasting
Both regulators are overseen by the Ministry of Information and Broadcasting,
Government of India, which is in charge of all media regulation.
Applications:
FM Radio
FM radio uses a modulation index, m > 1, and this is called
wideband FM. As its name suggests the bandwidth is much larger
than AM.
Television Sound:
In terrestrial TV broadcasts, the video information is transmitted using AM .
However the sound information is transmitted using FM, in order to reduce possible
interference between the video and sound signals. In this case, the maximum deviation of
the carrier,  fc , is chosen to be 50kHz, and the information baseband is again the high
fidelity range 20Hz to 15kHz. Therefore the bandwidth required for TV Sound is:
BW Of TV Sound=2(50k+15k)
=130khz
Satellite TV.
Some satellite TV transmissions broadcast an analogue video signal using FM. This
helps to obtain an acceptable signal at the receiving station In this case, the maximum deviation
of the carrier  fc , is chosen to be about 10 MHz, with a video baseband of around 5MHz.
Therefore the bandwidth required for Satellite TV is:
BW of satellite TV =2(10+5)
=30Mhz
Phase Modulation(PM)
• PM is the modulation technique in which carrier phase varies based
on analog baseband information signal to be transmitted using
wireless device.
• If a constant amplitude as well as constant frequency sine wave
carrier is given to the phase shifter the output is phase modulated
signal.
• Phase modulation is referred as indirect frequency modulation due
to the fact that phase modulation produces frequency modulation.
• The effect of variation in amount of phase shift is proportional to
change in the carrier frequency.
PM
Phase Modulation (PM)
In phase modulation the angle is varied linearly with the message signal m(t) as :
FM & PM
• In FM, modulation index is inversely proportional to the modulating
frequency but In PM that is independent of the modulating
frequency.
• Phase and frequency are inseparably linked as phase is the integral of
frequency.
• Frequency modulation can be changed to phase modulation by
simply adding a CR network to the modulating signal that integrates
the modulating signal.
Waveform
Relation between FM & PM
ADVANTAGES OF FM
• Resilient to the noise : AM waves do not have constant
envelopes and therefore more affected by static or noise
than FM. Unwanted electromagnetic waves do not cause
the frequency of FM carrier wave to change.
• Resilient to signal strength variations
• Does not require linear amplifiers in the transmitter
• Enables greater efficiency than many other modes
ADVANTAGES OF AM
• It is simple to implement
• it can be demodulated using a circuit consisting
of very few components
• AM receivers are very cheap as no specialized
components are needed.
• .
Comparison of FM with PM
Sr.
No.
FM
PM
1
Frequency deviation is proportional to
modulating voltage
Phase deviation is proportional
to the modulating voltage
2
Noise immunity is better than AM and PM
Noise immunity is better than
AM but worse than FM
3
SNR is better than PM
SNR is worse than FM
4
FM is widely used for radio broadcasting
PM is only used in some mobile
systems
5
It is possible to receive FM on PM receiver
It is possible to receive PM on
FM receiver
6
Modulation index is proportional to
modulating voltage as well as the modulating
frequency .
Modulation index is
proportional to modulating
voltage
Comparison of FM and AM
Sr.
No.
FM
AM
1
FM receivers are immune to noise
AM receivers are not immune to noise
2
It is possible to decrease noise by
increasing deviation
This feature is absent in AM
3
Bandwidth is higher and depends on
modulation index
Bandwidth is lower compared to AM but
independent of modulation index
4
FM transmission and reception
equipment are more complex
FM transmission and reception
equipment are less complex
5
All transmitted power is useful
Carrier power and one sideband power is
useless
Frequency Spectrum and Eigen Values
• The Equation of FM Mathematically is expressed as a sine of sine or
cosine of sine by using the Bessel Functions then equation
• The trigonometric series of above equation becomes
e(t)= Ecmax{Jo(β) cos wct
+ J1(β) cos (wc±Wm)t
+ J2(β) cos (wc±2Wm)t
+ J3(β) cos (wc±3Wm)t
+ J4(β) cos (wc±4Wm)t……..}
e(t )  Ec max

J
n  
n
(  ) cos(c  nm )t
• It has infinite no pairs of sidebands with a coefficients J called as
Bessel function coefficient
• The FM waveform has a component at the carrier frequency and an unlimited series of
frequency, above and below the carrier frequency
• Characteristics of Bessel Function


2 1
J

n n
OR
J ( n)  (1) J n
n
2J n2 Vc2( power)
E

n c
Bessel Function Coefficient Table
Bessel Function
FM Bandwidth
Eigen Value (career component zero)
The spectra of various values of β are shown in above fig
,in each case the spectral lines are spaced by fm
BFM =2 n fm Hz
Significant Sidebands – Spectrum
• The table below shows the number of significant
sidebands for various modulation

0.1
0.3
0.5
1.0
2.0
5.0
10.0
No of sidebands  1% of
unmodulated carrier
2
4
4
6
8
16
28
Bandwidth
2fm
4fm
4fm
6fm
8fm
16fm
28fm
Example:
For  = 5,
16 sidebands
(8 pairs).
Observations from FM Spectrum of Bessel’s Function
• Unlike AM, has large no. of sidebands which are separated
from carrier by Fm, 2Fm,….
• J coefficients eventually decrease as in value as n increases
• In AM increased depth of modulation increases side-band
power & hence the total transmitted power.
• In FM total transmitted power always remains constant, but
with increased depth of modulation the required bandwidth
is also increased.
• Amplitude of carrier component does not remains same.
• For certain values of modulation index, carrier component
disappears completely, these are called Eigen values.
FM Bandwidth
• The n is highest order of side frequency for which the amplitude is
significant
• The order of side frequency is greater than
( β + 1) and the amplitude is 5% less
• Hence ,
BFM = 2 n fm = 2 ( β +1) fm Hz
Substituting β = Δf/ fm
BFM = 2(Δf + fm) Hz……….. Carson’s Rule
Carson’s rule is an approximation and gives transmission bandwidth that
are slightly narrower than the bandwidths determined using the Bessel
table
• “FM is called as a constant bandwidth system”
Problems
• 1. Calculate BW of FM for modulating signals
a) 0.1 Khz
b) 1Khz
c) 10Khz
With maximum deviation of 75Khz
2. A FM wave is expressed as V=10sin[5*108 𝑡+4sin1250t]
Find: 1. Carrier Frequency & Modulating frequencies
2. Modulation Index & Maximum deviation
MCQ’s
• The amount of frequency shift in FM is directly proportional to the ---- of
the modulating signal
• The general name given to FM & PM is ------- modulation
• The FM produced by phase modulator is known as----• When the modulating signal crosses zero, the phase shift & frequency
deviation in a phase modulator are
a. At a maximum
b. At a minimum
c. Zero
• The bandwidth of FM signal is proportional to -----• The amplitudes of sidebands in an FM signal are dependent upon a
mathematical process known as -------
Types of FM
Narrow band FM
1.
Narrow band FM is defined as the situation where the
modulation index  is small.
2.
From the table of Bessel functions it may be seen that for
small , (  0.3) there is only the carrier and significant
sidebands, i.e. BW = 2fm.
FM with   0.3 is referred to as narrowband FM (NBFM).
3.
Maximum modulating frequency is usually 3kHz
4.
maximum frequency deviation is =75 kHz.
Types of FM
Wide band FM
1.
Wideband FM is defined as the situation where the
modulation index  is larger.
2.
For  > 0.3 there are more than 2 significant sidebands. As 
increases the number of sidebands increases. This is referred
to as wideband FM (WBFM).
3.
Modulation frequencies extend from 30 Hz to 15 kHz.
4.
Maximum permissible deviation is=75 kHz.
5.
Wideband FM system need large bandwidth, typically 15 times
that of narrowband FM system.
Generation of FM using PM
 x(t )dt
Modulating Wave x(t)
Integrator
FM Wave
Phase
Modulator
Carrier
Oscillator
Ec cos( 2fct )
Relation between FM & PM
METHODS OF FM GENERATION
DIRECT
METHOD
REACTANCE
MODULATOR
INDIRECT
METHOD
VARACTOR
MODULATOR
ARMSTRONG
METHOD
DIRECT METHODIn direct method, the modulating (base band) signal directly modulates the
carrier .The carrier signal is generated using a LC oscillator circuit.
Frequency of oscillator of carrier
𝜔𝐶 =
1
𝐿𝐶
The instantaneous frequency of the carrier wave is directly varied in accordance
with the message signal by means of an voltage controlled oscillator.
The frequency determining network in the oscillator is chosen with high quality
factor (Q-factor) and the oscillator is controlled by the incremental variation of
the reactive components in the tank circuit of the oscillator.
Varactor diode Generation
A varactor diode is a semiconductor diode whose junction capacitance
varies linearly with applied voltage when the diode is reverse biased.
FM Transmitters
NBFM
Crystal
oscillator
phase
modulator
Audio
source
WBFM
frequency
multiplier
ANTENNA
Power amplifier
Crystal oscillatorCrystal oscillator generates the stable carrier signal.
Phase modulatorThe phase modulator modulates the carrier signal and the
massage signal in the low power range to generate a
narrowband FM.
Frequency multiplierThe frequency multiplier is used to increase the frequency
deviation and carrier signal frequency to a desired level.
Power amplifierThe power amplifier gives the required power level to the
signal which passes through the antenna.
AntennaAntenna is a device which is used for sending and
receiving the information.
Indirect Method- Armstrong Method
FM stereo Transmitter
Broadcasting standards
• VHF band 88-108 MHz
• Deviation ± 75KHz
• Channel Spacing 200KHz
• Line of Sight Propagation
• Modulating Frequency 50Hz - 15 KHz
• Coverage 50 miles
• Power output 100kW
Imp links
• https://www.elprocus.com/voltage-controlled-oscillatorworking-application/
• http://www.radio-electronics.com/info/rf-technologydesign/pm-phase-modulation/what-is-pm-tutorial.php
• http://www.ques10.com/p/11412/how-can-you-use-avaractor-diode-in-the-generation/
• http://www.diffen.com/difference/AM_vs_FM
Unit 4: FM Reception
Communication Systems
• We have studied the basic blocks of any communication system
• Modulator
• Demodulator
• Modulation Schemes:
• Linear Modulation (DSB, AM, SSB, VSB)
• Angle Modulation (FM, PM)
Syllabus
• Block diagram of FM Receiver, FM Stereo Receiver , Two
way FM Radio Receiver, FM detection using Phase lock
loop(PLL) ,Slope detector, Balanced Slope detector etc.
AM/FM Radio System
• Different audio sources have different bandwidth
• Speech- 4kHz
• High quality music- 15kHz
• AM radio the baseband bandwidth limits to 5kHz
• FM radio uses baseband bandwidth upto 15kHz
AM/FM Radio System
• For AM radio, each station occupies a maximum bandwidth of 10 kHz
for transmission.
• Carrier spacing is 10 kHz
• For FM radio, each station occupies a bandwidth of 200 kHz, and
therefore the carrier spacing is 200 kHz
AM/FM Radio System
• Transmission Bandwidth:
• It is the bandwidth occupied by a message signal in the radio
frequency spectrum
• This is also the carrier spacing
• AM:
2W
• FM:
2( f  W ) (Carson’s Rule)
AM/FM Radio Spectrum
• Recall that AM and FM have different radio frequency (RF) spectrum
ranges:
• AM: 540 kHz – 1600 kHz
• FM: 88 MHz – 108 MHz
• Therefore, two IF frequencies
• AM: 455 kHz, with BW of 10 KHz
• FM: 10.7 MHz, with BW of 200 KHz
Radio Receiver
• Requirements:
– Has to work with both AM and FM signals
– Tune to and amplify desired radio station
– Filter out all other stations
– Demodulator has to work with all radio
stations regardless of carrier frequency
FM Radio Receiver
FM receiver is also super-heterodyne type of receiver with a
bandwidth of 200Khz and maximum frequency deviation of
±75Khz frequency deviation.
The propagation is restricted to line of sight as VHF band(88 to
108Mhz) is used with a radius of approximately 50 miles
around the transmitter.
FM Radio Receiver Block Diagram
Pre-emphasis & De-emphasis
• Pre-emphasis refers to artificially boosting relative amplitudes of the
modulating voltage for higher audio frequency.
• De-emphasis means attenuating those amplitudes & Frequencies by
amount they are boosted
Pre-emphasis
De-emphasis
• RF Section
– Tunes to the desired RF frequency.
– It passes the desired radio station.
– The signal sensitivity of the RF amplifier is
typically 1 to 10 µV.
– The RF tuned circuit and local oscillator are tuned
by variable ganged capacitors.
– The local oscillator frequency varies from 98.7 to
118.7Mhz (10.7Mhz more than signal frequency)
• IF amplifier Section:
• It consists of high gain stages with limiters.
• Limiters are necessary as discriminator circuits respond to
amplitude variations in FM signal, introducing unwanted
noise.
• The limiter circuit works when the input signal becomes
larger than required to drive the amplifier from cut-off to
saturation.
• The point at which the signal input exceeds the active range
is the threshold of the limiter, beyond which it works.
• This threshold level for limiter typically is 1mv and the
quieting level is set to sensitivity of amplifier, at which the
receiver quiets typically is 10µV or lower.
The output of Limiter is then applied to FM discriminator, which detects the
original signal, however this signal is not the original modulating signal. Thus
it is de-emphasized. De-emphasizing attenuates the higher frequencies to
bring back to their original amplitude as these are emphasized at the
transmitter. With ratio detector used instead of discriminator limiter is not
required as ratio detector itself limits the amplitude of the received signal.
Discriminator Circuit : The Foster-Seeley discriminator , the ratio detector are commonly
found in older receivers. They are based on the principle of slope detection using
resonant circuits.
FM
Detectors
Indirect Type
Direct Type
Slope Detectors
Balanced slope
Detectors
Foster seeley
discrimintor
PLL
Indirect- PLL
PLL is an electronics Module, that locks the phase of
output to the input
Phase Locked Loop
Vi
Vo
Phase locking
• PLL is just a feedback system, that detects the phase error and
then adjusts the phase of output
• VCO adjusts the phase difference
𝑉𝑖
𝑉𝑜
Δϕ
Phase detector
VCO
The basic PLL block diagram consists of three components
connected in a feedback loop :
• A phase detector (PD) or phase frequency detector (PFD)
• A voltage-controlled oscillator (VCO)
• A loop filter (LF)
IMP Conditions of PLL
• A basic property of the PLL is that it attempts to maintain the
frequency lock fosc= fi between Vosc and Vi even if the frequency fi of
the incoming signal varies in time.
• Suppose that the PLL is in the locked condition, and that the
frequency fi of the incoming signal increases slightly. The phase
difference between the VCO signal and the incoming signal will begin
to increase in time.
• As a result, the filter output voltage Vo increases  the VCO output
frequency fosc increases until it matches fin, thus keeping the PLL in
the locked condition.
Locked Range and Capture Range of the PLL
Locked condition: fosc = fi
Unlocked condition: fosc = fo = const
Lock Range of the PLL: The range of frequencies from fi = fmin to fi = fmax where the locked PLL remains in the locked
condition. The lock range is wider than the capture range.
• If the PLL is initially locked, and if fi < fmin, or fi > fmax  the PLL becomes unlocked fi ≠ fosc. When the PLL is unlocked,
the VCO oscillates at the frequency fo called the center frequency, or the free-running frequency of the VCO.
Capture Range of the PLL: The lock can be established again if the incoming signal frequency fi gets close enough to fo.
The range of frequencies fi = fo- fc to fi = fo+ fc such that the initially unlocked PLL becomes locked. Sometimes a
frequency detector is added to the phase detector to assist in initial acquisition of lock.
Locked Range and Capture Range of the PLL
• Once the PLL is in the locked condition, it remains locked as long as the VCO output frequency fosc can be
adjusted to match the incoming signal frequency fi  fmin ≤ fi ≤ fmax.
• When the lock is lost, the VCO operates at the free-running frequency fo,  fmin ≤ fo ≤ fmax.
• To establish the lock again, i.e. to capture the incoming signal again, the incoming signal frequency fi must be
close enough to fo  fo– fc ≤ fi ≤ fo+ fc . The 2fc is called the capture range.
• The capture range 2fc is an important PLL parameter because it determines whether the locked condition can
be established or not. Note that the capture range 2fc < the lock range fmax – fmin.
• The capture range 2fc depends on the characteristics of the loop filter. For the simple RC filter, a very crude,
approximate implicit expression for the capture range can be found as:
where fp is the cut-off frequency of the filter, VDD is the supply voltage, and Ko is the VCO gain.
• If the capture range is much larger than the cut-off frequency of the filter, fc/fp >> 1, the expression for the
capture range is simplified as:
• Lower fp is desirable in order to better attenuate high frequency and improve noise rejection but it cause
capture range smaller as usually desirable to have wider capture range.
Phase Detector (PD)
A simple phase detector is an XOR gate with logic low output
(Vφ = 0V) and the logic high output (Vφ = VDD).
An example below shows the PLL is in the locked condition
where Vi and Vosc are two phase-shifted periodic square-wave
𝟏
signals at the same frequency fosc = fi = T , and with 50% duty
i
ratios. The output of the phase detector is a periodic squarewave signal Vφ(t) at the frequency 2fi , and with the duty ratio
Dφ that depends on the phase difference φ(t) = [φosc(t) - φi(t)]
between Vi and Vosc  Dφ =
φ
𝝅
(for XOR)
The output of the XOR phase detector can be written as the
Fourier series:
The dc component Vo of the phase detector output can be
T
found easily as the average of Vφ(t) over a period 𝟐i
 Vo =
VDD
𝝅
where KD =
φ = KD φ
VDD
𝝅
volts/rad
KD is called PD gain
for 0 ≤ φ ≤ π
V
The average output rise to Vout = πDD ΔΦ = 0 → VDD when ΔΦ goes from 0 → π.
For ΔΦ > π , the average output begins to drop.
Loop Filter
The output Vφ(t) of the phase detector is filtered by the low-pass loop filter. The purpose of the low-pass filter is to
pass the dc and low-frequency portions of Vφ(t) and to attenuate high-frequency ac components at frequencies 2kfi .
The simple RC filter has the transfer function:
F(s) =
𝟏
𝟏+𝒔 𝑹 𝑪
=
1
𝟏
𝟏 + 𝒔/ω𝒑
ω
where ωp = 𝑅 𝐶 and fp = 2𝛑𝑝 is the cut-off frequency of the filter.
If fp << 2fi  the output of the filter Vo is approximately equal to the
dc component V𝝓 of the phase detector output.
In practice, the high-frequency components are not completely eliminated
and can be observed as high-frequency ac ripple around the dc or slowly-varying Vo.
In general, the filter output Vo as a function of the phase
difference. Note that Vo = 0 if Vi and Vosc are in phase (𝝓 = 0),
and that it reaches the maximum value Vo = VDD when the two
signals are exactly out of phase (𝝓 = π).
For 0 ≤ 𝝓 ≤ π, Vo increases, and for 𝝓 > π, Vo decreases. The
characteristic of periodic in 𝝓 with period 2π.
The range 0 ≤ 𝝓 ≤ π is the range where the PLL can operate in
the locked condition.
Voltage Controlled Oscillator (VCO)
In PLL applications, the VCO is treated as a linear, time-invariant system. To obtain an arbitrary output frequency (within
the VCO tuning range), a finite Vo is required. Let’s define φosc– φi = φ.
The XOR function produces an output pulse whenever there is a phase misalignment. Suppose that an output frequency
ω1 is needed. From the upper right figure, we see that a control voltage V1 will be necessary to produce this output
frequency. The phase detector can produce this V1 only by maintaining a phase offset φ at its input. In order to minimize
the required phase offset or error, the PLL loop gain, KD KO, should be maximized, since φ =
𝑽𝟏
𝑲𝑫
=
𝒇𝟏 − 𝒇𝟎
𝑲 𝑫𝑲 𝑶
The VCO gain is defined as:
Ko =
VCO Example
The filter output Vo controls the VCO, i.e., determines the frequency fosc of the VCO output Vosc . From PLL 4046 circuit
below, the voltage Vo controls the charging and discharging currents through capacitor C1. As a result the frequency fosc
of the VCO is determined by the Vo. The VCO output Vosc is a square wave with 50% duty ratio and frequency fosc.
The VCO characteristics are adjustable by three components:
R1, R2 and C1.
When Vo = 0, the VCO operates at the minimum frequency
fmin given approximately by:
fmin =
𝟏
𝑹𝟐(𝑪𝟏+𝟑𝟐 𝒑𝑭)
When Vo = VDD, the VCO operates at the minimum frequency
fmax given approximately by:
𝟏
fmax = fmin+ 𝑹𝟏(𝑪𝟏+𝟑𝟐 𝒑𝑭)
For fmin ≤ fosc ≤ fmax, the VCO output frequency fosc is ideally a
linear function of the control voltage Vo.
Δf
The slope Ko = osc of the fosc(Vo) characteristic is called the
Δ𝑽𝒐
gain or the frequency sensitivity of the VCO, in Hz/V.
Slope Detector:
• Tuned Circuit produce an output voltage proportional to the input
frequency.
• Center frequency is place at the center of the most linear portion of
the voltage versus-frequency curve
• When IF deviates above or below fc , output voltage increases or
decreases
• Tuned circuit converts frequency variation to voltage variation
Balanced Slope detector
•Two single-ended slope detectors connected in parallel and
fed 180 degree out of phase is used
•Phase inversion accomplished by center-tapping secondary
winding
• Top tuned circuit is tuned to a frequency above the center
frequency and below to lower frequency.
•At the center frequency, the output voltage from the two tuned
circuits are equal in amplitude but opposite in polarity, v out = 0 V
•When IF deviate above resonance, top tuned circuit produces a
higher output voltage than the lower circuit and voltage goes
positive
•When IF deviate below resonance, lower tuned
circuit produces
higher output than upper, and output goes negative.