AM and FM Transmitters and receivers

14 Marks
Ms. Kavita Giri
Lecturer (ET)
Government Residential Womens Polytechnic,
Latur

1. Explain with sketches the working of given type of
AM generation technique.
2. Explain the function of given blocks of AM super
heterodyne receiver.
3. Explain with sketches given type of AM
demodulation technique.
4. Explain with sketches principle of given type of FM
generation technique.
5. Compare the working of given type of FM detectors.
22334 – Principles of Electronic Communication
Ms. Kavita Giri, Lecturer (ET), GRWP, Latur

 3.1 Generation of AM
 3.2 Block diagram of AM super heterodyne receiver and its working
with waveforms.
 3.3 Demodulation of AM : Diode detector and Practical Diode detector
 3.4 Automatic gain control and its types
 3.5 Concept of Pre-emphasis and De-emphasis
 3.6 Generation of FM using direct and indirect method
 3.7 Block diagram of FM receiver and its working with waveforms.
 3.8 FM detector circuits: Ratio detector and PLL as FM detector

Low Level Modulation
• AM generation at low
power levels
• Generated AM is then
amplified using a chain of
linear amplifiers
High Level
Modulation
• AM generation at high
power levels
• Carrier and modulating
signals are amplified
first and then
modulated in the last
RF amplifier stage
AM Modulators

3.1.1 Low Level AM Modulator
FET acts
as a
variable
resistance
Op-amp
as a non-
inverting
amplifier

3.1.1 Low Level AM Modulator
Negative
bias at
gate
source
junction
FET
reverse
biased
Modulating
signal applied
to gate through
coupling cap.
C1
Resistance of
JFET changes in
proportion to
change in
amplitude of
modulating signal
Gain changes
in proportion
with
modulating
signal
Carrier signal
applied at non-
inverting terminal
will get amplified
more for positive
going modulating
signal
Gain increases
as Ri
decreases
when
modulating
signal is
positive
Less
amplification is
provided to the
carrier for
negative going
modulating
signal.
Thus AM wave
is produced at
the output of
op-amp
amplifier

3.1.2 Principle of High level modulation
Property of a tuned Circuit
• If we apply a current pulse
to a tuned circuit, then it
generates damped voltage
oscillations at its output.
• The amplitude of
oscillations is proportional
to the size of current pulse
and the decay rate is
proportional to the time
constant.
EC403E – Basics of
Communication
Ms.Kavita Giri, Lecturer(EC),Govt. Polytechnic, Nagpur

3.1.3 Requirements of High level modulation
A tuned Circuit
A circuit which
supplies current
pulses to the
tuned circuit
If the amplitude
of current pulses
is made
proportional to
the modulating
signal, then AM
wave will be
generated at the
output of the
tuned circuit.

3.1.4 High level collector modulator
• The class C amplifier
conducts for only a
portion of the positive
half cycles of the carrier
signal applied at the
base of transistor Q1.
It is a high power
RF Class C
amplifier.

Q1 receives the RF signal
at its base. It will conduct
only for a portion of
positive half cycle of carrier
signal.
Collector current of Q1 is in
the form of current pulses.
These pulses are supplied
to the tuned circuit.
The high power audio
amplifier amplifies the
modulating signal to high
power level.
Secondary winding of
modulating transformer T1
is connected in series with
the dc supply voltage Vcc.
Therefore modulating
voltage will add or subtract
from Vcc

3.1.4 High level collector modulator This varying supply
voltage is then applied to
the class C amplifier.
Naturally the amplitude of
collector current pulses
will vary in accordance
with the modulating
signal.
These current pulses pass
through the tuned circuit.
They will cause the tuned
circuit to oscillate at the
desired output frequency.
As per the property of the
tuned circuit, the AM wave
is produced at the output
of the tuned circuit.

3.1.5 Advantages of High level collector modulator
1. Better linearity
2. Higher efficiency
3. Higher output power per transistor
3.1.6 Limitations of High level collector modulator
It wont be possible to get 100% modulation using this circuit.
• To get 100% modulation, Em = Vcc
• When the modulating signal goes negative, it subtracts from the Vcc.
• At the negative peak point, the subtraction is zero, so zero voltage is
applied to transistor Q1 producing zero output.

AM transmitters
Low Level AM
transmitter
performs the process of
modulation near the beginning of
the transmitter
High Level AM
transmitter
performs the modulation step last,
at the last or "final" amplifier
stage in the transmitter

Low Level Modulated AM Transmitter
AF
modulating
Signal
Stabilized
RF crystal
oscillator
Class A
Buffer
amplifier
Modulator
Linear
Amplifier
Power
amplifier
Audio
processing
and
filtering
Class A AF
amplifier

Low Level Modulated AM Transmitter
RF
Oscillator
Produces
carrier
signal
Stabilised
to
maintain
deviation
within
prescribe
d limits.
Frequenc
y equal to
transmitte
d
frequency
Buffer
Amplifier
To
maintain
constant
input
resistanc
e at
modulato
r
Prevents
loading of
the
modulato
r stage
Modulator
Amplified
modulatin
g signal
is applied
to
modulato
r along
with the
carrier.
At the
output we
get AM
wave.
Linear
Amplifier
only
provides
a voltage
gain, and
not
necessari
ly a
current
gain
The
power
levels are
quite
small
Power
Amplifier
Increase
both the
voltage
and
current of
the AM
signal.
provides
current
gain.

High Level modulated AM Transmitter
Stabilized
RF crystal
oscillator
Class A RF
amplifier
Class C RF
power
amplifier
Class C RF
output
amplifier
Audio
processing
and filtering
Class A AF
amplifier
Class B AF
power
amplifier
AF
modulating
signal

High Level modulated AM Transmitter
Carrier is generated by the stabilized
crystal oscillator and amplified to
adequate power level using Class C RF
power amplifiers.
Modulating signal is also amplified to a
high power level before modulation
takes place.
Modulation takes place in the last class
C RF amplifier. The modulator output is
AM wave which is directly transmitted.
The collector modulated transistorized
circuit or plate modulated vacuum tube
modulator is used.

Comparison of Low level and High level modulation
Sr.
No.
Parameter Low level modulation High level modulation
1
Modulation takes
place at
Low power level High power level
2 Types of amplifiers
Linear amplifiers (A,
AB or B) are used after
modulation
Highly efficient class C
amplifiers
3 Efficiency
Lower than high level
modulators
Very high
4 Devices used
Transistors, JFET, Op-
Amps
Vacuum tubes or
transistors for medium
power applications
5
Design of AF power
amplifier
Easy as low power
involved
Complex due to high
power involved
6 Applications
Sometimes used in TV
transmitters
High power broadcast
transmitters
7
Power handling
capacity
Low High

Superheterodyne is basically a process of designing and
constructing wireless communications such as radio
receivers by mixing two frequencies together in order to
produce a difference frequency component called as
intermediate frequency (IF), so as to reduce signal
frequency prior to processing.

6.2.2 Superhetrodyne Receiver
•Receiving antenna: The receiving antenna receives the signal which was
sent by the transmitter. It sends the received signal for further
processing.
•RF amplifier: The received signal is fed to the RF amplifier stage so as to
amplify it, as the signal gets attenuated during long-distance
transmission. It is tuned in such a way that it can choose the desired
carrier frequency and amplify it.
•Local Oscillator: This circuit basically generates a signal with a fixed
frequency and the output is then fed to the mixer. When we talk
about AM broadcast system, the intermediate frequency is 455 KHz that
simply means that local oscillator should select such a frequency which is
455 KHz above the incoming signal frequency.

Mixer: A mixer simply mixes the carrier frequency with the frequency of the signal
generated by the local oscillator.
Here, two different frequencies are to be mixed so as to have another frequency
component of lower value. Now the thing that first comes to our mind is why the
mixer produces a lower frequency value, which is the difference between the two
frequencies.
The summation of the carrier and local oscillator frequency at the output of the
mixer will give rise to image frequency which is treated as a type of noise or
distortion in the signal. This is the reason why the mixer generates a frequency
difference at its output. This difference frequency is a constant value irrespective of
the variations in the input, known as the intermediate frequency. The constant
frequency at its output is gained by capacitance tuning. In capacitance tuning,
several capacitances are arranged together and operated by a controlling knob. It
doesn’t matter what the incoming signal frequency is, the RF amplifier and local
oscillator must be tuned to it. 22334 – Principles of Electronic Communication

•IF amplifier: This section basically amplifies the output of the mixer. IF
amplifier provides sensitivity(gain) and selectivity (bandwidth requirement)
to the receiver. As it consists of several transformers consisting of pairs of
the tuned circuit. Here, the sensitivity and selectivity are uniform and does
not show variations as in case of TRF receivers because IF amplifier’s
characteristics are independent of that of the received signal frequency as it
works on the intermediate frequency.
Due to this, the system design is quite easy so as to provide constant
bandwidth along with high gain. This section has narrow bandwidth and
due to its lower bandwidth, it rejects all other frequency so as to reduce the
risk generated from interference. The lower bandwidth accepting nature
supports Superheterodyne receivers to give much better performance than
other types of receivers. 22334 – Principles of Electronic Communication

•Demodulator: Demodulator is placed exactly after the IF
amplifier so that the constant frequency signal is demodulated
and the message signal can be extracted from it.
•Audio amplifier: The original signal is fed to the audio amplifier
which does not hold distortion or noise so that it can amplify
audio signal to a particular level.
•Power amplifier: Here, the signal is further amplified to a
particular power level which can activate the loudspeaker. The
amplified signal is finally fed to the loudspeaker circuit which
converts the electrical form of the signal into an audio sound
signal which can be heard by the listeners.

Select the desired station at frequency fs by tuning
the RF amplifier and local oscillator.
Local oscillator is tuned to frequency f0 with f0 > fs.
Mixer produces IF. Note that IF = (f0 – fs)
Output of mixer is an AM signal with two sidebands
and carrier equal to IF. The IF amplifier amplifies this
signal.
Detector will demodulate this signal to recover the
modulating signal.
The audio amplifier and power amplifier will amplify
the AF signal and apply it to loudspeaker.
Summary
of
superheterodyne
action

Waveforms
at
different
points
of
superheterodyne
receiver

Advantages
1. No variation in bandwidth. The bandwidth remains constant over
the entire operating range.
2. High sensitivity and selectivity.
3. High adjacent channel rejection.
Frequency Parameters of AM Receivers
1. Frequency bands: a. Medium wave (MW) band
b. Short Wave (SW) band
2. RF Carrier Range : a. 535 KHz to 1650 KHz (MW Band)
b. 5 to 15 MHz (SW Band)
3. Intermediate Frequency IF : 455 KHz
4. IF bandwidth B : 10KHz

 The process of detection is also called as demodulation.
 It is the process exactly opposite to modulation.
The process of recovering the message signal from the
received modulated signal is called as detection /
demodulation.

AM
detectors
Envelope
Detector
Square Law
detector

 Simple and efficient method for demodulation of
narrowband AM signals.
 A narrowband AM is one in which carrier frequency fc is
much higher than bandwidth of the modulating signal.
 An envelope demodulator produces an output signal
that follows the envelope of the input AM signal exactly.
 It is used in all the commercial AM radio receivers.
3.3.1 Envelope Detector

The envelope demodulator consists of a diode and RC filter.

In every
positive
half cycle
of the
input, the
demodul
ator
diode is
forward
biased.
charge the filter
capacitor C
connected
across the load
resistance R to
almost the peak
value of the
input voltage.
As soon
as the
capacitor
charges
to the
peak
value, the
diode
stop
conducti
ng.
The
capacitor
will now
discharge
through R
between the
positive
peaks.
The
discharging
process
continues
until the
next
positive half
cycle.

It shows the charging discharging of the filter capacitor and the
approximate output voltage .
It may be observed from these waveforms that the envelope of the AM
wave is being recovered successfully .

Selection of RC time constants
• The capacitor charges through D and Rs when the diode is on and it
discharges through R when the diode is off.
• The charging time constant RsC should be short compared to the
carrier period 1/fc .
Thus, RsC << 1/fc
• On the other hand, the discharging time constant RC should be long
enough so that the capacitor discharges slowly through the load
resistance R .
• But, this time constant should not be too long which will not allow
the capacitor voltage to discharge at the maximum rate of change of
the envelope .
• Therefore, 1/fc << RC << 1/W
• where, W = Maximum modulating frequency

Distortions in envelop detector
There are two types of distortions which can occur in the detector
output such as :
1. Diagonal clipping
2. Negative peak clipping
1. Diagonal Clipping
 This type of distortion
occurs when the RC time
constant of the load
circuit is too long.
 Due to this, the RC
circuit cannot follow the
fast changes in the
modulating envelope .

2. Negative peak Clipping
 This distortion occurs due to a fact that the modulation index
on the output side of the detector is higher than that on its
input side.
 Hence, at higher depth of modulation of the transmitted
signal, the overmodulation may take place at the output of the
detector.
 The negative peak clipping will take place as a result of this
overmodulation.

3.3.2 Practical Diode Detector

The diode has been reversed, so that now the
negative envelope is demodulated. This has no effect
on detection, but it does ensure that a negative AGC
voltage will be available.
The resistor R of the basic circuit has been
split into two parts R1 and R2 to ensure
that there is a series DC path to ground for
the diode

A low pass filter has been
added, in the form of R1-C1.
This has the function of
removing any RF ripple that
might still be present.
Capacitor C2 is
coupling capacitor,
which is used to
prevent the diode dc
output from reaching
the volume control R4.
R3-C3 is a low pass filter to the carrier
strength, and which may be used for
automatic gain control

 The signals from various radio stations reaching at the receiver
input are not of same strength.
 The signals from strong stations are strong and those from
weak stations are weak.
 If the receiver gain is constant then the receiver output will
fluctuate proportional to the strength of input signal.
 This is not desirable.
 So the automatic gain control is used to adjust the receiver
gain automatically so as to keep the receiver output constant
irrespective of strength of input signal.
AGC
Simple AGC
Delayed AGC
Types
of
AGC

3.4.1 Simple AGC
 Simple AGC is a system which will change the overall
gain of a receiver automatically.
 This is done in order to keep the receiver output
constant even when the signal strength at the input of
the receiver is changing.
 In the AGC system, a dc voltage (AGC bias) is derived
from the detector. This AGC bias is thus proportional to
the strength of the received signal.
 The AGC bias is applied to a selected number of RF and
IF amplifiers and mixer stage.
 The transconductance and hence the gain of the
devices connected to these stages is dependent on the
applied AGC bias.

AGC Characteristics
Advantages of Simple AGC
1. Simplicity
2. Low cost.
Hence used in domestic radio
receivers.
Disadvantages of Simple
AGC
1. Not only strong signals and the
weak signals also are
attenuated.

3.4.2 Delayed AGC
Ideal AGC
• No Delayed Automatic
Gain Control would be
applied until signal
strength was considered
adequate, and after this
point a constant average
output would be
obtained no matter how
much more the signal
strength rose.

3.4.2 Delayed AGC
Delayed Automatic Gain Control
curve.
• This shows that AGC bias
is not applied until the
signal strength has reached
a predetermined level, after
which bias is applied as
with normal AGC, but more
strongly.
• As the signal strength then
rises, receiver output also
rises, but relatively slightly.
• The problem of reducing
the gain of the receiver for
weak signals has thus been
avoided, as with “ideal”
AGC

3.4.2 Delayed AGC

3.4.2 Delayed AGC
• It uses two separate diodes: the detector and the AGC detector.
• These can be connected either to separate transformer winding,
as shown, or both may be connected to the secondary without
too much interference.
• A positive bias is applied to the cathode of the AGC diode, to
prevent conduction until a predetermined signal level has been
reached.
• A control is often provided, as shown, to allow manual
adjustment of the bias on the AGC diode, and hence of the
signal level at which Delayed Automatic Gain Control is applied.
• If weak stations are mostly likely to be received, the delay
control setting may be quite high (i.e., no AGO until signal level
is fairly high).
• Nevertheless, it should be made as low as possible, to prevent
overloading of the last IF amplifier by unexpected stronger
signals.

Pre-Emphasis
In FM, the noise has a greater effect on the
higher modulating frequencies. This effect
can be reduced by increasing the value of
modulation index (mf ) for higher modulating
frequencies (fm).
This can be done by increasing the
deviation Δf and Δf can be increased by
increasing the amplitude of modulating
signal at higher modulating frequencies.
Thus, if we boost the amplitude of higher
frequency modulating signals artificially then
it will be possible to improve the noise
immunity at higher modulating frequencies.
Why
Pre-
Emphasis
???

Pre-Emphasis The artificial boosting of higher
modulating frequencies is called as pre-
emphasis.
Boosting of higher frequency modulating signal is achieved by using the pre-
emphasis circuit as shown in fig.1(a).
As shown in the fig.1, the modulating AF signal is passed through a high pass RC
filter, before applying it to the FM modulator.
As fm increases, reactance of C decreases and modulating voltage applied to FM
modulator goes on increasing.
The frequency response characteristics of the RC high pass network is shown in
fig.1(b).
The boosting is done according to this pre arranged curve.
The amount of pre-emphasis in US FM transmission and sound transmission in TV
has been standardized at 75 μsec.

Pre-Emphasis
• The pre-emphasis circuit is basically a high pass filter. The
pre-emphasis is carried out at the transmitter.
• The frequency for the RC high pass network is 2122 Hz as
shown in fig.1 (b).
• Hence, the pre-emphasis circuit is used at the transmitter as
shown in fig.2.

De-Emphasis
The process that is used at the
receiver end to nullify or
compensate the artificial
boosting given to the higher
modulating frequencies in the
process of pre-emphasis is
called De-emphasis.
• That means, the artificially
boosted high frequency signals
are brought to their original
amplitude using the de-
emphasis circuit.
• The 75 μsec de-emphasis
circuit is standard and it is as
shown in fig. 3.
It shows that it is a low pass filter. 75
μsec de-emphasis corresponds to a
frequency response curve that is 3 dB
down at a frequency whose RC time
constant is 75 μsec.i.e.,
The demodulated FM is applied to the
De-emphasis circuit. With increase in fm
the reactance of C goes on decreasing
and the output of de-emphasis circuit
will also reduce as shown in fig.3.

Methods of
FM
Generation
Direct
Methods
Reactance
Modulators
Varactor
Diode
Modulators
Indirect
Methods
Armstrong
method

Direct Method of FM Generation
• In direct method or parameter variation method, the
baseband or modulating signal directly modulates
the carrier.
• The carrier signal is generated with the help of an
oscillator circuit.
• This oscillator circuit uses a parallel tuned L-C
circuit.
• Thus the frequency of oscillation of the carrier
generation is governed by the expression:
• Now, we can make the carrier frequency ωc to vary
in accordance with the baseband or modulating
signal x(t) if L or C is varied according to x(t).

A) Reactance Modulator
In direct FM generation, the instantaneous frequency of the
carrier is changed directly in proportion with the message signal.
For this, a device called voltage controlled oscillator (VCO) is
used.
A VCO can be implemented by using a sinusoidal oscillator with
a tuned circuit having a high value of Q.
The frequency of this
oscillator is changed
by changing the
reactive components
involved in the tuned
circuit. If L or C of a
tuned circuit of an
oscillator is changed in
accordance with the
amplitude of
modulating signal then
FM can be obtained
across the tuned circuit
A two or three terminal
device is placed across
the tuned circuit. The
reactance of the device
is varied proportional to
modulating signal
voltage. This will vary
the frequency of the
oscillator to produce FM.
The devices used are
FET, transistor or
varactor diode.
Principle of Working

Varactor Diode Modulator Principle of Working
A varactor diode is a
semiconductor diode whose
junction capacitance varies
linearly with the applied bias
and the varactor diode must be
reverse biased.
The varactor diode is reverse biased by the
negative dc source –Vb.
The modulating AF voltage appears in
series with the negative supply voltage.
Hence, the voltage applied across the
varactor diode varies in proportion with the
modulating voltage.
This will vary the junction capacitance of the
varactor diode.
The varactor diode appears in parallel with
the oscillator tuned circuit.
Hence the oscillator frequency will change
with change in varactor diode capacitance
and FM wave is produced.
The RFC will connect the dc and modulating
signal to the varactor diode but it offers a
very high impedance at high oscillator
frequency. Therefore, the oscillator circuit is
isolated from the dc bias and modulating

Advantages
• Simple Circuits
• Low Cost
Disadvantages
• The LC oscillator frequency is
not stable enough.
• Oscillator cannot be used for
broadcast purposes.
• Crystal Oscillator needs to be
used.
• We have to use automatic
frequency control scheme.

Armstrong Frequency Modulation System
In this method, the FM is obtained through phase modulation.
A crystal oscillator can be used hence the frequency stability is very high
and this method is widely used in practice.

This method needs to be divided into three parts:
Armstrong
method
Part I: How to obtain
FM from phase
modulator?
Part II: Implementation
of Phase modulator
Combining Parts I and II to
obtain indirect method

Part
I:
How
to
obtain
FM
from
phase
modulator?
In PM, along with the phase variation, some frequency variation also
takes place. Higher modulating voltages produce greater phase shift
which in turn produces greater frequency deviation.
Higher modulating frequencies produce a faster rate of change of
modulating voltage hence they also produce greater frequency
deviation.
Thus in PM the carrier frequency deviation is proportional to the
modulating voltage regardless of its frequency.
To correct this problem the modulating signal is passed through a
low pass RC filter. Due to this the high frequency modulating signals
are attenuated but there is no change in amplitudes of low
frequency modulating signals.
The filter output is then applied to a phase modulator along with the
carrier as shown.

Part
II:
Implementation
of
Phase
modulator

Part
II:
Implementation
of
Phase
modulator
The crystal oscillator produces a stable unmodulated carrier which
is applied to the 90° phase shifter as well as the combining
network through a buffer.
The 90° phase shifter produces a 90° phase shifted carrier. It is
applied to the balanced modulator along with the modulating
signal.
Thus, the carrier used for modulation is 90° shifted with respect to
the original carrier.
At the output of the product modulator, we get DSB SC signal i.e.,
AM signal without carrier.
This signal consists of only two sidebands with their resultant in
phase with the 90° shifted carrier .
The two sidebands and the original carrier without any phase shift
are applied to a combining network (∑). At the output of the
combining network, we get the resultant of vector addition of the
carrier and two sidebands as shown in figure 2.

Part
II:
Implementation
of
Phase
modulator
Now, as the modulation index is increased, the amplitude
of sidebands will also increase. Hence, the amplitude of
their resultant increases. This will increase the
angle Φ made by the resultant with unmodulated carrier.

Part
III:
Combining
Parts
I
and
II
The FM signal produced at the output of phase modulator
has a low carrier frequency and low modulation index.
They are increased to an adequately high value with the
help of frequency multipliers and mixer.

Block Diagram
RF
Amplifie
r
Local
Oscillator
Mixer
IF
Amplifier
Limiter
FM
Detector
De-
emphasis
AF and
Power
amplifier
Receiving
Antenna
It also works on the principle of
‘Super heterodyning’ as in AM receivers
Difference between AM and FM receivers:
1. The operating frequencies in FM are much
higher than in AM.
2. FM receivers need limiter and de-emphasis
3. FM demodulators are different than AM.
4. Method to obtain AGC is different.

Working
RF Amplifier
• Improves Signal to Noise ratio
• Matches receiver input impedance to
antenna impedance
Mixer (Frequency Changer)
• Input signal frequency fs and local
oscillator frequency f0 are mixed to down
convert received signal frequency to
intermediate frequency (IF)
• IF = f0 – fs
• = 10.7 MHz
IF Amplifier
• Amplifies IF or mixer output
• Due to large bandwidth, gain per stage is
low. Therefore, two or more stages of IF
amplifier are used.
Limiter
• Removes unwanted amplitude variations
in original FM signal.
FM Detector
• Recovers original modulating signal
back from FM signal.
De-Emphasis
• Artificially boosted high frequencies at
transmitter are removed.
AF and Power amplifier
• Modulating signal is voltage amplified.
• Power is increased so as to drive the
loudspeaker.
AGC
• Ensures that the signal fed to the
limiter is within its limiting range .
• Prevents overloading of last IF
amplifier.
Loudspeaker
• Converts modulating signal into
sound.

Waveforms at various points of FM receiver

An FM detector or demodulator is a circuit which receives an FM
wave at its input and produces the message signal or modulating
signal at its output.
• Demodulation or detection is exactly opposite to the
modulation process.
• The AM detector is basically an envelope detector.
But FM detector is basically a frequency to amplitude
converter.
• It is expected to convert the frequency variations in
FM wave at its input into amplitude variations at its

Requirements of FM detector
It must convert frequency variations into amplitude
variations.
This conversion must be linear and efficient.
The detector circuit should be insensitive to
amplitude changes. It should respond only to the
frequency changes.
It should not be too critical in its adjustment and
variation.
1
2
3
4

Types of FM detectors

AM and FM Transmitters and receivers

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to AM and FM Transmitters and receivers

Similar to AM and FM Transmitters and receivers (20)

Recently uploaded

Recently uploaded (20)

AM and FM Transmitters and receivers