 AM Transmitters:

AM transmitters are classified into two categories:

1. Low level AM transmitter
2. High level AM transmitter

1. Low level AM transmitter:

The block diagram of a low-level AM transmitter is shown below.

 The RF crystal oscillator is most stable mechanical device suitable to generate high
frequency carrier signal of particular frequency.

 The amplified modulating signal is applied to the modulator signal along with carrier. At
the output of the modulator we get the AM wave.

 This AM signal is then amplified using a chain of linear amplifiers to raise its power
level.

 The linear amplifiers can be class A, AB or B type amplifiers. The linear amplifiers are
used in order to avoid the waveform distortion in AM wave.

 The amplitude modulated signal is then transmitted using transmitting antenna.

 The transistorized modulator circuits can be used for low level modulator due to the low
power which is to be handled.

 The low-level transmitter circuit does not require a large AF modulator power so its
design is simplified.
  However, the overall efficiency is much lower compared to high level modulation. This
is due to the use of less efficient linear amplifiers.

2. High level AM transmitter:

 Here the carrier generated by the stabilized crystal oscillator is first amplified to the
adequate power level using class C RF amplifiers.

 The modulating signal also is amplified to a high-power level before modulation takes
place. If we want 100% modulation then the power of modulating signal must be 33

% of the total power. So, if 1500 W total power is to be transmitted, the modulating
power will be 500 W. This highlights the need to amplify the modulating signal to an
adequate power level.

 The modulation takes place in the last class C RF amplifier. The modulator output is Am
wave which can be directly transmitted.
 The collector modulated transistorized circuit or plate modulated vacuum tube modulator
is used as modulator stage.
 The advantage of high-level modulation is its high due to the use of highly efficient
efficiency class C amplifiers.
 The disadvantage is that a large AF power amplifier is needed to raise the modulating
signal to the adequate power level.
 Radio Receivers:
There are only two types of receivers,
1. Tuned radio frequency (TRF) Receiver
2. Superheterodyne Receiver
1. Tuned radio frequency (TRF) Receiver:

The block diagram of TRF receiver is as shown in fig. this receiver consists of two or more
tunable RF amplifiers. All these amplifiers are tuned simultaneously to the desired signal
frequency.

Operation:

The AM transmission take place in medium wave MW band or SW band.

The frequency range for MW band is from 530 kHz to 1640 kHz. Different radio stations operate
at different frequencies in this range of frequency.

The operation of TRF receiver is as follows,

 Due to electromagnetic waves passing over the receiving antenna, voltage is induced in it.
This induced signal consists of signals from various transmitting stations.
 The RF amplifiers are tuned simultaneously to select and amplify the desired signal and
reject all the other signals. Tuning means we adjust the resonant frequency of the circuits
equal to the desired station frequency. Ganged tuning means simultaneous tuning of tuned
circuits in all RF amplifiers stages using a gang capacitor.
 The amplified signal is then demodulated by the detector. The carrier signal is bypassed and
only the modulating signal is recovered in this process.
 The detected signal is amplified to the adequate voltage and power level using the audio
amplifier and power amplifier and given to the loud speaker for reproduction.

Problems in the TRF Receivers:

1. Instability
2. Variation in the bandwidth over the tuning range
3. Insufficient selectivity at high frequencies and poor adjacent channel rejection.

2. Superheterodyne receiver:

RF amplifier and Tuner Section

The amplitude modulated wave received by the antenna is first passed to the tuner
circuit through a transformer. The tuner circuit is nothing but a LC circuit, which is also
called as resonant or tank circuit. It selects the frequency, desired by the AM receiver. It
also tunes the local oscillator and the RF filter at the same time.

Mixer
The signal from the tuner output is sent to the RF-IF converter, which acts as a mixer. It
has a local oscillator, which produces a constant frequency. The mixing process is done here,
having the received signal as one input and the local oscillator frequency as the other input. The
resultant output is a mixture of two frequencies fs, f0, (f0 + fs), (f0 - fs) produced by the mixer,
out of these differences of frequency component i.e. (f0 - fs) is selected, which is called as
the Intermediate Frequency (IF= 455 kHz).
I.F = (f0 - fs)
This frequency contains the same modulation as the original signal fs.

The production of IF helps in the demodulation of any station signal having any carrier
frequency. Hence, all signals are translated to a fixed carrier frequency for adequate selectivity.

IF amplifier and Filter

Intermediate frequency filter is a band pass filter, which passes the desired
frequency. It eliminates all other unwanted frequency components present in it. This is the
advantage of IF filter, which allows only IF frequency.

AM Demodulator
The received AM wave is now demodulated using AM demodulator. This
demodulator uses the envelope detection process to receive the modulating signal.

Audio Amplifier
This is the power amplifier stage, which is used to amplify the detected audio signal.
The processed signal is strengthened to be effective. This signal is passed on to the
loudspeaker to get the original sound signal.

Waveforms at various points of a superheterodyne Receiver:

Advantages of Superheterodyne Receiver:

 No variation in bandwidth
 High sensitivity and selectivity
 High adjacent channel rejection

 Characteristics of the AM Radio Receiver:

1. Sensitivity
2. Selectivity
3. Fidelity
4. Image frequency rejection

1. Sensitivity:
Sensitivity of a radio receiver is defined as its ability to amplify weak signals. Minimum
RF signal level that can be detected at the input to the receiver and still produce a
usable demodulated information signal. radio receivers should have reasonably
high sensitivity so that it may have good response to the desired signal but should
not have excessively high sensitivity otherwise it will pick up all undesired noise
signals. It is function of receiver gain and measures in decibels.
Sensitivity of a receiver is expressed in microvolts of the received signal.
Typical sensitivity for commercial broadcast-band AM receiver is 50 μV.
Sensitivity of the receiver depends on: Noise power present at the input to the
receiver noise figure Bandwidth improvement factor of the receiver. The best way
to improve the sensitivity is to reduce the noise level.

The sensitivity curve is as shown in fig. indicates that the receiver input (µv)
required to obtain the same standard output power changes with carrier frequency.
 The required input voltage is minimum at 850 kHz and increases on both sides of
850 kHz.
2. Selectivity:
Selectivity of radio receiver is its ability to reject unwanted signals.

It is expressed in the graphical form as shown in fig. the radio receiver is tuned to
the frequency fs= 950 kHz. It shows that the receiver offers a minimum rejection at
950 kHz i.e. at tuned frequency but the rejection increases as the input signal
frequency deviates on both sides of 950 kHz. The selectivity decides the adjacent
channel rejection of a receiver. Higher the selectivity better is the adjacent channel
rejection and less is the adjacent channel interference.
3. Fidelity:
Fidelity is defined as a measure of the ability of a communication system to produce
an exact replica of the original source information at the output of the receiver. Any
variations in the demodulated signal that are not in the original information signal
is considered as distortion. Radio receiver should have high fidelity or accuracy.
Example- In an A.M. broadcast the maximum audio frequency is 5 kHz. hence
receiver with good fidelity must produce entire frequency up to 5kHz.
 4. Image frequency rejection:
The image rejection ratio, or image frequency rejection ratio, is the ratio of the
intermediate-frequency (IF) signal level produced by the desired input frequency to
that produced by the image frequency. The image rejection ratio is usually
expressed in dB.

 Tracking:
The receiver has a number of tunable circuits such as the antenna or mixer or a
local oscillator tuned circuits. All this circuits must be tuned correctly if any station
is to be tuned. For this reason, the capacitors in the various tuned circuits are
ganged (mechanically coupled to each other). Due to this arrangement it is possible
to use only one tuning control to vary the tuning capacitors simultaneously. The
local oscillator frequency (fo), must be precisely adjusted to a value which is above
the signal frequency(fs) by IF. i.e. fo=fs+IF. If this tuning is not done precisely then
the frequency difference i.e.(fo-fs) is not correct.
There are two types of Tracking.
 Two Point tracking:
1. Padder Tracking
2. Trimmer Tracking
 Three Point Tracking

1. Padder Tracking:
 A Small variable capacitor Cp called as the
padder capacitor is connected in series with
the oscillator coil. Due to the series connection of Cp and Cosc the effective capacitance
will be less than Cosc alone. This will increase the oscillator frequency making the tracking
error positive. The padder capacitor is adjusted to have zero tracking error on two
extreme points on the frequency dial.

2. Trimmer Tracking:

A small variable capacitor CTr called as the trimmer capacitor is connected in parallel
with the main capacitor Cosc. Due to parallel connection of CTr and Cosc the effective
capacitance will be greater than cisc alone. This decreases the oscillator frequency
 making the error negative. The Trimmer is adjusted to get zero error at two points on
the frequency dial.
 Three Point Tracking:

This combines the padder and trimmer tracking. The three frequencies of correct tracking
i.e. at which zero tracking error exists are normally 600 kHz,1500 kHz and the geometric
mean of the two i.e.950 kHz. It is possible to keep the tracking error below 3 kHz.

 Demodulation of AM signals:
The process of extracting an original message signal from the modulated wave is known
as detection or demodulation. The circuit, which demodulates the modulated wave is
known as the demodulator.
The following demodulators (detectors) are used for demodulating AM wave.
 Envelope Detector
 Practical Diode Detector

 Envelope Detector:
Conventional DSB-AM signals are easily demodulated by an envelope detector It consists
of a diode and an RC circuit, which is a simple lowpass filter During the positive half-cycle
of the input signal, the diode conducts and the capacitor charges up to the peak value of
the input signal When the input falls below the voltage on the capacitor, the diode becomes
reverse-biased and the input disconnects from the output During this period, the capacitor
discharges slowly through the resistor R On the next cycle of the carrier, the diode again
conducts when the input signal exceeds the voltage across the capacitor. The time constant
RC must be selected to follow the variations in the envelope of the modulated signal If RC
is too large, then the discharge of the capacitor is too slow and again the output will not
follow the envelope If RC is too small, then the output of the filter falls very rapidly after
each peak and will not follow the envelope closely.

 Practical Diode Detector:

The circuit operates in the following manner. 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, as will be shown. 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, but at the same time 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 a coupling capacitor, whose main function is to prevent the diode dc output
from reaching the volume control R4. The combination R3 – C3 is a low-pass filter designed
to remove AF components, providing a dc voltage whose amplitude is proportional to the
carrier strength, and which may be used for automatic gain control.

 Automatic Gain Control (AGC):

An Automatic Gain Control (AGC) circuit is a circuit that is designed to maintain a
constant output signal level after amplification, despite variations in signals at the
input of the amplifier or system. This is achieved by providing more amplification to
weak signals and less amplification to strong signals thus maintaining a constant signal
amplitude level at the output.

 Types of AGC:
1. Simple AGC:
 Simple AGC is a system which will change the overall gain of a receiver
automatically. This is done to keep the receiver output constant even when the
signal strength at input of the receiver is changing. In the AGC system a dc voltage
(AGC bias) is derived from the detector. This AGC bias is 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 receiver gain is automatically reduced as the input signal becomes more
and more strong.
The simple AGC is improvement over the no AGC situation. but its disadvantage is
that the reduction in the gain of the receiver will take place even for the weak
signals.
The simple AGC circuit is used in all low-cost domestic radio receivers.

2. Delayed AGC:
In delayed AGC, the AGC remains inoperative below a predetermined input carrier
voltage (point- B). If predetermined level, it is considered a weak signal. The received
signal strength is below the predetermined level, it is considered a weak signal. The AGC
bias voltage is applied to the RF and IF amplifiers only if the level or input carrier voltage
goes above this predetermined level. In other words, the AGC is delayed in applying the
bias voltage to the amplifiers the bias voltage to the amplifier for a certain predetermined
level.

Advantages of delayed AGC:

1. Gain is not reduced for weak signals.
2. Gain is reduced for strong signals.
3. The delayed AGC characteristics is close to the ideal AGC characteristics.

FM Transmitters and Receivers

 Generation of FM Waves:
The FM modulator circuits used for generating FM signals can be divided into two
categories such as:
(i) The direct method or parameter variation method
(ii) The Indirect method or the Armstrong method
The classification of FM generation methods is shown below:

(i) The direct method or parameter variation method:

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). An oscillator circuit
whose frequency is controlled by a modulating voltage is called voltage-controlled
oscillator (VCO). The frequency of VCO is varied according to the modulating signal
simply by putting a shunt voltage variable capacitor with its tuned circuit. This voltage
variable capacitor is called varactor or varicap.
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 as
shown in figure 1 below.

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. An
example of direct FM is shown in figure 1 which uses a Hartley oscillator along with a
varactor diode. The varactor diode is reverse biased. Its capacitance is dependent on the
reverse voltage applied across it. This capacitance is shown by the capacitor C(t) in figure
2.

Frequency of oscillations of the Hartley oscillator shown in figure 2 is given by:

where C(t) = C + Cvarector

This means that C(t) is the effective capacitance of the fixed tuned circuit capacitance C
and the varactor diode capacitance Cvarector.

Varactor Diode Modulator:

The varactor diode FM modulator has been shown below in figure

A varactor diode is a semiconductor diode whose junction capacitance varies linearly with
the applied bias and the varactor diode must be reverse biased.

Working Operation

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 dioide 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 signal.

Transistor Reactance Modulator:

  Figure shows a circuit of an LC oscillator that is frequency modulated by a
capacitive reactance (RC) transistor modulator.
 As shown, the audio frequency voltage is applied at the base of the transistor Q1.
 The amplitude variations of this driving voltage vary the forward bias to change the
transistor collector current.
 This changes β of the transistor which results in a proportionate change in the
equivalent capacitance across the oscillator tank circuit. Note the use of RF chokes
in the circuit.
 They are used to isolate various points of the circuit for ac current while providing a
dc path.

Effect of Mixing and Multiplication on FM Wave:

 Effect of Mixing: If sum component is selected

FM wave Mixer (fc + Δ f + f0)

f0

Local New center frequency is (fc + f0)

Oscillator
Deviation = Δ f, No change in mf

Mixing the FM wave with a local oscillator frequency will produce sum and difference
frequency component at the output of the mixer. We can select the sum or difference
frequency component. In fig., an FM wave (fc + Δ f) and local oscillator output f 0 are
applied to a mixer.

At the mixer output, we get four frequency components namely the two frequencies and
their sum & difference components. i.e., f0, (fc + Δ f), (fc +f0 + Δ f), (fc – f0 + Δ f).

As shown in fig., the frequency deviation remains unchanged but the center frequency fc
will change. It will increase to (fc + f0) if sum component is selected and it will reduce to (fc
– f0) if difference component is selected.

Due to constant deviation the modulation index remains unchanged in the process of
mixing.

Effect of Multiplication:

If FM wave with the center frequency fc and deviation Δ f are applied to a frequency
multiplier then the frequency and deviation both are multiplied equally.

Due to this modulation index (mf) of the FM wave will also get multiplied in the process of
multiplication.

xN FM wave with center frequency

Frequency
FM Wave Multiplier = N fc deviation = N Δf and
modulation index = N mf

Armstrong Method for the Generation of FM:

In this method, the FM is obtained through phase modulation. A crystal oscillator can be
used hence the frequency stability is very high.

The Armstrong method uses the phase modulator to generate a frequency modulated wave.

Working Principle

The working operation of this system can be divided into two parts as follows:

Part I: Generate a narrow band FM wave using a phase modulator.

Part II: Use the frequency multipliers and mixer to obtain the required values of frequency
deviation, carrier and modulation index.

Part I: Implementation of the Phase Modulator:

Figure.2 shows the block diagram of phase modulator circuit.

Working Principle

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.

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.

The angle Φ decreases with reduction in modulation index as shown in figure 3.

Thus, the resultant at the output of the combining network is phase modulated. Hence, the
block diagram of figure.1 operates as a phase modulator.

Part II: Use of Frequency Multipliers Mixer and Amplifier

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.

 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.

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.

The pre-emphasis circuit is basically a high pass filter. The pre-emphasis is carried out at
the transmitter.

 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.
 FM Receiver:

The block diagram of an FM receiver is illustrated in Figure (a). The RF amplifier

amplifies the received signal intercepted by the antenna. The amplified signal is then
applied to the mixer stage. The second input of the mixer comes from the local oscillator.
The two input frequencies of the mixer generate an IF signal of 10.7 MHz This signal is
then amplified by the IF amplifier. Figure (a) shows the block diagram of an FM receiver.

The output of the IF amplifier is applied to the limiter circuit. The limiter removes the
noise in the received signal and gives a constant amplitude signal. This circuit is required
when a phase discriminator is used to demodulate an FM signal.

The output of the limiter is now applied to the FM demodulator, which recovers the
modulating signal. However, this signal is still not the original modulating signal. Before
applying it to the audio amplifier stages, it is de-emphasized. De-emphasizing attenuates
the higher frequencies to bring them back to their original amplitudes as these are boosted
or emphasized before transmission. The output of the de-emphasized stage is the audio
signal, which is then applied to the audio stages and finally to the speaker.

It should be noted that a limiter circuit is required with the FM discriminators. If the
demodulator stage uses a ratio detector instead of the discriminator, then a limiter is not
required. This is because the ratio detector limits the amplitude of the received signal. In
Figure (a) a dotted block that covers the limiter and the discriminator is marked as the
ratio detector.

In FM receivers, generally, AGC is not required because the amplitude of the carrier is
kept constant by the limiter circuit. Therefore, the input to the audio stages controls
amplitudes and there are no erratic changes the volume level. However, AGC may be
provided using an AGC detector. This generates a dc voltage to control the gains of the RF
and IF amplifier.

RF Amplifier:

The RF amplifier in FM receivers uses FETs as the amplifying device. A bipolar junction
transistor can also be used for the purpose, but an FET has certain advantages over BJT.
These are explained below:

An FET follows the square law for its operation, the characteristics; curves of an FET
have non-linear regions. Due to the non-linearity, higher harmonics of the signal frequency
are generated in the output. The major advantage of an FET is that it generates only the
second harmonic components of the signal. This is known as the square law. Harmonics
higher than the second harmonic is nearly absent in the output of an FET amplifier. The
higher harmonics produce harmonic distortions and arc undesirable. In FETs, as only the
second harmonics are present; it is easy to filter these out by using the tuned circuits. BJTs
also generate higher harmonics, but they do not follow the square law. Therefore, they
provide more harmonic distortion than FETs. Thus, FETs are always preferred in the RF
amplifier of an FM receiver.

In BJT amplifiers, cross-modulation occurs if a strong signal of an adjacent channel gets

through the tuned circuits in the presence of a weak desired signal. The adjacent channel
will generate higher harmonics, which may come within the pass-band of the desired
signal. This will produce noise and distortions at the output. On the other hand, The effect
of cross-modulation is minimized in FET amplifiers, as the unwanted adjacent channel will
also produce only its second harmonic components, which may not fall into the pass-band
of the desired channel and thus are easily filtered out.
The input impedance of an FET becomes small due to the small input capacitive reactance
of FET at very high FM frequencies. This makes it easy to match the small impedance of
the antenna, typically 100 ohms, with the small input impedance of PET. This is not
possible with BJTs.

Limiter Circuit:

Limiter circuit is used in FM receiver to remove the noise present in the peaks of the
received signal and to remove any amplitude variation in the received signal; the output of
the limiter has constant amplitude. This is very in important in FM receivers because at
amplitude variation in the received carrier will result in unfaithful reproduction of the
audio signals. Figure (b) shows the typical circuit diagram of a limiter circuit used in an
FM receiver.

A typical circuit diagram of a limiter using FET is illustrated in figure (b). This circuit has
a leak-type bias at the gate, through R. and C, the source resistance is RS and the source
bypass capacitor is C, the capacitor CN provides the neutralization of the signal passing
through the internal capacitance between the gate and the drain. The limiting action is
provided by the gate and drain circuits.

FM Detectors:
The radio receiver recovers the modulated signal and recovers the message signal in its original
form by the process of demodulation.
The function of FM demodulator, is to change the frequency deviations of the carrier wave into
AF amplitude variations. The basic requirement of an FM detector is that the conversion should
be linear and insensitive to amplitude variations.

Types of FM Detectors:

1. Simple Slope Detector

2. Balanced Slope Detector
3. Phase Discriminator [ Foster Seeley Discriminator]
4. Ratio Detector

1. Simple Slope Detector:

Slope detector is a tuned circuit, whose resonant frequency is set at one side of the
centre frequency of FM signal, say fc + Δf. To get good linearity in the response of the
detector circuit, this frequency is chosen greater than the highest frequency deviation
in the incoming FM wave. Fig 1 shows circuit of slope detector in which FM signal is
applied to the tuned transformer (combination of T1, C1 and C2).
The output of the tuned circuit will have an amplitude that depends on the frequency of
the incoming signal. The resonant frequency is kept much higher than the largest
frequency deviation in the FM wave to ensure that the entire frequency range falls in the
linear part of the selectivity curve.
As the frequency of the signal varies up and down the central frequency, signal moves up
and down the selectivity curve (Fig 2). This causes the amplitude of the output voltage to
vary in line with the frequency variations. At this point, the signal is somewhat like an AM
wave.
The final stage in the process is to demodulate the amplitude modulated wave using a
simple diode detector circuit with an RC load of suitable time constant. This circuit is, in
fact, identical to that of an AM detector. The time constant of circuit C3 R1 must be slow
enough to keep the RF ripple as small as possible, but sufficiently fast for the detector
circuit to follow the fastest variations.
Disadvantages:
1. It is inefficient
2. It is linear only over a limited frequency range.
3. it is difficult to adjust as the primary and secondary windings of the transformer
must be tuned to slightly different frequencies.
2. Balanced Slope Detector:
The circuit diagram of the balanced slope detector is shown in Figure.

As shown in the circuit diagram, the balanced slope detector consists of two slope detector
circuits.

The input transformer has a center tapped secondary. Hence, the input voltages to the two slope
detectors are 180° out of phase. There are three tuned circuits. Out of them, the primary is tuned
to IF i.e., fc

The upper tuned circuit of the secondary (T 1) is tuned above fc by Δf i.e., its resonant frequency
is (fc+ Δf). The lower tuned circuit of the secondary is tuned below fc by Δf i.e., at (fc – Δf).
R1C1 and R2C2 are the filters used to bypass the RF ripple. Vo1 and Vo2 are the output voltages of
the two slope detectors.

The final output voltage Vo is obtained by taking the subtraction of the individual output
voltages, Vo1 and Vo2, i.e.

Working Operation of the Circuit

The circuit operation can be explained by dividing the input frequency into three ranges as
follows:

(i) fin = fc: When the input frequency is instantaneously equal to fc, the induced voltage in
the T1 winding of secondary is exactly equal to that induced in the winding T 2.
Thus, the input voltages to both the diodes D1 and D2 will be the same.
Therefore, their dc output voltages Vo1 and Vo2 will also be identical but they have opposite
polarities. Hence, the net output voltage Vo = 0.
(ii) fc < fin < (fc + Δf): In this range of input frequency, the induced voltage in the winding T 1 is
higher than that induced in T2.Therefore, the input to D1 is higher than D2. Hence, the positive
output Vo1 of D1 is higher than the negative output Vo2 of D2. Therefore, the output voltage Vo is
positive. As the input frequency increases towards (fc + Δf), the positive output voltage increases
as shown in 3.

If the output frequency goes outside the range of (fc – Δf) to (fc + Δf), the output voltage will
fall due to the reduction in tuned circuit response.
Advantages:
(i) This circuit is more efficient than simple slope detector.
(ii) It has better linearity than the simple slope detector.
Disadvantages:
(i) Even though linearity is good, it is not good enough.
(ii) This circuit is difficult to tune since the three tuned circuits are to be tuned at different
frequencies i.e., fc, (fc+Δf) and (fc – Δf).
(iii) Amplitude limiting is not provided.

3. Phase discriminator (Foster seelay discriminator):

 Fig.1 Fooster seeley discriminator
 It uses a double-tuned RF transformer to convert frequency variations in the received fm
signal to amplitude variations.
 These amplitude variations are then rectified and filtered to provide a dc output voltage.
This voltage varies in both amplitude and polarity as the input signal varies in frequency.
 Fig.1. shows a typical Foster-Seeley discriminator. The primary tank circuit consists of
C1 and L1. C2 and L2 form the secondary tank circuit. Both tank circuits are tuned to the
center frequency of the incoming fm signal.
 Choke L3 is the dc return path for diode rectifiers D1 and D2. Resistors R3 and R4 are
the load resistors and are bypassed by C3 and C4 to remove rf.
 To obtain the different phased signals a connection is made to the primary side of the
transformer using a capacitor, and this is taken to the centre tap of the transformer. This
gives a signal that is 90° out of phase.
 When an un-modulated carrier is applied at the centre frequency, both diodes conduct, to
produce equal and opposite voltages across their respective load resistors. These voltages
cancel each one another out at the output so that no voltage is present.
 As the carrier moves off to one side of the centre frequency the balance condition is
destroyed, and one diode conducts more than the other. This results in the voltage across
one of the resistors being larger than the other, and a resulting voltage at the output
corresponding to the modulation on the incoming signal.
 The choke is required in the circuit to ensure that no RF signals appear at the output. The
capacitors C1 and C2 provide a similar filtering function.
  The operation of the Foster-Seeley discriminator can best be explained using vector
diagrams fig. 2 that show phase relationships between the voltages and currents in the
circuit. Let's look at the phase relationships when the input frequency is equal to the
center frequency of the resonant tank circuit.

Fig.2 Phasor diagram of fooster seeley discriminator

 A typical discriminator response curve is shown in fig.3.

Fig.3. Typical discriminator response

 The output voltage is 0 when the input frequency is equal to the carrier frequency (FR).
 When the input frequency rises above the center frequency, the output increases in the
positive direction. When the input frequency drops below the center frequency, the
output increases in the negative direction.
 The output of the Foster-Seeley discriminator is affected not only by the input frequency,
but also to a certain extent by the input amplitude. Therefore, using limiter stages before
the detector is necessary.
Advantages of Foster-Seeley FM discriminator:
  Offers good level of performance and reasonable linearity.
 Simple to construct using discrete components.
 Provides higher output than the ratio detector
 Provides a more linear output, i.e. lower distortion than the ratio detector

Disadvantages of Foster-Seeley FM discriminator:

 Does not easily lend itself to being incorporated within an integrated circuit.
 High cost of transformer.
 Narrower bandwidth than the ratio detector

4. Ratio detector:

 In the Foster-Seeley discriminator, the amplitude of the resulting output voltage

varies as per varying magnitude of the input signal. This discriminator circuit is
modified is to provide amplitude limiting of the incoming signal and is termed as
Ratio detector.
 The Ratio detector circuit is given below in Fig1.

Fig1. Ratio detector circuit

 The Ratio detector circuit is almost same as Foster-Seeley discriminator, except for the
below modifications:

1. Diode D2 is reversed in direction.

2. A large capacitor C9 is connected across the output voltage of the two diodes.
3. The output voltage of the detector is taken across P and Q.

 The primary and secondary circuits are tuned to carrier frequency and C3 is the coupling
capacitor while L3 is the RFC element.
 The two generated frequency dependent output voltages are applied to the two
diodes D1 and D2. The capacitor C9 has large value, typically 10μF which charges to the
peak value of voltage across L2 and due to the large time constant, it holds this voltage.
 The effect of any amplitude variations due to noise and other interference is minimal on
the charge of capacitor C9 and the voltage remains constant.
 Additional limiter circuit is not required since the output voltage is not affected by
amplitude variations in the incoming signals.

 Conversely, when D1 conducts more than D2, V01 exceeds V02 again resulting in the
sum of these voltages as constant. Thus the output voltage Vout is negative.
 Therefore in this circuit the sum of the voltages V01 and V02 always remains constant,
but their ratio changes depending on the signal frequency. Hence the circuit is called ratio
detector.