Armstrong Method for the Generation of
FM
• In the direct methods of generation of FM, LC
oscillators are to be used. The crystal oscillator
cannot be used.
• The LC oscillators are not stable enough for the
communication or broadcast purpose. Thus, the
direct methods cannot be used for the broadcast
applications.
• 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.
• Figure.1 shows the block diagram of the
Armstrong method.
Fig.1 : Armstrong Method for FM Generation
• 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,
Part I: Generate a narrow band FM using
Phase Modulator
• As discussed earlier, we can generate FM using a
phase modulator.
• The modulating signal x(t) is passed through an
integrator before applying it to the phase
modulator as shown in figure 1.
• Let the narrow band FM wave produced at the
output of the phase modulator be represented by
where Vc1 is the amplitude and f1 is the frequency of
the carrier produced by the crystal oscillator.
• The phase angle Φ1(t) of s1(t) is related to x(t) as
follows:
where k1 represents the frequency sensitivity of the
modulator.
• Hence, the approximate expression for s1(t) can
be obtained as follows:
• After approximation, we get,
• Substituting,
• This expression represents a narrow band FM. Thus,
at the output of the phase modulator, we obtain a
narrow band FM wave.
Implementation of the Phase Modulator
• Figure.2 shows the block diagram of phase
modulator circuit.
E
Fig.2 : 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
• 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
Fig.3: Phasors explaining the generation of PM
• 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
• The angle Φ decreases with reduction in
modulation index as shown in figure 4.
Fig.4 : Effect of modulation index on frequency
f
• Thus, the resultant at the output of the combining
network is phase modulated. Hence, the block
diagram of figure.2 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.
• So both are increased to an adequately high value
with the help of frequency multipliers and mixer.
Example
Exercise
Pre-emphasis and De-
emphasis
• Pre and de-emphasis circuits are used only in frequency modulation.
• Pre-emphasis is used at transmitter and de-emphasis at receiver.
1. 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
(β), for higher modulating frequencies.
Pre-emphasis and De-emphasis
Continue
• This can be done by increasing the deviation ‘ f’ and ‘ f’ can be increased by
increasing the amplitude of modulating signal at higher frequencies.
Definition:
The artificial boosting of higher audio modulating frequencies in accordance with
prearranged response curve is called pre-emphasis.
Pre-emphasis circuit is a high pass filter as shown in Fig. 1
Fig. 1: Pre-emphasis Circuit
• As shown in Fig. 1, AF is passed through a high-
pass filter, before applying to FM modulator.
• As modulating frequency (fm) increases, capacitive
reactance decreases and modulating voltage goes
on increasing.
• fm Voltage of modulating signal applied to FM
Fig. 2: P re-emphasis Curve
CCIR standards. Centre for Counseling Innovation and
Research
• In systems employing American FM and TV standards,
networks having time constant of 75 sec are used.
The pre-emphasis is used at FM transmitter as shown
in Fig. 3.
Fig. 3: FM Transmitter with Pre-emphasis
De-emphasis
• De-emphasis circuit is used at FM receiver.
Definition:
The artificial boosting of higher modulating frequencies in
the process of pre-emphasis is nullified at receiver by
process called de-emphasis.
• De-emphasis circuit is a low pass filter shown in Fig. 4.
Fig. 4: De-emphasis Circuit
Fig. 5: De-emphasis Curve
• As shown in Fig.5, de-modulated FM is applied to
the de-emphasis circuit (low pass filter) where with
increase in fm, capacitive reactance Xc decreases.
• Fig. 5 shows the de-emphasis curve corresponding to a
time constant 50 s. A 50 s de-emphasis corresponds to
a frequency response curve that is 3 dB down at frequency
given by,
f = 1/ 2πRC
= 1/ 2π x 50x 1000
= 3180 Hz
• The de-emphasis circuit is used after the FM
demodulator at the FM receiver shown in Fig. 6.
Fig. 6: De-emphasis Circuit in FM Receiver
Comparison between Pre-emphasis and De-
Parameter Pre-emphasis De-emphasis
emphasis
1. Circuit used High pass filter. Low pass filter.
2. Circuit diagram
3. Response curve
4. Time constant
T = RC = 50 s T = RC = 50 s
5. Definition Boosting of higher frequencies Removal of higher frequencies
6. Used at FM transmitter FM receiver.
