Angle Modulation . Introduction. There are three parameters of a carrier that may carry information: Amplitude Frequency Phase Frequency and Phase modulation are closely related and grouped together as Angle modulation. Frequency Modulation.
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In this the instantaneous frequency of the carrier is caused to vary by an amount proportional to the amplitude of the modulating signal. The amplitude is kept constant.
e(t) = E sin (t +)
kf= frequency deviation/V = kf kHz/V
Maximum freq deviation
e(t) = E sin (t +)
Maximum freq deviation
Where modulation index is
% Modulation = Actual freq deviation/ allowed freq deviation
An FM broadcast-band transmitter has a peak deviation of ±60 kHz for a particular input signal. Determine the percentage of modulation.
-Frequency Deviation is maximum departure of instantaneous freq. of FM wave from career frequency
Maximum Freq of FM is fmax= fc+
is independent of modulating freq. and proportional to only amplitude of information
Modulation index is proportional to deviation and inversely proportional to modulating freq.
This decides the BW of the FM wave also decides the no of side bands In FM the modulation index can be greater than 1
DR =Maximum deviation /maximum modulating freq
in FM is 75KHz
In an FM system when the audio frequency is 300 Hz and the audio voltage is 2.0V, the deviation is 5kHz. If the audio voltage is now increased to 6V what is the new deviation? If the voltage is now increased to 9V and the frequency dropped to 100Hz what is the deviation? Find the modulation index in each case.
Find the carrier and modulating frequencies, the modulating index, and the max. deviation of an FM wave below. What power will the wave dissipate in a 10 ohm resistor?
Compare this with:
Modulating index =5 as given.
Frequency spectrum of FM Wave –
Sin of sin function is solved by Bessel function
This is an approximate method used to predict the required bandwidth necessary for FM transmission
About 98% of the total power is included in the approximation.
What bandwidth is required to transmit an FM signal with intelligence at 12KHz and max deviation 24 kHz
Consult Bessel function table to note that for modulating index of 2, components which exist are J1,J2,J3,J4 apart from J0.
This means that apart from the carrier you get J1 at +/-10kHz, J2 at +/- 20kHz, J3 at +/- 30kHz and J4 at +/- 40 kHz.
Total bandwidth is therefore 2x40=80kHz.
For an FM signal given by
If this signal is input into a 30 ohm antenna, find
AM Vs FM systems
In both systems a carrier wave is modulated by an audio signal to produce a carrier and sidebands. The technique can be applied to various communication systems eg telephony and telegraphy
Special techniques applied to AM can also be applied to FM
Both systems use receivers based on the superheterodyne principle
In an AM transmitter, provision must be made for varying the carrier amplitude whilst for FM the carrier frequency is varied.
AM and FM modulators are therefore essentially different in design. FM can be produced by direct frequency modulation or by indirectly phase modulation.
The FM carrier must be high usually in the VHF band as it requires large bandwidth which is not available in the lower bands.
The FM and AM receivers are basically the same, however the FM receiver uses a limiter and a discriminator to remove AM variations and to convert frequency changes to amplitude variations respectively. As a result they (FM) have higher gain than AM.
FM receivers give high fidelity reproduction due to their large audio bandwidth up to 15 kHz compared with about 8 kHz for AM receivers.
Another term common to FM is the modulation index, as determined by the formula:
In phase modulation, the phase shift is proportional to the instantaneous amplitude of the modulating signal, according to the formula:
Frequency is the derivative of phase, or, in other words, frequency is the rate of change of phase
The modulation index is proportional to frequency deviation and inversely proportional to modulating frequency
Insert fig. 4.4
This solution may be shown to be given by
To evaluate the individual values of J is quite tedious and so tables are used.