Q1) why modulation? Compare between FM & PM?
Modulation is an important step of communication system. Modulation is defined as the process whereby some
characteristic (line amplitude, frequency, phase of a high frequency signal wave (carrier wave) is varied in accordance
with instantaneous value intensity of low frequency signal wave (modulating wave.)
1. Reduction in the height of antenna
2. Avoids mixing of signals
3. Increases the range of communication
4. Multiplexing is possible
5. Improves quality of reception
Frequency Modulation (FM):
is the modulation technique in which carrier frequency varies based on analog baseband information signal to be
transmitted using wireless device. Frequency modulation is considered to be superior compare to the Amplitude
modulation due to better noise immunity and its ability to reject the interfering signals due to the capture effect.
• Advantages: Increased immunity to noise.
• Disadvantage: Requires larger bandwidth.
• Application: Radio Broadcasting, Direct Satellite Broadcasting
Phase modulation (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.
• Advantages: Increased immunity to noise.
• Disadvantages: More complex hardware at receiver.
• Applications: Used in data communication systems.
Q2) discuss the problem
A quadrature demodulator for FM signals consists out of two parts; A frequency dependent phase shift network and a
phase detector. This article is going to take a closer look at the phase shifting network itself.
Two signals are considered to be in phase quadrature when their phase difference is exactly 90°.
A quadrature detector splits the signal, that is to be demodulated, up into two signals. The first signal remains unchanged
and is applied to the input of a phase detector. The second signals is passed through a capacitor. The capacitor will create
a 90° phase shift. The phase shifted signal is then applied to a frequency dependent phase shifter, a LC tank circuit. If the
LC tank circuits center frequency equals the signals frequency, it imposes a further phase shift of 0° on the signal.
Therefore the overall phase difference will remain 90°.
If the signals frequency does vary from the LC circuits center frequency, the LC tank will further shift the phase of the
signal from the capacitor, so that the signal’s total phase shift will be the sum of the 90° shift, that’s imposed by the
capacitor, and the additional positive or negative phase change that is imposed by the LC tank circuit. If the frequency
increases, the phase shift of the LC circuit decreases and vice versa. Therefore, the overall phase difference decreases as
the frequency increases and vice versa.
If the the original signal and the phase shifted signal are applied to the input of a phase detector, the modulation signal
(i.e. audio or data) can be recovered easily.
Q3) what’s the modulation index? Discuss what’s the effect of changing it’s value?
FM modulation index:
In terms of a definition: the FM modulation index is equal to the ratio of the frequency deviation to the modulating
frequency.
FM deviation ratio:
The modulation index will vary according to the frequency that is modulating the transmitted carrier and the amount of
deviation. However when designing a system it is important to know the maximum permissible values. This is given by
the deviation ratio and is obtained by inserting the maximum values into the formula for the modulation index.
Q4) there are two methods for the generation
1) Direct method
2) Indirect method
Q5) the super heterodyne:
Q6)
In order to be able to receive FM a receiver must be sensitive to the frequency variations of the incoming signals. As
already mentioned these may be wide or narrow band. However, the set is made insensitive to the amplitude variations.
This is achieved by having a high gain IF amplifier. Here the signals are amplified to such a degree that the amplifier runs
into limiting. In this way any amplitude variations are removed.
In order to be able to convert the frequency variations into voltage variations, the demodulator must be frequency
dependent. The ideal response is a perfectly linear voltage to frequency characteristic. Here it can be seen that the center
frequency is in the middle of the response curve and this is where the un-modulated carrier would be located when the
receiver is correctly tuned into the signal. In other word there would be no offset DC voltage present.
The ideal response is not achievable because all systems have a finite bandwidth and as a result a response curve known
as an "S" curve is obtained. Outside the bandwidth of the system, the response falls, as would be expected. It can be
seen that the frequency variations of the signal are converted into voltage variations which can be amplified by an audio
amplifier before being passed into headphones, a loudspeaker, or passed into other electronic circuitry for the
appropriate processing.
