18 Questions
What happens to modulation when the signal loudness is doubled?
Modulation doubles
In frequency modulation, sidebands are formed on which sides of the centre frequency?
On both sides of the centre frequency
What is the modulation index of an FM carrier with a carrier swing of 100 m kHz and a modulating signal of 5 kHz?
25
Calculate the percent modulation for an FM transmission with a frequency deviation of 18.75 kHz in the 88-108 MHz band.
75%
If an FM signal has a resting frequency of 105 MHz and a highest frequency of 105.03 MHz when modulated by a 5 kHz signal, what is the carrier swing?
30 kHz
How are the sidebands spaced from the center frequency in FM?
At equal distances from the center frequency
In frequency modulation, the information signal changes the _____ of the carrier wave.
frequency
The process of amplitude modulation involves varying the _____ of the carrier wave in proportion to the instantaneous amplitude of the information signal.
amplitude
In phase modulation, the information signal changes the _____ of the carrier wave.
phase
The modulation index is a measure of the _____ in a modulated signal.
frequency variation
In amplitude modulation, the envelope of the modulated carrier wave is an exact replica of the _____ signal wave.
information
Which of the following modulation techniques is used in standard broadcast band (550-1550 kHz)?
Amplitude Modulation (AM)
What happens if modulation exceeds 100% in a transmitter?
It produces interference known as splatter.
In amplitude modulation, if the maximum carrier wave amplitude is 750 mV and the minimum is 250 mV, what is the percentage modulation?
66.67%
What is heterodyning action in frequency modulation?
The generation of side frequencies when a carrier wave is mixed with a modulating signal.
What are the two additional frequencies produced in an AM wave due to heterodyning action?
Sum and difference components.
Where does the modulating frequency appear in an amplifier output in frequency modulation?
It remains unchanged but does not appear in the output.
What do USF and LSF represent in frequency modulation?
Upper sideband frequency and lower sideband frequency.
Study Notes
Modulation and Demodulation
- In FM, when a carrier is modulated, a number of sidebands are formed, with infinite theoretical number, but their strength becomes negligible after a few sidebands.
- Sidebands in FM lie on both sides of the centre frequency spaced fm apart, with equal amplitudes at equal distances from f0.
Types of Modulation
- There are three types of sine-wave modulations: Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (PM).
- In AM, the information or AF signal changes the amplitude of the carrier wave without changing its frequency or phase.
- In FM, the information signal changes the frequency of the carrier wave without changing its amplitude or phase.
- In PM, the information signal changes the phase of the carrier wave without changing its other two parameters.
Amplitude Modulation (AM)
- In AM, the amplitude of the carrier wave is varied in proportion to the instantaneous amplitude of the information signal or AF signal.
- The process of amplitude modulation is shown graphically in Fig. 66.5, with the AF signal, carrier wave, and resultant modulated wave.
- All stations broadcasting on the standard broadcast band (550-1550 kHz) use AM modulation.
- Modulation in excess of 100 per cent produces severe distortion and interference (called splatter) in the transmitter output.
Frequency Modulation (FM)
- The modulation index of an FM carrier can be calculated using the formula: CS = 2 × f × f_m
- The frequency deviation of an FM transmission can be calculated using the formula: f_d = (f_max - f_c) / 2
- The percent modulation of an FM transmission can be calculated using the formula: m = (f_d / f_max) × 100
Examples and Calculations
- Example 66.12: Modulation index of an FM carrier having a carrier swing of 100 m kHz and a modulating signal of 5 kHz.
- Example 66.13: Calculate percent modulation if it is broadcast (i) in the 88-108 MHz band (ii) as a portion of a TV broadcast.
- Example 66.14: An FM signal has a resting frequency of 105 MHz and highest frequency of 105.03 MHz when modulated by a signal of frequency 5 kHz. Determine (i) frequency deviation, (ii) carrier swing, (iii) modulation index, (iv) percent modulation, (v) lowest frequency reached by the FM wave.
Calculate the percentage modulation of a carrier wave with given maximum and minimum amplitudes. Understand how modulation affects the transmitter output and learn to avoid distortion and interference (splatter).
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
