distinguish between frequency modulation and amplitude modulation.?

Question

this question is from physics class 12 cbse

Answer 1

Here is a really good explanation of how it works-:

http://en.wikipedia.org/wiki/Amplitude_modulation

http://en.wikipedia.org/wiki/Frequency_modulation

Probably more much information than you need.

Amplitude Modulation keeps the frequency the same and changes the amplitude, Frequency modulation changes the carrier frequency but keeps the same amplitude - thats about as simple as I can make it.

Answer 2

Amplitude Modulation is the technique used to send out a "message" signal over a "carrier" signal by "embedding" the "message" signal in the carrier's amplitude.

What this means is that suppose that you have a Message signal say M(t)= A cos(2*PI*F *t), generally message signals are in the low frequency region that's why a carrier(of much higher frequency) is needed to send out this message signal. This is done to avoid the use of very large size attennas to recieve the message because size of attenna needed for a W wavelength signal is about W/4. When freq is low W is high.

Coming back to AM......
Now consider that you have a very High Frequency signal
S(t) = C cos(2*PI*f * t), we can modulate this signal's amplitude that is we can embed the message signal in the carrier's amplitude like this....
S'(t) = ( C + A cos(2*PI*F*t) ) cos (2 * PI *f * t).
This is a general AM wave equation......

Now you may ask how to do this....well its very easy.... just use a transistor suppose n-p-n type and connect the base to the message signal + some other sinosuidal voltage.....do the rest of connections like....emitter to ground....and collector to some DC voltage.....and you will get an AM output!!! try on some simulator if you have 1 put C= 2*A for a good view.

Just like we did AM by modulating the Amplitude...if you put the message signal in the frequency term of the carrier signal like....

S'(t)= C cos( 2 * PI * ( f + A cos(2 * PI * F * t) ) * t) this is a FM....same used in radio transmissions . This is little harder to explain but i guess you don't need that so that's the general idea...

Differences......

1.More bandwith is required in FM than in AM about 10 times the frequency of message signal in FM is required!!!

2.AM is prone to errors due to propagation. That's because the message is embedded in the amplitude and due to heat currents and a lot of other frequencies(near about the carrier not the message signal freq!!), the amplitude information can be distorted very easily.

3.Very sharp detectors are needed for AM for achieving good resolution of recieved signal.

Answer 3

Both modulations involve the superimposition of an audio signal over a radio frequency signal to ensure that it travels a longer distance.
If u understand whats amplitude , and what is frequency, and how it is shown on a sine curve, it will be easier to understand.
The max deivation from the null point, is the max amplitude, and the distance between any two similar points on the wave curve over a complete cycle is the wavelength. The speed of light divided by the wavelength, gives the frequency.
1) Amplitude Modulation:-
This is a much older method of modulation, in which the amplitude of the radio frequency is made to vary with the amplitude of the audio frequency, so basically what you have is the amplitude is of the audio signal, but the frequency is of the radio signal. The bandwidth required to properly transmit and receive an AM wave, is exactly twice of the spectrum of audio frequencies being transmitted.
Here the modulation index is defined as the ratio of the amplitude of the audio signal, over the amplitude of the radio signal, ie, Va/ Vr
this gives the ratio of modulation index, which shows that the radio signal completely disappears, for a value of modulation index = 1, hence the AM wave cannot be modulated by more than 100%.
Advantages:- since AM waves operate in the lower frequency spectrum, ie 540-1620 khz, they have a much longer range as compared to FM.
2) AM transmitters are more complex, but the receivers are relatively simple to construct, hence to make it more popular, and easily available to everyone.
3) AM is easier to generate than fm.

2) Frequency Modulation - This is more difficult to understand, as in this, the frequency of the radio signal is made proportional to the amplitude of the audio signal, and the frequency of the audio signal determines, the rate of change of the frequency of the radio signal.
So a high frequency in a particular segment of the signal will indicate, the amplitude of the particular signal is high, and a high rate of change of the frequency in a segment, will indicate which frequency of audio signal is being used.
It is more complex to understand, but can be summed up in that higher the deviation of the frequency, greater is the modulation index, and hence more info can be transmitted, since FM allows for a modulation index of greater than 1.
This also makes it less prone to noise, unlike AM

Adv:
1) more info can be transmitted, more voice clarity can be achieved
2) relatively immune to noise and interference as compared to AM
3) Because FM came after AM, it has been implemented with better planning and proper channel spacing etc.

Dis adv-
1) FM transmitters are simple, but receivers have to be more complex.
2) They are limited to LOS or line of sight operations, hence more antennas needed to cover a certain area, than AM.

Answer 4

The Modulation is required to carry the informtion - could be an audio signal for example.. In case of Amplitude modulation, the AMPLITUDE of the Carrier wave is changes according to the modulating signal - (say audio frquency ). Whereas in case of Frequency modulation, the FREQUENCY of the carrier wave is changed according to the modulating signal.

AM is easy to build, demodulation is simple but is prone to noise pick up from lightening, other electrical appliances. FM is better in clarity and stubborn against noise but little more complex to build. FM occupies more bandwidth than AM.

Answer 5

A radio transmitter must send a signal that changes between silence and varying sounds that a radio receiver can detect and convert back into the original audio signal (voice into microphone or music, etc.). Because the transmitter sends sinusoidal radio waves, the only aspects that can be easily altered are the height of the waves (amplitude) or the wave length (frequency) of the waves. Such modulation is labeled AM and FM (not to be confused with AM and PM).

Answer 6

in frequency modulation the width of the carrier wave-sine wave is equal to frequency of the modulated wave.

in amplitude modulation, the amplitude of the carrier wave is equal to signal or modulated wave.

Answer 7

FM is far more efficient as compared to AM transmission. Also FM is almost immune to noise as compared to AM reception. FM broadcast operate in very high frequency and ultra high frequency ranges-where noise is much less than in Amplitude modulation transmission.

Answer 8

am-amplitude of carrier wave changes in accordance withamplitude of modulating signal
fm-frequency of carrier wave changes in accordance with amplitude of modulating signal