Any additional bandwidth as required to carry information in the modulating signal on one side of the carrier will usually have a different phase change of phase with respect to time from any reference point from the carrier. To neutralize this phase difference, AM modulation has to add some additional matching bandwidth on the opposite side of the carrier to carry a signal that will exactly cancel any phase shift of the spectrum the first side, so that AM doesn't become PM.
With FM, modulating a carrier changes the signal frequency to new frequencies. You can also call those additional new frequencies so generated "side bands". I interpret the question as follows: If we modulate a carrier with a pure tone using AM, we get a single set of sidebands, but if we modulate with phase modulation, we get an infinite number of sidebands, spaced at the modulation frequency. It is easy to see why amplitude modulation at a single frequency gives exactly two sidebands. Simply multiply out the expression for AM:.
Now, phase modulation. I refer you to this animation generated by this matlab script of the phasor diagram:. As seen in the animation, the higher order sidebands are necessary to keep the amplitude of the resultant phasor in red constant and thus produce pure phase modulation.
You can see how each pair of higher order sidebands is needed to correct the deviation from a circular arc introduced by the lower order sidebands.
Mixing audio with a carrier is exactly the same as mixing an incoming signal with a local oscillator to get an intermediate frequency. In both cases you result with the original frequencies, the sum of the frequencies, and the difference between the two frequencies.
Whenever you mix frequencies, this results. When two people sing together, harmonics result. If the difference between their notes is in the audible range, you will hear it. I have heard quartets sing, and a deep bass note emerge that wasn't sung or played. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
For the short wave bands interference levels are often high - some broadcast stations have experimented and used single sideband with full carrier.
This effectively reduces halves the bandwidth of the overall signal for a given audio bandwidth. For this to provide gains within a channelized band plan, all stations need to adopt the same plan. Aeronautical communications often use amplitude modulation. Channel bandwidths of 25kHz and 8. Audio can be tailored to suit the channel spacing as a typical communications audio bandwidth of Hz to around 3 kHz can be adopted.
The bandwidth of amplitude modulation can be seen to be twice that of the highest audio signal to be carried. This makes in relatively poor in terms of spectral efficiency, but nevertheless AM is still used for some applications in view of its simplicity, especially in terms of demodulation.
Amplitude modulation sidebands When a carrier is modulated in any way, further signals are created either side of the steady carrier. Spectrum arising from carrier modulated by 1 kHz tone If the steady state tones are replaced with audio like that encountered with speech of music, these comprise many different frequencies and an audio spectrum with frequencies over a band of frequencies is seen.
Amplitude Modulation Spectrum Amplitude modulation, AM bandwidth It can be seen that if the top frequency that is modulated onto the carrier is 6 kHz, then the top spectra will extend to 6 kHz above and below the signal. Typical AM channel and signal bandwidths Some of the main uses of amplitude modulation these days are for broadcasting and for aeronautical communications.
Shopping on Electronics Notes Electronics Notes offers a host of products are very good prices from our shopping pages in association with Amazon. Note: Electronics Notes receives a small commission on sales at no cost to you. Community Bot 1. Yes, exactly : And you can just send the carrier wave, but then by definition it wouldn't change a carrier wave has a constant amplitude so no data would be transferred.
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