# DSB signal

#### nikhilthunderlion

Joined Oct 1, 2007
23
in an DSB or AM signal the frequency is kept constant ,only the amplitude(of the carrier) is varied in accordance with the modulating signal, than how can there be frequency (fc+fm) and (fc-fm)which is the frequency of the sidebanbs (if this is true ,it means that in the wave form of the AM signal the frequency will be varying) .According to the me the frequency should be only fc (If the frequency is changing than it is some type of FM wave.).so tell me the how the frequency of the side band have come.

#### bloguetronica

Joined Apr 27, 2007
1,541
In a real signal, the frequency may vary due to some error margin when the carrier wave is generated before modulation, in the transmitter device. So, even in a AM signal, it is expected the signal frequency to vary, but any variations will be ignored, as the band-pass filter in the receiver allows reception of carrier waves with frequencies close to the tuned one.

#### beenthere

Joined Apr 20, 2004
15,819
The rate of change of the modulating signal does impose a frequency shift in the carrier. An FFT will demonstrate the effect. The amount of shift determines the signal bandwith, and is tightly regulated in the case of commercial AM radio transmissions.

#### Papabravo

Joined Feb 24, 2006
21,253
The sum and difference components come about because multiplication, a non-linear operation, produces four output frequencies from two input frequencies. Do the math.