[questio]what relations between data rate and spectrum width

what relations between data rate and spectrum width?
e.g:120m/s data rate=120mhz spectrum width?
ths!

The spectrum depend mainly on the modulation used and the bit rate
for example
for ook or FSK or ASK a 1 M bit per second take BW = 0.5 MHZ

In general the Bandwidth is almost equal with Symbol Rate.
Some digital modulations have more than 1 bit per symbol (e.g. QPSK = 2 bit/symbol, 8PSK = 3bit/symbol, etc) but this will not affect the bandwidth, even if the Bit Rate in those cases is 2 or 3 times higher.

I think there is a dual understanding in the original question.

The answer of ramy gad was simple , but accurated.

The relation betwen bandwidth and bit rate depends on the modulation schemes. This is one of the main reason people spend a lot of time and money researching for modulations techniques that more and more results in improvemts of spectral usages and higher throughputs.

The concept of simbol rate or Baud Rates raised by vfone is crucial in digital communication but must be dealt with care. It usually causes confusion: if more bits can be send with each symbol it means that the spectrum needed is narrower to transmit the same information( bit rate), so it improves bandwidth efficiency . I see data here related to amount of information, so regarding to bit rate.

But what I did not understand was the unit used to data rate m/s ?

The best way to get the most accurate result is to transform the time-domain signal into frequency domain using discrete fourier transform(DFT) and take a look at its power spectrum.

If a good guess is needed, why don't we start from a simple case, the 1010 pattern? The 1010 pattern with 50% duty cycle is a typical pattern contains the "hightest frequency components" as compared to signal patterns such as PRBS7/23/31. The bandwidth of the ideal 1010 pattern is infinite since it has a rise/fall time of 0. In the real life, due to different kinds of losses and due to the fact that higher frequency components contained in the 1010 pattern subjects to higher degree losses, at certain harmonics, the higher frequency components contained in the 1010 pattern will become very small and can be neglect, where, that frequency point can be regarded as the bandwidth of the signal.

In order to relate time-domain signal to its frequency-domain counterparts, a lot of terms are used and most of which are very confusing in the definition. You must be careful when interpret their meanings. e.g.

The fundamental frequency of a 1010 pattern with a bit rate of X Gbps is 0.5X GHz due to the fact that it repeats itself 0.5XG times in one second. However, after performe the DFT to the signal and look at its spectrum, you will find that the bandwidth of the signal is not directly related to this "fundamental frequency" but relate to the fastest signal edges: rise/fall time. As a rule of thumb, the signal bandwidth(BW) relates to its Tr/Tf by BW=0.35/Tr. Someone prefer BW=0.5/Tr. Actually both of them has some assumptions behind regarding the rise/fall time(whether 80% or 90%) and the power losses that tolerated (70%-->3dB or 90%). Again, the most accurate results requires the DFT and the examination of the harmonics of its spectrum.

Another rule of thumb assuming a 7% rise/fall time of the signal period of a 1010 pattern is: BW=5th harmonic=2.5X GHz (for BR=XGbps) for clock signal, if the signal propagate down a long interconnect, the BW is roughtly 0.5X GHz (for BR=X Gbps) due to the fact that most of the high frequency components subjected to very large losses, thus is negligible.

For all the theory behind the above-comments, please refer to the following two SI books for details:

1. Signal Integrity - Simplified
https://www.edaboard.com/viewtopic.p...+circuit+board

2. High-Speed Digital Design: A Handbook of Black Magic
Author: Howard Johnson Graham Martin Graham
ISBN: 0133957241
https://www.edaboard.com/viewtopic.p...ht=black+magic