Bandwidth of Fm signal

[madd]

Hello

i wnat to calculate Bandwidth of a FM/Any signal how can i do this. i have acquired the FM radio band(88-108Mhz) now i have many peaks of signal at different freq (e.g 91,96,99 e.t.c). but i dont Know the bandwidth of any signal. i want to calculate Bandwith of every signal automatically.not theoratically but how do labview going to impliment it.

Regards

Madd

[Brinoceros]

Hi madd,

The bandwidth of a signal is the frequency range where a signal is above some cutoff level.  So what you would need to do is just see over what frequency range each your peaks is above a certain level.  There are a number of ways you can do this depending on what form your frequency spectrum data is currently in.  You can just sift through your data and find crossover points where the magnitude of your signal crosses some threshold.

Hope this helps!

[madd]

Hello

Tnx for the reply. but my spectrum is sweeping on time axis what i want is while sweeping i can calculate the band width of the signal depending on the peak  of the signal.

Regards

Madd

[johnsold]

Madd,

As you may know the spectrum of an FM signal can become rather complicated. If you have a sinusoidal carrier modulated by a sinusoidal signal, the resulting signal can be described by a Bessel function (although I do not recall the exact form).

Why can you not use the threshold technique described by Brain? Apply it to a segment of the data as the data is acquired.

Your sweep time and sampling rates must be such that the modulating signals have time to cover the spectrum and enough samples acquired to give meaningful results.

Lynn

[Yardov]

Hi Madd,

You can very well estimate the bandwidth of an FM signal using Carson's rule:

Pass Band BW = 2 * (Fd + Fmax)

Where Fd is the frequency deviation and Fmax is the maximum frequency content of the signal.
For example, for a sine wave of frequency 5k Hz and modulated with a frequency deviation of 10k Hz will have an approximate bandwidth of 30k Hz. This approximation is widely used in the industry so feel free to use it to calculate your bandwidhts this way.

Good luck,