FM Modulation: Solving Problems with 400KHz and 40 Amplitude

[mohlam12]

Hello,

So I was studying for my exam and came across this exercice we did in class and which I really don't know how we got the answer.

It says that if we have a FM with a carrier frequency of 4MHz, and the bandwidth is 400KHz, what is then the maximum deviation.. The answer is 200KHz.

So how did they figure it out? I know that BW = 2 ( Freq. Dev. + Signal Freq.)

And, the second question states: if the maximum amplitude of the source signal is 40, what would the modulation index be? The answer is 5.

Anyone can tell me how they solved it ? Thanks.

 

[fss]

It says that if we have a FM with a carrier frequency of 4MHz, and the bandwidth is 400KHz, what is then the maximum deviation.. The answer is 200KHz.

So how did they figure it out? I know that BW = 2 ( Freq. Dev. + Signal Freq.)

You can just think about this conceptually. If you have a signal 400 kHz wide centered on 4 MHz, you can only deviate ±200 kHz in either direction around the center frequency.

And, the second question states: if the maximum amplitude of the source signal is 40, what would the modulation index be? The answer is 5.

Anyone can tell me how they solved it ? Thanks.

This question seems unclear to me. Amplitude only matters in AM as far as the modulation index.

 

[skeptic2]

And, the second question states: if the maximum amplitude of the source signal is 40, what would the modulation index be? The answer is 5.

Anyone can tell me how they solved it ? Thanks.

The modulation index is the maximum frequency deviation divided by the maximum modulating frequency. We already know the maximum deviation is 200 kHz and 200 kHz divided by 5 = 40 kHz.

I suspect the problem was misstated. Instead of maximum amplitude it should have said maximum frequency and instead of 40 it should have been 40 kHz.

 

[mohlam12]

Thank you. I think the problem was misstated.