# FM question

Discussion in 'Electronics Homework Help' started by jdouglasusn, Aug 6, 2012.

1. ### jdouglasusn

29
0
May 29, 2012
My instructor gave a problem and it is not making much sense to me.

(This is word for word how he wrote it)

"FM has a deviation of 7kHz & intelligence frequency of 2kHz. Si/Ni = 20 dB.
Calculate So/No at the output."

I know mf = deviation/intelligence frequency which is 3.5

What I am stumped on is Si/Ni = 20dB and how that exactly factors in to calculate the answer.

2. ### Laplace

1,252
184
Apr 4, 2010
Suppose that the modulator/demodulator was perfect and no noise was added, then the signal (including noise) at the demod output would be a perfect reproduction of the signal at the modulator input; Si/Ni=So/No. Is there any modulation technology that can recognize the noise component of an input signal and remove it? If not, then how would modulation noise be added to the noise already in the input signal? Are the two noise components uncorrelated?

3. ### jdouglasusn

29
0
May 29, 2012

That is my hitch. This question he wrote made no sense. It doesn't matter anyways. I will ask him to clarify tomorrow. Thanks anyways

4. ### foTONICS

332
9
Sep 30, 2011
I thought FM had a deviation of +- 75kHz

5. ### davennModerator

13,901
1,971
Sep 5, 2009
depends on the use you are referring to
for broadcast in the 88 - 108 MHz band it can be up to 150kHz

for FM on land mobile on other bands it is commonly 7.5, 12.5 or 25 kHz

Dave

6. ### foTONICS

332
9
Sep 30, 2011
your referring to narrow band?

7. ### davennModerator

13,901
1,971
Sep 5, 2009
well the OP referred to 7kHz which is definately narrow band

you referred to + - 75kHz which is wide band broadcast

I was making you aware that FM deviation can vary depending on the radio/TV service that its used for

Dave

8. ### john monks

693
3
Mar 9, 2012
FM deviation of 7kHz means that the carrier frequency is being deviated +7kHz and -7kHz.
Intelligence frequency means the modulating frequency or what?
Does the question relate to the affect the modulating has on the signal to noise ration or the deviation or what?

I,m very confused by the nature of the question.

9. ### foTONICS

332
9
Sep 30, 2011
intelligence frequency and modulating frequency are the same thing. if I had to take a stab at what he meant I would say the teacher wants to know what affect the mf has on the signal to noise ratio at the output