FM Transmission

@ashishsharma30-bJCohB • Oct 23, 2024

Oct 23, 2024

921

hello.
As it is a known fact that CCIR has put a limit on the bandwidth of transmission of FM for radio broadcasting from 88MHz to 108MHz, with maximum frequency deviation of +-75Khz with maximum bandwidth allowable for each channel being 200KHz.
Now, my question is that within this specified range of transmission, even with the same frequency of operation, in different places, why these signals do not interfere??😕
Anyone Please help.
thanks in advance.

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reachrkata

@reachrkata-FOcqhH • May 7, 2010

Its just because of the distance between the two sources. Just like there are norms governing bandwidth, deviation, etc, there are also those governing maximum transmission power and min distance between 2 stations allowed to transmit at the same frequency.

-Karthik

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ashish.sharma30

@ashishsharma30-bJCohB • May 8, 2010

thank you.
clarify if i am wrong, you are saying that the signals at same frequencies are transmitted with different carrier power content?
if that is the case, then there is a capture effect which occurs in FM receivers to select the having signal with more power content when signals with same frequency are received.
Please correct where ever i am wrong.
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durga ch

@durga-TpX3gO • May 8, 2010

Hi Ashish

Each Station has its own frequency right? and in between two frequency ranges there is always a guard band which avoids the interference as much as possible.
As I can see, there is a possibility of two types of interference here - between same station using same freq of transmission in two different areas and interference between signals of different stations serving same area.
As far as second is concerned , but guard band should be doing the job.
For the first one, you have same signals being received multiple times from diff stations at different locations. The receiver at this point, should be capable of filtering out the child signals
As well as Karthik mentioned, the power of the tx is always maintained to control the recieved power.In terms of maths- I think you must be aware that receiver power is related to the dist between tx and rx .
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reachrkata

@reachrkata-FOcqhH • May 9, 2010

@Ashish - You understood me wrong. 2 stations can be transmitting at the same signal level. But if one of the stations is in Bangalore and the other say in Mysore, distance ensures that the station is Bangalore is very very weak in Mysore. So in that way, what you mentioned about "capture effect" is correct and thats how it works.

@durga - true, the kind of interference you mentioned is saying it broadly. typically FM receivers follow the Superheterodyne principle. here the incident FM carrier is down mixed to an IF stage. And the receivers use very precise local oscillators for this. In addition down mixing not only gives the IF but also a range of beat frequencies. So interferences typically possible are 2 different stations having a beat frequency at the IF, harmonics of the local oscillator and the incident station beating, etc etc. There are generally limits defined for allowable levels of interference, keeping in mind clarity of FM audio perception by the user.

- Karthik
😁
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