AM and FM frequencies

[fran1942]

Hello, looking at the attached graph of noise voltage to frequency, I am trying to understand (from a noise point of view) why AM and FM broadcasts at the frequencies they do ?

Can anyone offer any suggestions please ?



n.b. ignore second attachment. It is incorrect.

 

[bhav]

Thermal and shot both are inherent noise that is always present in a real electrical system and represent fundamental limitations and difficulties in making sensitive electrical measurements.
Both are proportional to Bandwidth of the system. that's why FM is kept in higher frequency range than AM.

 

[fran1942]

Thanks, that covers why FM is placed where it is from a noise point of view.

So, is there any particular reason (from a noise point of view) why AM is placed at 521 to 1612 kHz ?

Thanks.

 

[godfreyl]

The choice of frequency bands for AM and FM broadcasts had little or nothing to do with noise.

Each AM station uses 9 or 10 KHz of bandwidth, so there is space for up to about 100 stations between 600KHz and 1.6MHz. Operating at a fairly low frequency allows good reception at a distance, even with obstacles in the way.

Each FM station uses about 200KHz of bandwidth, so again there is space for about 100 stations between 88 and 108 MHz.

If you want 100 stations using 200KHz each, you need a band 20MHz wide, so for FM, you can't use medium wave frequencies. e.g. If you allocated a band from 2 to 3 MHz, there would only be space for five radio stations.

If you allocated a band from say 5MHz to 25MHz, you run into other problems. The 5 to 1 ratio of minimum to maximum frequency would make receiver and receiving antenna design difficult and impractical.

For those reasons, it's pushed up into the VHF range, and we have to live with the fact that reception is more or less "line of sight" only, with any obstacles (even a building) between you and the transmitter causing problems.

 

[betwixt]

To add to that, AM was the mode of choice in the very early days of broadcasting and at the time, even the best vacuum tubes would only work up to a few MHz. The technology to use FM or VHF didn't exist and AM on low frequencies became an established system.

Brian.