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I came across a picture ( shown below ) comparing the SNRs of FM and AM. There was also some explanation of the FM threshold effect and how the threshold effect is the reason as to why the FM graphs start at about (S/N) of 17dB.

The explanation here - http://www.allsyllabus.com/aj/note/ECE/Analog_Communication/Unit8/FM%20Threshold%20Effect.php

Would just like to ask, what is the FM threshold effect and why is this the case? Why does the carrier-to-noise ratio matter?

Performance of Analog Systems

Thanks!

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    \$\begingroup\$ " There was also some explanation of the FM threshold effect and how the threshold effect is the reason as to why the FM graphs start " Did you try reading those explanations? \$\endgroup\$
    – PlasmaHH
    Commented Mar 4, 2016 at 21:53
  • \$\begingroup\$ @PlasmaHH Well, if I understood the explanations I wouldn't be asking this question would I? \$\endgroup\$
    – Jian W
    Commented Mar 4, 2016 at 21:54
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    \$\begingroup\$ If you read the text, you would ask explicit questions about the specific parts you did not understand, wouldn't you? \$\endgroup\$
    – PlasmaHH
    Commented Mar 4, 2016 at 21:55
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    \$\begingroup\$ Can you provide a link to or copy of the explanations? Perhaps there are people here that don't know the answer but know enough to understand the explanations. \$\endgroup\$
    – Justin
    Commented Mar 4, 2016 at 21:56
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    \$\begingroup\$ My sympathy is with the OP. I've had a quick look at the link, and though I understand the threshold effect, the link tells me zilch. Try wikipedia, it's not much better, but is a little bit better. Just hit google and read all the links. \$\endgroup\$
    – Neil_UK
    Commented Mar 4, 2016 at 22:20

1 Answer 1

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Background first...

FM is better at delivering higher demodulated signal to noise ratios (SNR) than AM across a large range of different carrier SNRs: -

enter image description here

The vertical axis is output (demodulated) SNR and the horizontal axis is carrier SNR.

As you can see, there is a term called "FM improvement factor" - this is basically how much quieter the FM demodulated output is compared to an AM scheme. Above a certain carrier SNR the FM improvement factor is constant compared to AM.

However, as carrier SNR drops below a certain level, the improvement offered by FM starts to also drop.

What is the FM threshold effect?

Usually this is defined as the carrier SNR at which the FM improvement has reduced by 1 dB. In the picture above the carrier SNR would be a little below 12 dB

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  • \$\begingroup\$ Is the "threshold" where phase-jumps occur, and entire cycles are added or lost, thus CLICKS occur? \$\endgroup\$
    – analogsystemsrf
    Commented Aug 23, 2018 at 3:10
  • \$\begingroup\$ @analogsystemsrf I'm unsure about that question. \$\endgroup\$
    – Andy aka
    Commented Aug 23, 2018 at 7:04

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