When making comparisons between AM and FM there is a vast difference in performance of each depending upon the received signal strength. Generally we tend to think that FM will always outperform AM but that is not always true and here is why.
The improvement in S/N ratio for FM can be calculated using the formula dB = 10*Log 3*m^2, where m is the modulation index of the FM signal. The modulation index in FM is calculated by this formula m = deviation / highest mod frequency.
In a typical NFM system like a two-way radio, the deviation is 5 KHz deviation and the highest modulating frequency is 3 KHz so m = 1.667. If you plug 1.667 into the first formula the S/N ratio gain for FM over a 100% modulated AM signal is about 9.2 dB. In an FM broadcast system the gain is almost 19 dB!.
But there is one caveat, in order to achieve this gain, all of the FM sidebands need to exceed a minimum signal level. This is sometimes called the FM improvement threshold but more commonly called the quieting point. As the FM signal level sinks below this threshold, the S/N gain quickly disappears. There is a limit point at which AM then has a higher S/N ratio than FM.
This is because of two factors: 1. FM has multiple sidebands and all of them need to be detectable in order to have the gain in S/N ratio. 2. An FM receiver bandwidth is typically in the 15 KHz range where as an equivalent AM receiver bandwidth can be as narrow as 6 KHz. That fact by itself gives AM an approximate 4.0 dB advantage in noise reduction. This is one of the reasons why single sideband, a form of AM, works best for weak signal Dxing.
To summarize: If the FM signal exceeds the quieting point then it will typically have a higher signal to noise ratio than an equal level AM signal. As the FM signal level drops below this point, the AM signal will eventually have a higher signal to noise ratio.
Dan
wa8vzq@yahoo.com
