Rud Merriam wrote: > Jose, > > Just as you were posting this message I was stumbling on a web site that > agreed with your comment. > > With further searching I think I have the relationship. The QEX article has > the statement that to go from the 3kHz bandwidth used you "subtract 34 dB > and add 10 log of the desired bandwidth in Hz". But I think he has it wrong.
I have not seen such article yet. > My search found that you adjust by taking 10log(BWoriginal/BWdesired) and > adding it to the given figure. Makes sense, in the way it takes the extra bandwidth into consideration. > I think the author neglected to consider that > the power of the signal is unchanged during the calculation. The result is > you need to add 19.82 dB to the reported values to obtain the SNR for a > 31.25 Hz signal. Seems to be in the ballpark. I had mentally derived some 17 dB as a correction factor, but did not actually calculate it. As Patrick explained, the 3 kHz bandwidth is a sort of equal yardstick to measure up the different modes. > As proof (I hope <g>): > > Signal: 3000 Noise (3kHz): 3000 SNR(dB): 0 > Signal: 3000 Noise (31.25Hz): 31.25 SNR(dB): 19.82 > > Where the noise is 1 Watt-s per Hz. > > The article reports that PSK-31 work down to -12 dB in AWGN this actually > means it work to 7.82 dB. The channel capacity for that SNR per > Shannon-Hartley is 88 bps. PSK-31 attains less that half the channel > capacity. Seems it is time to dust off my copy of Sklar's book.... 73, Jose, CO2JA __________________________________________ Participe en Universidad 2008. 11 al 15 de febrero del 2008. Palacio de las Convenciones, Ciudad de la Habana, Cuba http://www.universidad2008.cu
