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. My search found that you adjust by taking 10log(BWoriginal/BWdesired) and adding it to the given figure. 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. 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. Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net -----Original Message----- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Jose A. Amador Sent: Friday, October 26, 2007 2:26 PM To: digitalradio@yahoogroups.com Subject: Re: [digitalradio] Re: QEX Article on HF Digital Propagation Yes, a 3 kHz voice channel...not the inmediate environment of the digital signal, but much, much farther away. And as noise floor is related to bandwidth... Your mileage may vary... 73, Jose, CO2JA
