Ed, Thank you for a very clear explanation here... Well written out.
73s and thanks, Dave NK7Z http://www.nk7z.net On 11/16/2016 05:04 PM, Edward R Cole wrote:
Just a comment/correction about JT65: I don't know who is claiming 20-dB improvement over CW, but the creator of JT65, Joe Taylor - K1JT has stated that "about" ten dB improvement can be realized. I can hear -18 dB JT65 tones from eme signals. This fits well with the theory of receiving weak-signals, be they CW or digital. The typical (or maybe better than typical) CW op can hear copy signals down to 17 dB below what is SNR = 0 dB in a 2.5 KHz bandwidth. How that is determined is by reducing bw to 50-Hz. The improvement in SNR from 2.5 KHz to 50-Hz is given by 10Log(2500/50) = 10 Log(50) = 17 dB. JT-65 signals can be decoded down at -28 dB referenced to 2.5 KHz, thus about 10-dB improvement over hearing CW. I find I need to reduce radio bw to 100-Hz when trying to copy very-weak CW; maybe my ears/brain adds another 3-dB filtering to get to 50-Hz (maybe not as I seem to need SNR> 6 dB to copy CW. But I can make out tones at SNR=0 (or below). Narrowing radio bw to 50-Hz does not help me as the filter ringing confuses what my brain hears. Having severe hearing disability does not help, either. Nearly all the improvement in decoding signals comes from narrowing bw. JT-65 digital bw is about 5-Hz using a sequence of tones that vary in audio frequency so the FSK signal takes about 180 KHz bw in Tx. Note 10Log(2500/5) = 27 dB. One more note: JT-65 is run at SSB bw (typ. 2.5 KHz) since all the weak-signal work is done in sw. Some eme JT-65 stations run 4-KHz bw to watch several frequencies. I run MAP65 at 60-KHz bw to simultaneously watch the whole eme sub-band 144.095-144.155. My system does not use the K3 2nd IF or roofing filters. I've written before about that so visit my website if you want to know more about that. 73, Ed - KL7UW --------------------------- From: brian <[email protected]> To: "[email protected] >> 'Elecraft Reflector'" <[email protected]> Subject: Re: [Elecraft] AGC INFORMATION Message-ID: <[email protected]> Content-Type: text/plain; charset=windows-1252; format=flowed Hi Matt, I used to think I could copy CW signals below the noise. However when I put SPECTROGRAM in line and actually measured the S/N ratio the results were disappointing. According the SPECTROGRAM, those "in the noise" signals actually had an +8-10 dB S/N ratio. Try it and see for yourself. Likewise those WSPR and JT65 guys have been mislead by the -dB (e.g. -20) edited signal strengths. They need to know that those are calculated with a 2.4 - 2.8 KHz assumed noise power. In reality, many of the negative "dB" value signals can be copied by ear. 73 de Brian/K3KO 73, Ed - KL7UW http://www.kl7uw.com "Kits made by KL7UW" Dubus Mag Business e-mail: [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
