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73! de Eugene RA0FF http://www.qsl.net/ra0ff/ >Вторник, 25 декабря 2018, 8:17 +11:00 от Chuck Dietz <w5prch...@gmail.com>: > >I think I understand much of what you are saying, but I know that I was on 160 >meter FT=8 two nights ago with the speaker up fairly loud. I only heard noise. >I set the AGC off and adjusted the RF gain so that it did not overload. Still >no hint of any signals, but I decoded two stations! > >Just sayin’. > >Chuck W5PR > >Sent from Mail for Windows 10 > >From: K4SAV >Sent: Monday, December 24, 2018 2:10 PM >To: topband@contesting.com >Subject: Re: Topband: FT8 - How it really works > >Although I have finished my FT8 testing, there is one final thought I >would like to leave with you, and also to correct one statement I made >earlier. Someone thought FT8 measured the noise in the interval when >the FT8 signals were off, and I replied that would result in a real S/N >number. That is not true as you will see in the info below. You would >get a real S/N number if the RF was sampled, but not if the audio is >sampled. > >I spent many years designing electronic circuits professionally, so I >still think that way. So for a few minutes lets think about a circuit >that can decode something below the noise floor .If you think about FT8 >or anything similar, from a designers point of view, you suddenly >realize that making a statement of "the circuit can decode down to X dBs >below the noise floor" is almost an impossible task, that is, if you are >talking RF noise floor as most people will be assuming. > >Since you will be dealing with audio, not RF, the receiver will convert >the RF into audio and compress it into something that has a lot less >dynamic range. How much less? Say the volume is set to a level such >that the strongest signals do not clip, then how far down is the noise? >You can expect that to vary on each band too. > >Now comes a real complication. If you were taking samples in the RF >world, you could see the noise level on your S meter and estimate it >relative to the strongest signals. However your circuit will be dealing >with audio. Surprisingly, when the signals disappear, the receiver AGC >voltage drops and the receiver gain increases. That produces a lot more >audio signal. The audio noise in the case of no signals becomes higher >than the audio level for strong signals if you are using USB bandwidth >and receiving something similar to FT8. That condition is not nearly as >pronounced when using a narrow CW bandwidth. Even if you put the >receiver into AGC slow mode it won't hold for the 3 seconds when FT8 is >off, so you still get the increased audio in the off period. Then there >will be a sudden increase in audio when the first signal reappears, >until the ACG kicks in and lowers it. This happens even with fast AGC >selected. It's fast enough that you don't notice it when listening, but >if you put a scope on it you can see it. Yeah, all that surprised me >too when first thinking about it. Take a close listen and see if you >agree. If you can't hear it, put it on a scope or anything that displays >an audio waveform and it will become very obvious. > >If you made a statement that this circuit can decode X dBs below the >noise floor, most people will be thinking RF noise floor. So what is it >in the audio world that represents the noise floor in the RF world, and >what would your statement mean? > >Of course you could turn off the AGC and decrease the receiver RF gain >and that would make the audio very low when the signals disappear. That >would also severely limit the dynamic range for your circuit since you >would no longer have the compression supplied by the receiver.. Your >circuit would have to cover a much wider dynamic range, similar to what >a receiver does. So your circuit would need what? maybe 100 dB dynamic >range to cover the strongest signals to the weakest noise floor, >forgetting about decoding below the noise floor. Actually that wouldn't >really happen because receivers can't produce a dynamic range of 100 dB >in the audio. They may do it in the RF world, but not in audio. >Receivers have no need to do that. > >Jerry >_________________ >Searchable Archives: http://www.contesting.com/_topband - Topband Reflector > >_________________ >Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector