Topband: FT8 - How it really works

[Chuck Dietz]

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 at 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