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
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