[Elecraft] AGC INFORMATION

[Edward R Cole]

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 <alsopb at comcast.net>
To: "Elecraft at mailman.qth.net >> 'Elecraft Reflector'"
         <elecraft at mailman.qth.net>
Subject: Re: [Elecraft] AGC INFORMATION
Message-ID: <582C591C.10100 at comcast.net>
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
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