[Elecraft] AGC INFORMATION

[Dave Cole]

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 <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
>     "Kits made by KL7UW"
> Dubus Mag Business e-mail:
>     dubususa at gmail.com
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