http://elecraft.85.s1.nabble.com/K3-in-a-cw-pileup-needs-work-tp4523884p4551110.html
There's currently nothing variable after the SA612. Doesn't reducing
>
> Don,
>
>> Can you elaborate your reasons for this suggestion.
>
> Adding attenuation before the first active stage (amplifier)
> always reduces noise figure because that's where the S/N
> ratio is determined provided the first stage has enough
> gain to override the noise of the next stage.
>
>> I had come to believe that the best situation is to place
>> attenuation before the first mixer to give the greatest
>> amount of dynamic range (headroom).
>
> That assumes any IMD is produced only in the first mixer.
> In the K3, the limiting factor is almost certainly later in
> the chain because of the very good IMD performance of the KR
> (first) mixer. Without specific analysis, the weakest link
> is probably the ADC followed by the second mixer. If it was
> the first mixer, narrowing the roofing filter would not
> improve the dynamic range. That's why I would do two stages
> of gain reduction - 10 dB in the 30 kHz IF followed by 6 dB
> in the 8.125 MHz IF, if needed.
>
> A designer always wants to make sure any amplifier gain is
> sufficient for the noise figure of the succeeding stage
> but low enough to avoid overload down the line.
>
> 73,
>
> ... Joe, W4TV
>
>
>
>
>> -----Original Message-----
>> From: Don Wilhelm [mailto:
[hidden email]]
>> Sent: Wednesday, February 10, 2010 1:06 AM
>> To: Joe Subich, W4TV
>> Cc:
[hidden email]
>> Subject: Re: [Elecraft] K3 DSP AGC Question
>>
>>
>> Joe,
>>
>> Can you elaborate your reasons for this suggestion. I had come to
>> believe that the best situation is to place attenuation
>> before the first
>> mixer to give the greatest amount of dynamic range (headroom).
>>
>> Yes, I agree that the AGC Threshold could be raised in the K3
>> and would
>> be beneficial. I have felt the default of 005 was too low.
>>
>> 73,
>> Don W3FPR
>>
>> Joe Subich, W4TV wrote:
>> > Perhaps one should get the ambient well down in the ADC operating
>> > range but killing gain (adding loss) before the first mixer and
>> > the roofing filter is not the way to do it. If you want to
>> > better align the dynamic range of the receiver (or specifically
>> > the ADC) to conditions, the attenuation should come in the
>> > IF - preferably divided between the first IF and 2nd IF.
>> >
>> > However, there would still be no apparent reason that the K3
>> > should not be capable of sustaining an AGC threshold at least
>> > 10 db higher than currently set with AGC THR = 008 whether
>> > that be reached with preamp on, preamp off or attenuator on.
>> >
>> > 73,
>> >
>> > ... Joe, W4TV
>> >
>> >
>> >
>> >
>> >> -----Original Message-----
>> >> From:
[hidden email]
>> >> [mailto:
[hidden email]] On Behalf Of Guy
>> >> Olinger K2AV
>> >> Sent: Monday, February 08, 2010 1:49 PM
>> >> To:
[hidden email]
>> >> Cc:
[hidden email]
>> >> Subject: Re: [Elecraft] K3 DSP AGC Question
>> >>
>> >>
>> >> It may be a dance, but hopefully careful intelligent musing
>> >> over the matter is convincing people that they should get the
>> >> ambient well down in the analog-to-digital converter (ADC)
>> >> operating range. If they do, a -99 threshold is a -109
>> >> threshold, and further backing off RF gain can make it a -119
>> >> or -129 threshold as far as the ADC is concerned, and
>> >> headroom is being used for what headroom should be used for.
>> >>
>> >> One thing for sure, there has not been a good-enough
>> >> presentation of what is going on, and people really don't
>> >> seem to get it. For something that is plain indisputable
>> >> physics, people are still running PRE and max RFgain on 80
>> >> with fast AGC, and are honestly dismayed and confused about
>> >> the inevitable outcome, incorrectly blaming the rig for being
>> >> natively noisy, no-work NR and NB, filter "ringing", unable
>> >> to pull signals out of noise, and more.
>> >>
>> >> Maybe the variable gain by band in the MP (with a menu
>> >> override) was really a better strategy for a default. This
>> >> same issue dogged Orion owners, and by the chatter a lot of
>> >> them never understood either.
>> >>
>> >> There IS something entirely separate with the headphone audio
>> >> and HI-Z headsets, high volume levels, maybe a sharp
>> >> saturation point, and mush, just haven't nailed it yet.
>> >>
>> >> 73, Guy.
>> >>
>> >> On Mon, Feb 8, 2010 at 12:01 PM, Joe Subich, W4TV
>> >> <
[hidden email]> wrote:
>> >>
>> >>>
>> >>>> Likely? I don't know.
>> >>>>
>> >>> Likely is a better choice since raising the AGC threshold
>> >>>
>> >> seems to be
>> >>
>> >>> the primary goal of much of the "PRE/ATT/RF Gain dance" we've seen
>> >>> recently.
>> >>>
>> >>> The hardware AGC (HAGC) would seem to provide an ultimate limit on
>> >>> level to the ADC - unless the ADC can't handle the
>> >>>
>> >> transient peaks -
>> >>
>> >>> so a higher threshold for the DSP derived AGC would be
>> >>>
>> >> helpful in many
>> >>
>> >>> ways.
>> >>>
>> >>> 73,
>> >>>
>> >>> ... Joe, W4TV
>> >>>
>> >>>
>> >>>
>> >>>
>> >>>
>> >>>> -----Original Message-----
>> >>>> From:
[hidden email]
>> >>>> [mailto:
[hidden email]] On Behalf Of
>> Lyle Johnson
>> >>>> Sent: Monday, February 08, 2010 11:24 AM
>> >>>> To:
[hidden email]
>> >>>> Subject: Re: [Elecraft] K3 DSP AGC Question
>> >>>>
>> >>>>
>> >>>> Hello Joe!
>> >>>>
>> >>>>> Assuming that the threshold values have not changed in a
>> >>>>>
>> >> major way
>> >>
>> >>>>> since Jack's measurements, would it be possible to
>> >>>>>
>> >> extend the range
>> >>
>> >>>>> of threshold values - say to AGC THR=016 at about -80dBm
>> >>>>>
>> >> (assuming
>> >>
>> >>>>> 2 dB per step)?
>> >>>>>
>> >>>>>
>> >>>> Possible? Yes.
>> >>>>
>> >>>> Likely? I don't know. There are a lot of side effects
>> >>>>
>> >> that happen
>> >>
>> >>>> when the threshold is raised...
>> >>>>
>> >>>> 73,
>> >>>>
>> >>>> Lyle KK7P
>> >>>>
>> >>>>
>> >>> ______________________________________________________________
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