[Elecraft] K3 DSP AGC Question
Joe Subich, W4TV
lists at subich.com
Thu Feb 11 09:47:56 EST 2010
> In terms of what an operator can really do with the controls
> on a stock K3 (as opposed to a theoretical modified K3),
> would you be recommending leaving PRE set all the time, and
> only using the RF (IF) gain to keep ambient noise
> appropriately to the low end of the SA612/ADC ranges?
I have not considered keeping PRE on all the time though I
seem to need it above 14 MHz due to my poor antennas. I do
not know the range of the ADC but find it extremely odd that
the (relatively sharp) ACG threshold would be less than 50 dB
above the noise floor (-138 dBm MDS).
73,
... Joe, W4TV
> -----Original Message-----
> From: guyk2av at gmail.com [mailto:guyk2av at gmail.com] On Behalf
> Of Guy Olinger K2AV
> Sent: Wednesday, February 10, 2010 4:54 PM
> To: Joe Subich, W4TV
> Cc: don at w3fpr.com; elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 DSP AGC Question
>
>
> Well, that is a hefty board and circuit change to do what you
> suggest. There's currently nothing variable after the SA612.
> Doesn't reducing the RF (really IF) gain first do fairly much
> the same thing? Reducing level at Q1 output by reducing at
> RF gain knob is just barely in front of the SA 612 second
> mixer and lowers input to ADC at the same time. Seems like a
> pretty good pragmatic engineering choice.
>
> In terms of what an operator can really do with the controls
> on a stock K3 (as opposed to a theoretical modified K3),
> would you be recommending leaving PRE set all the time, and
> only using the RF (IF) gain to keep ambient noise
> appropriately to the low end of the SA612/ADC ranges?
>
> 73, Guy.
>
> On Wed, Feb 10, 2010 at 4:20 PM, Joe Subich, W4TV
> <lists at subich.com> wrote:
> >
> > 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:w3fpr at embarqmail.com]
> >> Sent: Wednesday, February 10, 2010 1:06 AM
> >> To: Joe Subich, W4TV
> >> Cc: elecraft at mailman.qth.net
> >> 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: elecraft-bounces at mailman.qth.net
> >> >> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of
> Guy Olinger
> >> >> K2AV
> >> >> Sent: Monday, February 08, 2010 1:49 PM
> >> >> To: nospam.lists at subich.com
> >> >> Cc: elecraft at mailman.qth.net
> >> >> 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
> >> >> <lists at subich.com> 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: elecraft-bounces at mailman.qth.net
> >> >>>> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of
> >> Lyle Johnson
> >> >>>> Sent: Monday, February 08, 2010 11:24 AM
> >> >>>> To: elecraft at mailman.qth.net
> >> >>>> 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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