> 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: [email protected] [mailto:[email protected]] On Behalf > Of Guy Olinger K2AV > Sent: Wednesday, February 10, 2010 4:54 PM > To: Joe Subich, W4TV > Cc: [email protected]; [email protected] > 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 > <[email protected]> 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:[email protected]] > >> Sent: Wednesday, February 10, 2010 1:06 AM > >> To: Joe Subich, W4TV > >> Cc: [email protected] > >> 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: [email protected] > >> >> [mailto:[email protected]] On Behalf Of > Guy Olinger > >> >> K2AV > >> >> Sent: Monday, February 08, 2010 1:49 PM > >> >> To: [email protected] > >> >> Cc: [email protected] > >> >> 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 > >> >> <[email protected]> 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: [email protected] > >> >>>> [mailto:[email protected]] On Behalf Of > >> Lyle Johnson > >> >>>> Sent: Monday, February 08, 2010 11:24 AM > >> >>>> To: [email protected] > >> >>>> 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 > >> >>>> > >> >>>> > >> >>> ______________________________________________________________ > >> >>> Elecraft mailing list > >> >>> Home: http://mailman.qth.net/mailman/listinfo/elecraft > >> >>> Help: http://mailman.qth.net/mmfaq.htm > >> >>> Post: mailto:[email protected] > >> >>> > >> >>> This list hosted by: http://www.qsl.net > >> >>> Please help support this email list: > >> http://www.qsl.net/donate.html > >> >>> > >> >>> > >> >> ______________________________________________________________ > >> >> Elecraft mailing list > >> >> Home: http://mailman.qth.net/mailman/listinfo/elecraft > >> >> Help: http://mailman.qth.net/mmfaq.htm > >> >> Post: mailto:[email protected] > >> >> > >> >> This list hosted by: http://www.qsl.net > >> >> Please help support this email list: > >> >> http://www.qsl.net/donate.html > >> >> > >> > > >> > ______________________________________________________________ > >> > Elecraft mailing list > >> > Home: http://mailman.qth.net/mailman/listinfo/elecraft > >> > Help: http://mailman.qth.net/mmfaq.htm > >> > Post: mailto:[email protected] > >> > > >> > This list hosted by: http://www.qsl.net > >> > Please help support this email list: > http://www.qsl.net/donate.html > >> > > >> > --------------------------------------------------------------------- > >> - > >> > -- > >> > > >> > > >> > No virus found in this incoming message. > >> > Checked by AVG - www.avg.com > >> > Version: 9.0.733 / Virus Database: 271.1.1/2677 - Release > >> Date: 02/09/10 02:35:00 > >> > > >> > > > > > ______________________________________________________________ > > Elecraft mailing list > > Home: http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/mmfaq.htm > > Post: mailto:[email protected] > > > > This list hosted by: http://www.qsl.net > > Please help support this email list: http://www.qsl.net/donate.html > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
