An AGC Story
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Bottom posted for context.
Geoff GM4ESD wrote "If I had a K3, before I changed the HAGC threshold my *first* measurement would be to look at the output of the SA612 mixer (with a spectrum analyser) while introducing two or more very close spaced signals into the antenna port, so that these signals do get through the roofer - as in a pileup. The objective, of course, would be to obtain an acceptably clean output from the mixer with non intrusive products over a wide range of input levels. IMHO the higher the HAGC's threshold can be the better, provided its control is tight enough when activated to protect all what follows." I made within DSP b/w IMD measurements on my K3 and comparable measurements on my K2. These were included in a post dated 5 December but you may have missed it. The following measurements relate to K3 S/N 4904 on 7MHz with 2.7kHz roofing filter and 2.7kHz DSP b/w. Default slow AGC was selected. The table shows input level at the K3 antenna connector for each of two carriers spaced 500Hz. At each input level, the level of the strongest IMD product observed at the AF output using an HP8568B is recorded alongside. -73dBm Better than -55dBc (Input S9) -63dBm -55dBc -53dBm -50dBc -42dBm -45dBc -33dBm -42dBc -23dBm -37dBc (Input S9+50) These figures are good and IMHO entirely acceptable. Some older transceivers would struggle to provide -35dBc at S9. I made a similar set of measurements with my old K2 S/N 997 -73dBm -50dBc -63dBm -48dBc -53dBm -45dBc -43dBm -48dBc -33dBm -45dBc -23dBm -15dBc Broadly speaking the K3 is as good or better than the K2, which is itself good. It is possible later mods to the K2 may have made it even better. The most recent mods to my K2 were carried out over 5 years ago. 73 Bob, 5B4AGN ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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Bob,
Yes I did see your post thanks, but have not yet had the opportunity to digest your results - but I will. When you made these measurements, what settings did you use for the 'audio' Hi-cut and Lo-cut? My thinking was that I would first look at the SA612's output wile injecting multiple signals, and then do the same at suitable points along the signal path as it made its way to the audio output. By doing this it should be possible to find the weakest link in the chain (in terms of IMD), and then attempt to do something to improve the 'weak link's' performance. May I take this opportunity to stress that my comments were not intended to be any kind of criticism of the K3. 73, Geoff GM4ESD . "Bob Henderson" <[hidden email]> wrote on Sunday, December 12, 2010 4:11 AM: <snip> > I made within DSP b/w IMD measurements on my K3 and comparable > measurements on my K2. These were included in a post dated 5 December > but you may have missed it. > > The following measurements relate to K3 S/N 4904 on 7MHz with 2.7kHz > roofing filter and 2.7kHz DSP b/w. Default slow AGC was selected. > The table shows input level at the K3 antenna connector for > each of two carriers spaced 500Hz. At each input level, the level of > the strongest IMD product observed at the AF output using an HP8568B is > recorded > alongside. <snip> ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Bob Henderson-2
Guy, have one (or several) on me for suggesting such a sensible idea :-)
73, Geoff GM4ESD Guy Olinger K2AV On Sunday, December 12, 2010, at 10:04 AM, Guy Olinger K2AV wrote: Thank you, Bob. But you have robbed Geoff of his favorite whipping boy. We will have to take Geoff out to a pub and help him drown his sorrows. What was your setup? 73, Guy ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Geoffrey Mackenzie-Kennedy
Speaking of criticism, I've done my share of complaining about AGC pumping
and pileup mush in the K3. That said, it's still the best contest receiver I've ever use, by a wide margin. It's sensitive, quiet and incredibly selective. In fact, I can get *too* close to adjacent stations without realizing it! In actual contest conditions, the AGC pumping can be reduced to more-or-less acceptable levels by using the 8-pole 400 Hz filter. It becomes a non-problem with the 5-pole 200 Hz filter. I suspect it could be reduced even more by increasing the HAGC threshhold, but evidently not without changing other components in the chain to avoid increased IMD. IMHO, the pileup mush is no worse than any other receiver I've used, and most of the time it's better (that sounds strange, but you know what I mean.) Considering that I had quite a few hours with my rate meter near 200/hr in CQ WW CW, I'd have to say the mush isn't reducing my rate a whole lot. One clue is that the mush effect is most noticable when there's a pileup of relatively weak signals and there's not a nearby strong station -- i.e., when the HAGC is not activated. If most of the signals are at the same pitch, it's mush. A bunch of really weak signals like that are simply darned hard to copy, and I'm beginning to think that no receiver can separate them unless AGC is turned off, with the attendant problem of ears being blown out when a strong station drops by. I have found, however, that the situation improves considerably with diversity reception using two antennas with opposite polarity. There's enough difference in the ever-changing arrival angles to be able to distinguish weak signals at the same pitch. No other receiver can do that for me. 73, Dick WC1M -----Original Message----- From: Geoffrey Mackenzie-Kennedy [mailto:[hidden email]] Sent: Sunday, December 12, 2010 6:30 AM To: Bob Henderson Cc: Elecraft Discussion List Subject: Re: [Elecraft] An AGC Story Bob, Yes I did see your post thanks, but have not yet had the opportunity to digest your results - but I will. When you made these measurements, what settings did you use for the 'audio' Hi-cut and Lo-cut? My thinking was that I would first look at the SA612's output wile injecting multiple signals, and then do the same at suitable points along the signal path as it made its way to the audio output. By doing this it should be possible to find the weakest link in the chain (in terms of IMD), and then attempt to do something to improve the 'weak link's' performance. May I take this opportunity to stress that my comments were not intended to be any kind of criticism of the K3. 73, Geoff GM4ESD . "Bob Henderson" <[hidden email]> wrote on Sunday, December 12, 2010 4:11 AM: <snip> > I made within DSP b/w IMD measurements on my K3 and comparable > measurements on my K2. These were included in a post dated 5 December > but you may have missed it. > > The following measurements relate to K3 S/N 4904 on 7MHz with 2.7kHz > roofing filter and 2.7kHz DSP b/w. Default slow AGC was selected. > The table shows input level at the K3 antenna connector for > each of two carriers spaced 500Hz. At each input level, the level of > the strongest IMD product observed at the AF output using an HP8568B is > recorded > alongside. <snip> ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Bob Henderson-2
Sorry Geoff.
My setup is fairly basic but I believe quite adequate. I use two +10dBm low noise xtal oscillators operated within the 40m band. The output of each is attenuated 20dB before combining via a Minicircuits SCP-2-1. Thereafter the signals pass through an 8MHz LPF to a step attenuator. The SCP-2-1 provides around 33dB port to port isolation at 7MHz. Isolation between the oscillator output stages is therefore around 73dB. 73 Bob, 5B4AGN On 12 December 2010 10:04, Guy Olinger K2AV <[hidden email]> wrote: > Thank you, Bob. > > But you have robbed Geoff of his favorite whipping boy. We will have > to take Geoff out to a pub and help him drown his sorrows. > > What was your setup? > > 73, Guy > > On Sat, Dec 11, 2010 at 11:11 PM, Bob Henderson <[hidden email]> wrote: > > Bottom posted for context. > > > > Geoff GM4ESD wrote > > > > "If I had a K3, before I changed the HAGC threshold my *first* > > measurement would be to look at the output of the SA612 mixer (with a > > spectrum analyser) while introducing two or more very close spaced > > signals into the antenna port, so that these signals do get through > > the roofer - as in a pileup. The objective, of course, would be to > > obtain an acceptably clean output from the mixer with non intrusive > > products over a wide range of input levels. IMHO the higher the > > HAGC's threshold can be the better, provided its control is tight > > enough when activated to protect all what follows." > > > > > > I made within DSP b/w IMD measurements on my K3 and comparable > > measurements on my K2. These were included in a post dated 5 December > > but you may have missed it. > > > > The following measurements relate to K3 S/N 4904 on 7MHz with 2.7kHz > > roofing filter and 2.7kHz DSP b/w. Default slow AGC was selected. > > The table shows input level at the K3 antenna connector for > > each of two carriers spaced 500Hz. At each input level, the level of > > the strongest IMD product observed at the AF output using an HP8568B is > recorded > > alongside. > > > > -73dBm Better than -55dBc (Input S9) > > -63dBm -55dBc > > -53dBm -50dBc > > -42dBm -45dBc > > -33dBm -42dBc > > -23dBm -37dBc (Input S9+50) > > > > These figures are good and IMHO entirely acceptable. Some older > > transceivers would struggle to provide -35dBc at S9. > > > > I made a similar set of measurements with my old K2 S/N 997 > > > > -73dBm -50dBc > > -63dBm -48dBc > > -53dBm -45dBc > > -43dBm -48dBc > > -33dBm -45dBc > > -23dBm -15dBc > > > > Broadly speaking the K3 is as good or better than the K2, which is > > itself good. It is possible later mods to the K2 may have made it even > > better. The most recent mods to my K2 were carried out over 5 years ago. > > > > 73 Bob, 5B4AGN > > ______________________________________________________________ > > Elecraft mailing list > > Home: http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/mmfaq.htm > > Post: mailto:[hidden email] > > > > 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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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