Hi Geoff The noise power ratio test(NPR) might show more clearly any mixer IMD problems. The NPR test might also better characterize any roofing filter IMD as well.
The noise power ratio test better characterizes dynamic receiver performance. The NPR test also does not require expensive low phase noise signal generators. 73 John --- On Sat, 12/11/10, Geoffrey Mackenzie-Kennedy <gm4...@btinternet.com> wrote: > From: Geoffrey Mackenzie-Kennedy <gm4...@btinternet.com> > Subject: Re: [Elecraft] An AGC story > To: "Kok Chen" <c...@mac.com> > Cc: "Elecraft Discussion List" <elecraft@mailman.qth.net> > Date: Saturday, December 11, 2010, 5:26 AM > Chen, > > It is not really my place to comment on this subject not > having a K3 to > measure, but I would like to offer the following thoughts > related to the > "naughty things" you mention. > > Whereas measuring the receiver's audio output level vs.the > level of a single > signal injected into the antenna input port will obviously > provide some good > information about the AGC's control characteristics, > unfortunately this > measurement does not provide any hard data as to whether or > not > intermodulation generated in the IF stages will become or > is an issue when > two or more strong signals get through the roofer in use. > > Given that the 1db compression point of a typical SA612 > mixer is reached > when the *composite* power of all signals reaching its > input is > approximately minus 25dbm, and given that positive gain > exists between the > antenna port and the SA612's input when both the Preamp and > Attenuator are > OFF, in the region of 6db I believe when a 400 Hz roofer is > in place, then > in the *absence* of any HAGC action the SA612 would enter > compression when > the *composite* power of all signals at the antenna port > that eventually get > through the roofer is only minus 31dbm. At this level > *without* HAGC applied > there would be, of course, a large number of of strong odd > order intermod > products appearing at the SA612's output should multiple > signals get through > the roofer. > > Now bring in the HAGC. > > Because part of the method used to control IF gain by the > HAGC involves > current starvation of the J309 IF amplifier, it is > *possible* that with HAGC > applied the Output Intercept (OIP3) of this amplifier might > decrease, > thereby increasing the level of IMD products generated by > this amplifier. > > So, even though the HAGC might prevent the SA612 from > approaching > compression, much of the HAGC's protective work in terms of > reducing > unwanted IMD products *might* be undone by the J309 when it > is starved of > current. Please understand that this is pure speculation on > my part. > > 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 > antenna input levels. > IMHO the higher the HAGC's threshold can be the better, > provided that its > control is tight enough when activated to protect all what > follows. > > 73, > Geoff > GM4ESD > > > Kok Chen wrote on Thursday, December 09, 2010, at 7:05 PM: > > > We know that the S9+ signals have to be kept from > saturating the A/D > > converter (or even kept in check so the large signal > doesn't do > > naughty things to the IF amplifier and the SA612 > second mixer -- I > > don't know which of these three is the wimpiest in the > chain). So > > some AGC is neccessary. > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > 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:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
