Wayne Burdick wrote: >Joe Subich, W4TV wrote: > >> Remember, IMD is driven by total *peak* signal level. The peak signal >> level is not simply the scalar sum of the levels ... it is a vector >> sum as the individual signals add in phase. > >And those peaks could be slightly modulating the AGC loop. >
Maybe that should say "modulating the AGC loops" (plural)? The HAGC loop and the DSP AGC loop interact strongly - especially around the HAGC threshold - and this creates the possibility for each loop to modulate the other. When the HAGC begins to act, we know that the firmware automatically adjusts the DSP AGC to give consistent audio levels and correct S-meter readings. The firmware monitors for HAGC action by digitizing the voltage on the HAGC1 line, but there has to be some delay before it can become "aware" of any change. The dynamics of this process create the possibility for the HAGC loop and the DSP AGC loop to modulate each other. As far as I'm aware, this feature is unique to the K3, which may help to explain the "rig-specific" nature of the problem. If this is a valid model, the effects would predictably be worst around the HAGC threshold where the response is inherently nonlinear. The dynamic interaction between the two loops would be quite sensitive to the rate(s) of amplitude variation in the composite incoming signal, as individual components are keyed on and off. It would also be affected by added noise, due to its dependency on the instantaneous vector sum of *all* incoming signal, noise and interference components hitting the HAGC detector (as identified by W4TV). The mathematics of two cascaded feedback loops are way beyond me, but the benefit of increasing the HAGC time constant by a very large factor (10-20) sounds at lot like avoiding competition between the dominant poles in the two loops. But the same model also explains why this wouldn't be a complete cure. This is all speculation, because much depends on hidden detail in the DSP AGC firmware... but might it make some sense? -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek ______________________________________________________________ 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
