Gee I wish this had been available last week - we just got back from a week in Nicaragua.
We used a K3 in the ARRL DX CW test for about 4700 QSOs. Well over half of them were under medium to heavy pileup conditions with AGC-F. I found at times turning on the attenuator helped .... probably not as much as AGC off and 'ride' the RF gain control, but I don't have a whole lot of patience with that either. 73 Hank K8DD (YN2DD) On Wed, Feb 25, 2009 at 6:45 PM, wayne burdick <[email protected]> wrote: > There's been a lot of discussion recently about what happens to > multiple closely spaced signals with AGC turned on. We have a new > field-test firmware revision available that offers a significant > improvement in such situations by reducing the IMD caused by > conventional AGC. If you'd like to try it, please send an email to > [email protected]. > > Details: > > Some operators described a "merging" of multiple, close-spaced signals > when listening to pileups, making individual signals hard to copy. > Assuming that a narrow crystal filter is in use (close to the DSP > bandwidth), this is not "desense". In most cases, we've discovered, the > cause is IMD (intermodulation distortion) related to the AGC algorithm. > > To simulate what happens under pileup conditions, we combined four > crystal oscillators with gaps between them of 5, 6, and 7 Hz (a total > spread of 18 Hz for the four signals). We then injected the signals > into the K3 at an equal level of about S5 and used spectral analysis to > examine what happens to the passband. With AGC-F turned on (AGC fast, > set to the K3's factory defaults), IMD products came up some 20-25 dB > as compared to AGC OFF. The situation could be far worse in an actual > on-air situation with more signals, noise, and key clicks. (AGC-S makes > only a small improvement.) We ran the same test on several other > radios, and verified the AGC-induced IMD characteristics are similar. > > IMD is generated because AGC can cause mixing between the tones and > their harmonics. The faster the decay rate, in general, the more > products will be generated. With just one or two signals, most products > will be outside the DSP's narrow IF or AF passband. But in a pileup, > where signals are greater in number and can be nearly on top of each > other, there are many more opportunities for signals to combine. This > results in IMD products spaced at 1 to 20 Hz, depending on the time > constants, location of the signals, etc. > > Turning AGC off is one strategy that operators have traditionally used > to combat the effects of AGC modulation. It then becomes necessary to > "ride" the gain controls. Neither Lyle nor I have that much patience :) > So we took a different approach: keep the AGC on, but reduce its > modulation effects to negligible levels by adding a very long "hold" > time. > > In the field-test firmware, two changes have been made. First, the AGC > HLD menu setting, which used to apply only to voice modes, now applies > to CW as well. (It still only affects AGC-S, the slow AGC setting.) > Second, we increased the range of AGC HLD to facilitate > experimentation. What we found is that an AGC hold time of about 1 > second, combined with a slower AGC-S decay time, reduces in-band IMD > with closely spaced signals by 20-25 dB, making it very similar to > turning AGC off. The *attack* time is unaffected, so there is no > "thumping" as you might have with audio-derived AGC. Any new signal > peak above the one that starts the "hold" timer will restart the timer. > > This "hold" AGC appears to be useful in many listening situations. It > can make voice signals sound cleaner, as well; after all, a voice > signal can have many components, and AGC-induced modulation can add IMD > products. It also helps with noisy band conditions. > > The down side to AGC "hold" is that large signals can charge it up to a > high level, reducing gain for the duration of the hold time. But in > many cases this is far preferable to the AGC IMD effects, especially if > you're listening to many signals at similar amplitudes -- as in a > pileup. > > 73, > Wayne, N6KR > Lyle, KK7P > > > --- > > http://www.elecraft.com > > > > ------------------------------------ > > Yahoo! Groups Links > > <*> To visit your group on the web, go to: > http://groups.yahoo.com/group/Elecraft_K3/ > > <*> Your email settings: > Individual Email | Traditional > > <*> To change settings online go to: > http://groups.yahoo.com/group/Elecraft_K3/join > (Yahoo! ID required) > > <*> To change settings via email: > mailto:[email protected] > mailto:[email protected] > > <*> To unsubscribe from this group, send an email to: > [email protected] > > <*> Your use of Yahoo! Groups is subject to: > http://docs.yahoo.com/info/terms/ > > -- "I knew that God put me on this earth to be on the radio." Ed Bradley "Me too!" K8DD - ______________________________________________________________ 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
