[Elecraft] [Elecraft_K3] New K3 Firmware: Pileup-inspired AGC changes

[hank.k8dd]

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 <n6kr at elecraft.com> 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
> n6kr at elecraft.com.
>
> 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:Elecraft_K3-digest at yahoogroups.com
>    mailto:Elecraft_K3-fullfeatured at yahoogroups.com
>
> <*> To unsubscribe from this group, send an email to:
>    Elecraft_K3-unsubscribe at yahoogroups.com
>
> <*> 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
-