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 n...@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 ______________________________________________________________ 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
