Date: 94-02-25 11:14:05 est From: draperbl@smtplink.mdl.sandia.gov Subj: results of AGC survey To: KA9FOX WOW! I was surprised at the number (>30 from 3 continents) and quality of the responses to my survey question about the use of AGC for CW contesting. But I was even more surprised by the outcome -- two thirds of the folks who responded use NO AGC in many if not all conditions! Here's the breakdown: Always use AGC: 35% Sometimes turn it off: 50% Never use AGC: 15% And if you're thinking "the guys who don't use AGC are the old-timers who aren't hard-core contesters and are still stuck in the 1950s," think again. Some of the no-AGC gang have calls that you've seen regularly in the top 5 of the SS, CQWW, Sprint, and ARRL DX contests in the last several years. Read all of the comments carefully and you'll learn something, I bet. After reading the messages, I think what many of these guys are saying is that we need radios with AGC circuit response that isn't so flat -- we need to add "depth" to the band under many circumstances. And that depth can't come entirely from pitch/frequency differences, some of it should be derived from amplitude differences. That kind of circuit won't take care of noise and pumping problems, of course (turn it OFF!), but it would make most operating better. Anyway, here are the edited comments (I've removed the calls 'cuz that's the way I think it should be). To those of you who responded, thanks for sharing your ideas. 73, Bruce AA5B draperbl@smtplink.mdl.sandia.gov ___________________________________________________________ Most of the time I leave it on, but I am always alert to times that the sensitivity is being degraded by a strong signal somewhere nearby as evidenced by AGC pumping. Then I immediately turn it off. Always use AGC. I tried this in the ARRL DX. I think that it works. When the piles got tough I just turned the AGC off and backed off the RF gain. I have been licensed for 35 of my 50 years and remember well when AGC on receivers hardly worked. Turning it off and riding the RF gain is a good technique even now when modern rigs have AGC circuits that do their intended job very well. If you think about the fact that the whole idea of AGC is to reduce the apparent differences between signals of different levels you begin to realize that if a 20-over signal and an S-3 signal are heard at about the same apparent level when in the clear you have no problem . . . but put both in your passband at the same time and there is a challenge. With less, or no AGC you hear one very loud signal and one much weaker signal . . . and although you may get a headache doing it, your brain can be trained to ignore the loud signal and copy the weaker one . . . with AGC on, the weaker one might be totally masked by the gain reduction needed to lower the level of the strong signal. As I have gotten lulled by the nice performance of my modern xcvr I sometimes forget that my brain can do a better job under some condx if I will just defeat the AGC and try it. After seeing the W3LPL note I began testing with no AGC on my FT1000 and found it to be useful when signals are close to or down in a fluctuating noise level that is activating the AGC. Another use seems to be when key clicks from strong adjacent signals just out of the passband are activating the AGC and masking weak signals. It has to do with not wanting to listen to noise that is as loud as any station you will hear. Bringing the RF gain down will increase the dynamic range of what you are hearing, so if a signal is above the noise, it won't be at the same audio level. You are missing something (if you always use AGC) . . . Yes, but usually when my DSP noise eliminator can't take care of static crashes on 40 thru 160. Great survey question! I've noticed the same thing. I pretty much keep AGC on fast all of the time, and never turn it off. I also generally keep the RF gain wide open, though I've seen a lot of world-class ops really ride it on the low bands. I know I've read more than once that keeping the volume level up in the headphones and turning AGC off is an unhealthy combination, and I'd sure like to still be able to copy the weak ones when I'm 85 (if there's anyone left on the planet who knows the code in 2050). I agree with you. I have found very few cases where turning off the AGC helps. I do run with fast AGC and back down the RF gain to keep signals down in the range where the rcvr can handle them. You bet, running without the agc on my FT1000 has its advantages! Was taught this method 40+ years ago by my Elmer. Oddly, it's difficult to explain why, though. The receiver is quieter, certainly. A disadvantage is getting your ears/head blasted by strong signals. I find myself turning the AGC off frequently. I do this primarily on 80 and 160, and almost never on any band above 40m, where I do it once in awhile. Probably sounds weird . . . . but it works. AGC off is the preferred method. Another factor is one's ability to separate signals by level. Modern agc systems are so flat that everyone sounds the same in a pile up. This makes it difficult to distinguish callers. Turning off the agc will add depth to the pile up (of stations calling you) and make it easier to pick out who you want (weak or strong). I always leave the agc off on CW. This allows aural differentiation by signal strength of multiple signals in the passband. For example, if you call cq and two guys call you, one of whom is loud and the other not so loud, agc compression will make them sound identical. When the loud guy is not sending, the weak guy will sound like the loud guy because the rx gain will increase. With the agc off, the loud guy stays loud, and the weak guy stays weak, allowing you to pick out either one. AGC on. Turned to min position. I use AGC. I also bathe regularly. I prefer to use the AGC. I always use *fast* AGC. Never turn it off. Saving my ears is more important than pulling QSOs out of the noise. I ran hundreds of piss weak stations on 10 meter scatter this weekend from 6D2X with the AGC turned off. Funny that you posted this request. During this past weekend's ARRL DX CW from K5OJI, I used the "no agc" technique on numerous occasions to copy weak stations that were just at or slightly above the noise on a '940. Only used this on the low bands. This trick did not work at all during Saturday night's thunderstorms. When the signals really get weak, you can turn off the AGC and pull them out of the noise a little easier. I do this occasionally, and in fact managed to pull some weak ones out over this past weekend this way. I usually switch back and forth between fast AGC and AGC off. I don't usually turn down the RF gain, however. At present, I use AGC-fast for phone and NO AGC for CW. I think the problem is the effect of QRM on AGC. Yeah. Old timer's secret to reducing the effect of static crashes on weaker sigs. Let your ears do the work. Obviously, you shouldn't do this for the entire 48 hours, or you'll end up slowly going deaf, like I am, with tinnitus. If you have a big pileup going, turning off the AGC and turning down the RF gain cuts it down to just the loud ones. As the pileup thins out, turn the RF gain back up and then the AGC. Saves blasting your ears and brain with too much extraneous garbage. If you do not have a close in very strong signal, you may never notice the advantage of shutting off the AGC and "riding the RF gain". This is particularly important on 80 and 160m when noise crashes will pull the agc, and weak signals then disappear. Disadvantage, it can be tiring and/or hazardous to your eardrums! You are not limiting the amplitude of the crashes except through audio amp distortion. On my main rig, I will work without agc only during lightning QRN or when a station is close and strong enough to pull the AGC. Tried turning AGC off years ago, didn't like it, so run in AGC FAST in both CW and SSB. I seems to me that AGC compresses the dynamic range, which is not good. Yes, Bruce, you're missing something. There are often circumstances under which AGC is a severe handicap. This applies mostly to CW, yet sometimes to fone. For 35 years of hamming I NEVER used the AGC on CW, but always rode the RF gain manually to hear weaker signals. Then I got an OMNI VI last year, and now ALWAYS leave fast AGC on except when trying to hear a very weak station in a non-contest pile up. I am sometimes turning the AGC off when there is lots of QRM from a nearby station which would modulate my AGC. I always operate with the AGC turned ON. And I always turn on much more attenuation than the other operators. ----------------------- Headers ------------------------