++++++++++++++++++ Date: Sun, 22 Sep 2002 10:18:16 -0700 From: Wayne Burdick Organization: Elecraft To: Elecraft Subject: [Elecraft] increasing receive gain Once a K2 is assembled and aligned correctly, its overall receive gain can be increased, if desired, by reducing the size of resistor R1 on the Control board. This resistor is intended to set the AGC threshold to the ideal level. But it also affects the nominal gain of the I.F. amp (MC1350), so a slightly smaller value may be preferable for some operators. (In hindsight, we could have used a trimmer potentiometer here.) If you do go with a smaller value of R1, be sure to re-test receiver performance on both quiet bands and bands with lots of noise or strong signals. Also, you'll need to readjust your S-meter settings (CAL S HI and CAL S LO). While there are other ways to increase overall receive gain, this method has the advantage of not compromising receiver dynamic range. 73, Wayne N6KR ++++++++++++++++ From: "Rod N0RC" To: "Wayne Burdick" , "Elecraft" Subject: Re: [Elecraft] increasing receive gain Date: Mon, 23 Sep 2002 08:58:53 -0600 Wayne & others making this mod: What would you suggest as a absolute minimum value for R1 considering overall RX performance? That said, I changed mine and tested it "under fire" during the NJQRP Sprint. After trying a few different values I settled on 33k ohms. With this value, and the K2 preamp switched on, my K2's S-meter reading matched closely with my Icom 746pro (preamp off). With the K2 preamp off the S-meters match within about 3 "S" units (I spot checked signals on 15, 20 and 30m). I thought about setting R1 lower, to get the Icom and K2 to match S-meter readings, both with preamps off. But, I feared the stage gain might be too high, and degrade overall RX performance. What test(s) would anybody suggest to evaluate the suitability of a particular R1 value? (tests that can be preformed without a full compliment of lab equipment preferred) What I noticed most during the Sprint was the elevated noise level on 40m, I was running the K2 with Preamp on. Usually I have the preamp on, RF gain at max, and AF control between 9-11 o-clock (depending on the other station's signal strength). With R1=33k, while on 40m with preamp on, I found setting the RF gain control to 3 o-clock a better choice to reduce background noise. Noise on other bands was not a noticeable and running RF gain at max, preamp on, and AF gain at ~9 o-clock worked well. 73, Rod N0RC ----- Original Message ----- From: "Wayne Burdick" To: "Elecraft" Sent: Sunday, September 22, 2002 11:18 AM Subject: [Elecraft] increasing receive gain > Once a K2 is assembled and aligned correctly, its overall receive gain can be increased, > if desired, by reducing the size of resistor R1 on the Control board. ++++++++++++++++ Reply-To: From: "Ron D'Eau Claire" To: "'Elecraft'" Subject: RE: [Elecraft] increasing receive gain Date: Mon, 23 Sep 2002 08:30:52 -0700 It's hard to beat the "disconnect the antenna and see if the background noise drops" test. No laboratory equipment can do better to tell you if you have enough "preamp" gain and a low enough receiver noise figure. If the "S-meter" on other rigs seems to indicate higher, it may be that those receivers have a less-desirable gain distribution than the K2, and so a lower tolerance for strong signals. Ideally ALL amplification in a receiver should come after the IF filters that set the receiver bandwidth (and so after the "S-meter" circuits too). The ONLY function of any amplification ahead of the IF filters is to make background "atmospheric" noise picked up by your antenna stronger than the noise generated in the receiver itself so the noise generated by the receiver won't limit what you can hear when you dig for a really weak signal. I've never found a case in which the preamp was desirable on 40, from the standpoint of adequate sensitivity. I can understand wanting it on if the K2 receiver, overall, is anemic, but if the noise level drops when you disconnect the antenna with the preamp off, turning the preamp ON will compromise the overall dynamic range of the receiver. The "RF Gain" on the K2 does not control the level of signals at the RF input. It is simply a manual control of IF amplifier gain. So turning it down to compensate for having the Preamp On still exposes the Preamp and receiver "front end" to overload from strong signals that you cannot hear because they are outside of the IF filter passband. Well, often you CAN hear them, but they come through as hash and QRN and you don't even realize they ARE signals. Ron AC7AC K2 # 1289 What test(s) would anybody suggest to evaluate the suitability of a particular R1 value? (tests that can be preformed without a full compliment of lab equipment preferred) -- snip -- 73, Rod N0RC ++++++++++++++++++ From: "Rod N0RC" To: , "'Elecraft'" Subject: Re: [Elecraft] increasing receive gain Date: Mon, 23 Sep 2002 09:55:45 -0600 Ron, ----- Original Message ----- From: "Ron D'Eau Claire" To: "'Elecraft'" Sent: Monday, September 23, 2002 9:30 AM Subject: RE: [Elecraft] increasing receive gain > It's hard to beat the "disconnect the antenna and see if the background > noise drops" test. No laboratory equipment can do better to tell you if > you have enough "preamp" gain and a low enough receiver noise figure. If Done that. ;-) Preamp on/Preamp off background noise drops in both cases with antenna disconnected. > > I've never found a case in which the preamp was desirable on 40, from > the standpoint of adequate sensitivity. I can understand wanting it on > if the K2 receiver, overall, is anemic, but if the noise level drops > when you disconnect the antenna with the preamp off, turning the preamp > ON will compromise the overall dynamic range of the receiver. > It's all a balancing act at my QTH. Attic antenna, in the middle of a small city, LARGE apartment complex to one side, commercial buildings less that 1/2 mile away.... Having the Preamp on helps me hear those weaker signals, but at the price of more noise...like I said in my case it is a balancing at. (That's why I like the Icom 746pro, the DSP functions give me some advantage at times) Now before anybody says move to a quieter location....I'll be happy to if you pay the freight! ;-) real estate commissions, bonus for inflated housing prices, perhaps provide a chauffeured limo to compensate for time lost commuting... ;-) > The "RF Gain" on the K2 does not control the level of signals at the RF > input. It is simply a manual control of IF amplifier gain. So turning > it down to compensate for having the Preamp On still exposes the Preamp > and receiver "front end" to overload from strong signals that you cannot > hear because they are outside of the IF filter passband. Understood, fortunately the K2 has wonderful close-in signal characteristics that I am exploiting. The only time it is a big problem, is during BIG contests with high power ops, such as field day, NOV sweeps., NA sprints...etc. During those time you will find me hiking, biking or on the WARC bands. Not much of a contester, but do enjoy a few short sprints now and then, that are at QRM levels. 73, Rod N0RC ++++++++++++++++ From: James Hammons To: elecraft at mailman.qth.net Subject: [Elecraft] Re: Increasing Receiver Gain Date: Mon, 23 Sep 2002 13:45:03 -0400 Reply-To: james-e-hammons at worldnet.att.net I tried a 47k pot in place of R1 and found that as you go to lower values the agc will soon cease to function as it should. If you have enough gain so you can clearly hear the receiver noise, with the antenna disconnected, AF pot max, in both pre on and off positions, you have more than enough gain. This is a worst case situation the noise with an antenna connected will always be more than this, If you don't hear an increase in noise when antenna is connected you have some other problem. The S meter must be calibrated after any change to read meaningful values. I had previously increased the gain of my audio filter, I restored this back to factory values, I wanted a little more gain since I had done the KI6WX variation of the 2nd filter mod. This gives a very flat passband but causes about 2db more insertion loss. I found that the change of R1 to 47k was adequate and so I put in a 47k fixed resistor in place of the trimmer pot. The original value was no doubt also adequate. James +++++++++++++++ Reply-To: From: "Ron D'Eau Claire" To: "'Elecraft'" Subject: RE: [Elecraft] increasing receive gain Date: Mon, 23 Sep 2002 12:09:59 -0700 Since the noise background drops when you disconnect the antenna with the Preamp OFF, then you don't need it to help hear weak signals. Adjusting R1 to increase the gain of the IF amp will do a better job since it comes after the IF filter. Or increase the audio gain. Of course, you want to run that test in the narrowest selectivity setting you ever use. The narrower the bandpass, the more noise generated in the stages after the IF filter will show up. As Bob pointed out, this is NOT an especially "quiet" IF amp, but as long as the rig passes the "antenna disconnect" test after tinkering with the resistor value, you ability to hear weak signals will not be hurt at all. As you pointed out, you are NOT using a "high gain" antenna there in your attic, so the deleterious effects of running the preamp when you don't need one won't show up as often as it might for someone with a giant gain antenna on a tower. Even so, local sigs like a CB'er can cause your preamp a lot of grief even on band well away from 27 MHz. Keep in mind when evaluating the effect of R1 that as you increase the gain, the S-meter will be LESS active on weak signals. Since the S-meter is measuring the control voltage used to turn down the gain if the IF amplifier, adjusting R1 to keep the AGC action from starting will also cause the S-meter to show stronger signals or noise levels as S-0 levels (or whatever you have set the "meter" to show with no signal input"). Ron AC7AC K2 # 1289 ---------------------------------------------- Done that. ;-) Preamp on/Preamp off background noise drops in both cases with antenna disconnected.... It's all a balancing act at my QTH. Attic antenna, in the middle of a small city, LARGE apartment complex to one side, commercial buildings less that 1/2 mile away.... Having the Preamp on helps me hear those weaker signals, but at the price of more noise...like I said in my case it is a balancing at. (That's why I like the Icom 746pro, the DSP functions give me some advantage at times)... 73, Rod N0RC +++++++++++++++++ From: "Rod N0RC" To: , "'Elecraft'" Subject: Re: [Elecraft] increasing receive gain Date: Mon, 23 Sep 2002 14:25:33 -0600 Ron, I must have been lucky and hit the sweet spot for my particular situation... ----- Original Message ----- From: "Ron D'Eau Claire" To: "'Elecraft'" Sent: Monday, September 23, 2002 1:09 PM Subject: RE: [Elecraft] increasing receive gain > Since the noise background drops when you disconnect the antenna with > the Preamp OFF, then you don't need it to help hear weak signals. More often than not, preamp on does help, an observation consistent with MDS specs. But I think it is due to the poor antenna system I have. Yes the noise level goes up, but so does the signal level > Adjusting R1 to increase the gain of the IF amp will do a better job > since it comes after the IF filter. Or increase the audio gain. Of Which is what I did, then with preamp on for more signal amplification, I can back off the RF[IF] gain until things are suitable for me. With the original R1 value I almost never switched the preamp off, and at times I wished I had another 1/4 turn on the RF[IF] gain. Again, like I said, I think I hit the sweet spot for my particular installation. > Keep in mind when evaluating the effect of R1 that as you increase the > gain, the S-meter will be LESS active on weak signals. Since the S-meter Funny I observed the opposite. With the new R1 value (33k), and the S-meter calibrated per the manual, weak signals "dance" the meter more readily in the S1-3 range. All of this of course is quite subjective, I'll just have to "live it" for a while and see how it plays out. But for now my gut say "sweet spot". I hope somebody with the proper equipment will make all the measurements found in the ARRL specs for different R1/Preamp combinations. That would be far more telling. 73, Rod N0RC ++++++++++++++++ From: "Rod N0RC" To: "Elecraft-list" Date: Thu, 26 Sep 2002 07:36:09 -0600 Subject: [Elecraft] More Control Board R1 experiments Folks, I now have R1 set to ~41k ohm, and the overall radio performance is much better. Recall I posted about changing R1 to 33K, and that I thought the radio sounded "harsh" on 40m. That post yielded a good discussion with Ron AC7AC and posts by others. After mulling things over I still wasn't satisfied, so back to the bench.... Still I wished for test equipment to make a better evaluation of different R1 values. No chance of getting any, but it occurred to me WWV might make a good "test signal". It is only 10-15 miles away, and the 10MHz freq. produces a SOLID/STEADY S9 signal at my QTH (on my 746pro). I checked WWV with the K2 and my 33k R1. WOW S9 +20, preamp-off, way too much. I reasoned that close in strong signals on the amateur bands would degraded K2 RX performance. But where to set R1? I checked the sensitivity S-meter specs in ARRL report. K2 at 14 MHz: preamp off, 115 uV to yield S9. 746pro S9 signal at 14.2 MHz: preamp off, 82 uV. So theory-wise if the 746 is reading S9, the K2 should read a little less. Again I set about trying different R1 values, and finally settled on ~41k ohms. (not a std value, 220k in parallel with the original 51k). This value causes the K2's "S9" bar to just flicker. Still a little high maybe, but I'll take a little extra gain & resulting noise to compensate for my poor antenna system. I've been using my K2 for a few days and find this latest change to be quite useable. The "harsh" sound on 40 is gone. I also notice that often I now have the pre-amp switched off. Before I would always have the pre-amp on. So that's the story here, submitted for peer review. And if the methodology is worthy, perhaps it can be useful to others. 73, Rod N0RC ++++++++++++++++++ From: "John McClain" To: "Rod N0RC" , "Elecraft-list" Subject: Re: [Elecraft] More Control Board R1 experiments Date: Thu, 26 Sep 2002 09:57:24 -0700 I have replaced R1 with a 47K and am very pleased with the results. I haven't done any experimenting with other values yet but I will as soon as I dig down deep enough in my spare parts drawer to find a pot that I can play with. John K7SVV K2 #2569, SST 20, Rock Mite 20 ++++++++++++++++