+++++++++++++++++++ See also K2 Extreme Signal +++++++++++++++++++ Fix for Extreme Receiver Overload Subject: [Elecraft] spitting distance QSOs: why the diodes are needed by HF Packers Here's the scenario. If you transmit a VERY large signal from a transmitter whose antenna is within a few feet of the K2's antenna, the signal will exceed 3 volts peak-to-peak at pin 4 of the K2's I.F. amplifier, an MC1350. When this happens, the MC1350's AGC range will be exceeded. +++++++++++++++++++ Date: Wed, 01 Jan 2003 18:58:22 -0800 From: Wayne Burdick Organization: Elecraft To: HF Pack , qrp , Elecraft Subject: [Elecraft] Simple K2 RX mod handles extremely strong on-frequency signals Several recent HF Pack postings described K2 received audio distortion ("Howling K2s") in the presence of nearby transmitters operating on the exact same frequency as the K2. ("Update on Stalking the Wild Howling K2" - WB6MLC and "Conclusion from the witness. (K2 Trouble - my $.02 worth)" - K2CPE, WB6ZQZ and others.) Over the past two days, we reproduced this in our lab and came up with a very simple modification that dramatically increases the K2's on-frequency extreme signal-handling capability. The mod uses just two 1N4148 or 1N914 diodes; details appear below. We have modified five K2s as described, and they all worked perfectly. In hindsight, we should have included the two diodes in the original design. The reason we didn't notice this sooner is that the test suite used by most RF labs, including ours and the ARRL's, does not include a test of extremely strong on-frequency signal handling. The K2 passes all of the usual dynamic range tests with flying colors, as has been well documented. But these tests all use weak signals *within* the K2's crystal filter passband; large signals are injected outside the passband (usually 5 to 20 kHz away). Those tests are looking for desensing of the receiver when strong off-frequency signals are present and for intermodulation products from strong off-frequency signals that produce on-frequency interference that masks desired weak signals. The K2 excels in these areas and tops most current rigs on the market. Of course if we had been doing our share of HF Pack operation, we would also have discovered this problem sooner! Operating multiple rigs on the same frequency, all within walking distance of each other, was something that never occurred to us when we designed the K2 and released it in 1999, prior to the big upsurge in HFPacking that occurred in 2000. Thanks to all who did tests and let us know what was going on. We believe someone demonstrated the effect to Wayne at SeaPac 2002, and we apologize for not looking into it then. Modification Details: This applies to all K2s regardless of serial number. 1. Solder a 1N4148 or 1N914 diode (or equivalent) between pins 4 and 6 of the I.F. amp (U12, MC1350). The cathode (banded end) should go to pin 4. Do this on the bottom of the board using very short leads. 2. Solder a second diode of the same type between the same two pins, but with the banded end toward pin 6. Results: With the diodes in place, we've been able to transmit at 100 watts into an antenna just a few feet from the K2 (with its own antenna) with no apparent K2 receiver problems. In these tests the receiving antenna was non resonant. But at the lower power levels used for HF pack operation, there should be no problem even with resonant antennas operating in each other's near field. Fast and slow AGC still work normally. Technical Details: The MC1350 used in the K2 for receive automatic gain control (AGC) can handle up to about 2.5-3.0 V peak-to-peak at its input, pin 4. Beyond this, the AGC becomes ineffective, and the product detector can be overdriven. Normal on-air, on-frequency signals are generally under 200 mV at pin 4 of the I.F. amp, even at "S9 + 40 dB" as indicated on the K2's S-meter. But when you inject an extremely large signal from a nearby transmitter on the same frequency, the signal can go as high as 7 Vpp unless it is hard-limited. The two diodes limit the signal to 1.4 V peak-to-peak. Even when the diodes are conducting, i.e. when the signal is so strong that it looks like a square wave at pin 4, there is no audible signal distortion. This is because the MC1350 is followed by a second crystal filter which removes any harmonic distortion products (i.e. multiples of 4.915 MHz). The diodes appear to have no other side-effects. The modification provides a large increase in on-frequency dynamic range by acting as a clean limiter. Most commercial rigs use multiple I.F. amp stages to achieve this, but this adds a lot of complexity, adds significant IF noise and increases current drain, which is not compatible with the K2's intended use as a battery-powered field radio. It is also unnecessary; the K2's gain distribution is such that the diode limiter will never interfere with received signal quality. If you make the modification, please let us know if it cures any observed audio anomalies as described earlier. If results are universally positive, we'll incorporate the mod into new K2 kits immediately. 73, Wayne, N6KR Eric, WA6HHQ ++++++++++++++++++ Date: Fri, 03 Jan 2003 11:55:16 -0800 From: "John, KI6WX" To: Elecraft Mailing List Subject: [Elecraft] Re: mod to allow K2 to handle huge in-band signals--please try Placing the diodes across the K2 antenna jack would decrease the dynamic range of the K2. Strong broadcast signals and other out of band signals would intermodulate each other producing lots of non-wanted signals right in the amateur bands. Placing the diodes after the Xtal filter guarantees that only inband signals would be strong enough to cause diode conduction. The filter following the IF amp removes some of the distortion caused by the diodes. -John KI6WX I had exactly the same reaction as Gary ... why would anyone transmit on the same frequency in such close proximity to each other? Now with Eric's explanation, I understand the HFpackers' application and it makes sense. However, I don't understand why the protection is applied to the IF amp rather than the antenna jack. Isn't it possible to fry stuff between the antenna and the IF? Back in May during our QRO (KW amps) multi-multi operation from 6Y, we were very concerned about damaging our K2's by harmonics from other bands getting into our RX Antenna jack (with RX antenna connected and in close proximity to all the other antennas). Eric and Wayne suggested a back-to-back diode solution and even a series resistor to clamp the voltage and limit the current at the jack itself. This has always been a concern with commercial rigs and it is common to blow up something in their front end. Perhaps the K2 front end is more robust? 73, Ed - W0YK +++++++++++++++ Date: Fri, 03 Jan 2003 21:28:34 -0800 From: Wayne Burdick Organization: Elecraft To: rondec at easystreet.com Cc: 'Elecraft Mailing List' Subject: Re: [Elecraft] Re: mod to allow K2 to handle huge in-band signals--please try That's correct, Ron. Stations across town won't cause the AGC range to be exceeded. Wayne Ron D'Eau Claire wrote: > > Is that enough to produce the "howling" effect? It sounded like it took > another transmitter practically sitting on the same table ... > > Ron AC7AC > K2 # 1289 > > -----Original Message----- > I have a couple operators that live very close (within a few miles of > me) that use their legal limit amps regularily and I'm always stumbling > across them at 'peg the needle' db over. > > Comments? > > Mike K5PU ++++++++++++++++ Date: Fri, 03 Jan 2003 21:38:57 -0800 From: Wayne Burdick Organization: Elecraft To: robert parker Cc: 'Earl Ritchie' , elecraft at mailman.qth.net Subject: [Elecraft] spitting distance QSOs: why the diodes are needed by HF Packers Robert, Here's the scenario. If you transmit a VERY large signal from a transmitter whose antenna is within a few feet of the K2's antenna, the signal will exceed 3 volts peak-to-peak at pin 4 of the K2's I.F. amplifier, an MC1350. When this happens, the MC1350's AGC range will be exceeded. It takes an admittedly HUGE signal to cause this problem. Normal on-air signals rarely rise above 100 mV at pin 4. But if you did as the HF Pack group does--operate on the same frequency while standing a few feet of another station, with your antennas mutually couple by being in their near fields--you can get signals that exceed the AGC range. (They do this because they all have independent back-pack mounted transceivers, and occasionally they both talk on the HF Pack net frequency even when standing right next to each other. We wanted the K2 to be well-behaved in this situation. Read on.) The new modification, a simple one, is to put two diodes (one in one direction, one in the other) across pins 4 and 6 of the MC1350. This clamps signal input to the 1350 at 1.4 Vpp. Yes, this is a hard limiter, but since the 1350 is followed by another crystal filter, harmonic distortion will be reduced to a level where it is almost inaudible. And of course the diodes have no effect on normal on-air signals--just "extreme" ones, on the same frequency. The net result is that you can literally use the K2 as a monitor receiver for a transmitter at the same station, with both on different antennas just feet apart, even if the transmitter is putting out 100 watts. It won't hurt the K2 even if the diodes are *not* used, but with them there, that collossal signal will sound just like another S9+40 one, even if in reality it is more like S9+80 ;) 73, Wayne N6KR robert parker wrote: > > What kind of diodes are you referring to? How many volts are we talking > about? Wouldn't this de-sense the receiver by quite a bit? Before a > diode even gets into full conduction, it enters a non-linear region. > All sorts of mixing products will be present. I'm guessing the > distortions you are referring to are the mixing products that occur. > Are you thinking of using small signal diodes? What would be the FV? > > Regards, > Robert VE3RPF> +++++++++++++++++ Date: Sat, 4 Jan 2003 12:51:47 +0000 From: "Julian (G4ILO)" Subject: Re: [Elecraft] Re: mod to allow K2 to handle huge in-band signals--please try To: "Elecraft List" Another ham lives a couple of hundred metres from me. I never knew he existed, never mind experienced problems when he transmitted, when I used the K2. I only found out about him when I had an FT-817, which was unusable when he came on the same band. (Actually, at one ham rally, I noticed that the 817 experienced problems on other bands too if someone transmitted from as close as 20m away.) I've reached the conclusion that this mod is unnecessary for me (and most of us.) 73, -- Julian, G4ILO. (K2 #392. Ex-FT817 owner) Homepage: http://www.qsl.net/g4ilo +++++++++++++++++++ Date: Sat, 04 Jan 2003 10:57:19 -0600 To: , "'Elecraft Mailing List'" From: Tom Hammond =?iso-8859-1?Q?N=D8SS?= Subject: RE: [Elecraft] Re: mod to allow K2 to handle huge in-band signals--please try Let's not confuse front end overload with what Wayne's trying to fix. They're not the same. Mere front end overload will probably NOT create the problem the addition of the diodes is designed to protect against. The two radios (one being a K2) must be VERY close together. I suspect that being a hundred YARDS apart, even at 1 KW, might NOT cause this problem. It takes a WHOPPING BIG signal input (as Wayne says, something like -6dB) to cause the specific problem to surface. AND they have to be INSIDE the filter passband to boot. As Julian, G4ILO, recently comments, paraphrasing... "I suspect that few of us really NEED this mod", and he is correct! However, it's too simple to install NOT to do so when the next opportunity presents itself. So, it's not a MUST HAVE mode, but at is a GOOD TO ADD MOD... sometime. 73, Tom N0SS Ron D'Eau Claire wrote: >Is that enough to produce the "howling" effect? It sounded like it took >another transmitter practically sitting on the same table ... ++++++++++++++++ Date: Sun, 05 Jan 2003 07:25:40 -0800 From: lhlousek To: elecraft at mailman.qth.net Cc: g4ilo at qsl.net Subject: [Elecraft] Re: mod to allow K2 to handle huge in-band signals A few ops seem to be missing the point of the huge-signal mod. It is not to help rejection of off-frequency in-band signals. It is to limit the gain for huge signals on the exact frequency you are tuned to. This could occur, for instance, if you were in a 3 way QSO and one of the STNs was very close (5 meters?) to yours...or if you tune to your next door neighbor's QRO signal. Lou W7DZN ++++++++++++++++ rom: "Kenneth A. Christiansen" To: Date: Sun, 5 Jan 2003 12:49:36 -0600 Subject: [Elecraft] Reply on Simple K2 RX Mod Wayne asked if this mod helps and here is my experience. I upgraded all of my K2 except the software to the "B" version and added the two diodes as suggested by Wayne. I belong to the North Dakota Goose River 160 meter SSB net that meets at 14:30 on Sunday mornings. I have three strong local stations and the rest of the stations are 50 to 200 miles away and weak. Every other rig I have owned in the last 20 years had problems with the strong local signals. The K2 would get distorted but with the pre amp off and attenuator on it didn't sound too bad. This morning I turned on the pre amp and set the volume for the net control that was 70 miles away. When the strong local stations came on they were clear as a bell and no louder sounding than the net control. Of course they had a better signal to noise ratio. The recovery of the AGC was instant so I never missed a word of the net control. My Omni-D and IC737 never could handle the 160 meter net and before I added these diodes to the K2 I had to turn off the pre amp, turn on the attenuator and ride the AF gain between the net control and the local stations. I am impressed enough I now plan to order the KPA100, KAT100 and the external cabinet so I can have the best of both worlds. Ken Christiansen W0CZ ++++++++++++++++ Date: Sun, 05 Jan 2003 12:32:52 -0800 From: Wayne Burdick Organization: Elecraft To: "Kenneth A. Christiansen" Cc: elecraft at mailman.qth.net Subject: Re: [Elecraft] Reply on Simple K2 RX Mod "Kenneth A. Christiansen" wrote: > ....added the two diodes as suggested....When the strong local stations came > on they were clear as a bell and no louder sounding than the net control..... > before I added these diodes to the K2 I had to turn off the pre amp, turn on the > attenuator and ride the AF gain between the net control and the local > stations. Thanks for trying the mod, Ken. Your situation (very strong nearby stations on the same freuqency) is a perfect exaxmple of what the diodes are good for. 73, Wayne N6KR +++++++++++++++ From: "Dave Sergeant" To: elecraft at mailman.qth.net Date: Tue, 07 Jan 2003 06:31:26 -0000 Subject: [Elecraft] Re: Simple K2 RX mod handles extremely strong on-frequency signals One other place where this strong signal overload situation occurs here in my own shack. As I have mentioned before, I use a homebrew 5W VXO transmitter for our 60m band. I am using the K2 as the receiver in conjunction with a Datong UC1 upconverter with the K2 on 28MHz. Perhaps because I have taken the lazy way out, and in any case because I need to monitor my own transmission for netting and sidetone purposes, I have not implemented any muting of the receiver in transmit other than switching the antenna over. Hence my s meter goes hard against the end scale (if leds can do that!) when I transmit and I get the overload audio effect Wayne mentions. Cure is easy, I either offtune the receiver 100Hz or so or I switch out the K2 preamp, in any case it is only when I tune spot on my transmit frequency that I hear the howl. I imagine putting in Wayne's diodes would cure the problem, but that means taking the lid off the K2 which is doing an excellent job working stations at the moment! I could equally do the job properly and mute the receiver quite a few db in transmit. So the problem is not such an unusual circumstance as some would lead us to believe. 73s Dave G3YMC dsergeant at iee.org dsergeant at btinternet.com http://www.dsergeant.btinternet.co.uk +++++++++++++++++++ Reply-To: "Indy" From: "Indy" To: Date: Tue, 18 Mar 2003 09:06:08 -0700 Subject: [Elecraft] EXTREME signals It is so difficult to use language in such a way as to be absolutely = clear to everyone. let me try agn. Wayne's back-to-back diode mod across the IF has absolutely no negative = impact upon the radio. The diodes do not conduct until an extremely = strong signal is present. When such is present, the diodes bleed off = the excess signal to ground. the result: Under normal receiving, one will not know the mod has been = done. In the extraordinary case that a nearby station, for example, QRP = to the field, four guys have their QRP radios set up near each other, if = you tune across one, that signal will sound no different than any other = S-9 signal. However, when you tune off of that nearby signal, the AGC = will be pumping with the keying of that extremely strong nearby signal. In my test, I was keying a 100 watt station a few INCHES away from the = K2. I could tune across that signal with impunity, but if I was ten khz = away, the K2 would pump with the keying of that 100 watt adjacent = station making copy of a weak signal impossible, though I could copy a = strong signal that would impact the AGC anyway. However, if the MP were at five watts, a few khz away there was no = longer any pumping ;-) If you take your radio to field day and accidently tune across one of = the other stations there, you would be glad to have the mod installed = ;-) 73, Fred +++++++++++++++++++