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CW to FM Remodulator?
I have been looking into gizmos that improve CW copy. Most are audio
tone detectors that ignore short impulse noise bursts and then regenerate the CW with a keyed tone oscillator. There are several of these designs around and they are all well and good, but I stumbled across something different and was wondering if any of you have had personal experience with it? An October 1971 article in Ham Radio magazine (pg 17) titled "high-performance CW processor for communications receivers", "Frequency modulating the telegraphy signals in your receiver provides an interesting and profitable addition to conventional receiver design". The idea is to sample the last IF of a receiver after as much IF filtering as you can muster, and then using this as the RF input to a FM modulator. The RF/IF is modulated at the audio frequency you like to hear while copying CW. The next step is to frequency multiply the FM modulated signal to increase the bandwidth and up the modulation index. The following step is to treat it like any normal FM receiver IF and run it through a limiter stripping off any amplitude information. The last step is to put the signal into a normal FM discriminator to recover the modulating tone you used. What this is supposed to do is reduce or eliminate QRN (not QRM) from the CW signal making a "quiet" background to copy the CW. Have any of you ever done this and how did it work out for you? - Jeff |
CW to FM Remodulator?
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CW to FM Remodulator?
Andy,
Thanks for your response. I guess I'm looking for a magic pill (though I know better). I agree that the human brain/ear combination is unsurpassed in digging out the really weak ones or the weak signal our of the pile up. My unfortunate situation is my degenerating hearing. I now wear hearing aids in both ears and have difficulty understanding spoken conversation no matter what the volume level. I have always been a CW only operator so the loss of SSB use doesn't seem so bad. I can copy CW much better than listen to voices, but I can never be sure that I can still dig the weak signals out of the mud and I'm pretty sure I have lost some ability to deal with pile-up QRM. So I'm grabbing at straws for gizmos to help me out as my hearing digresses. 73 - Jeff - KA9S On Nov 3, 6:27 am, "AndyS" wrote: wrote: Have any of you ever done this and how did it work out for you? - Jeff Andy writes: Jeff, I haven't tried this method, but one rule of thumb I have always believed in is: "No matter how much you shift, limit, amplify or divide noise, it still ends up as noise" The only effective way I have ever found is to narrow the bandwidth around the signal until the signal starts to get degraded. If done digitally, it can be done by digital processing, but that changes only the technique, not the principle... So, while I would really like to try out some of these "improved methods", I am not confident enough in them to spend a weekend wiring together some hardware.... In my younger years, I probly would've, tho.... Personally, as a CW operator of some 45 years, I have found that my ears/brain does a lot better job of filtering than one would suppose, especially if I am copying some standard message where I sort of know the words the other fellow will send. I only need 2 or 3 letters per word to fill in the pieces with devastating accuracy (grin)..... But, good luck on your efforts. If you do build up something, please come back and post it here. I am sure that there are many experimenters who try something like that if someone thinks it shows promise..... But, please, take some actual measurements. And with S/N ratios of around the 0 db level. My ear copy can still pick those out, and many of the "processors" can't deal with noisy sigs in that region....they tend to fall apart when anything below tangential sensitivity is received.... Andy W4OAH in Eureka, Texas |
CW to FM Remodulator?
Thanks for your response. I guess I'm looking for a magic pill (though
I know better). I agree that the human brain/ear combination is unsurpassed in digging out the really weak ones or the weak signal our of the pile up..... Interesting. I recall reading (in QST, no less) of PSK QSO's where the human ear could NOT even tell a signal was being received, yet the screen copy was "5x9" (if such a phrase even has meaning in this context!-) -- --Myron A. Calhoun. Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified Instructor Certified Instructor for the Kansas Concealed-Carry Handgun license |
CW to FM Remodulator?
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CW to FM Remodulator?
Paul Keinanen wrote: spot frequencies are undetectable due to selective hearing loss. One approach would be to run the CW signal through some kind of FM receive, limiting the carrier, but producing white noise when no signal is available (and you would have to learn to copy negative-CW) or alternatively use some amplitude detector to control a noise gate, i.e. when there is a carrier present, the white noise would get through, with no signal, the headphones would be silent. Just an idea. Paul OH3LWR Andy writes: A hard limiter decreases the signal to noise by about 5.6 db, and that's a mathematical fact... If we know the characteristics of the noise, and the characteristics of when it occurs and the distribution of the energy, then we have "a priori" experience , and it really isn't "noise" anymore, and it can be dealt with --- in some cases rejected - and it's effects on the intelligibiliy of a desired signal made less... There are two characteristics familiar to a radar engineer, which deals with detection of a signal in the presence of noise:: Probability of detection -- Which is the probablility that a signal will be detected in the presence of noise without an error. Probability of false alarm -- This is the probability that detection of a vailid signal will occur when there is, in fact, no signal present. Entire volumes have been written on Pd versus Pfa, since this means life or death to an aircraft (for instance) when a missle lock may happen..... If the other guy detects you and sends his missle before you do, you will probly be dead. If you fire your missles off at a ghost and have none left, you will probly be dead.. In the final analysis, CW is just On-Off signals, much like pulses. The bottom line in all this is that if you KNOW what the signal is going to do you can increase the chances of detecting it properly. For instance, if the signal is repetitive, it can be stored, integrated, differentiated, or accumulated with weighing functions to recover the intelligence ---- "a priori" knowledge is necessary.. If you know what the noise is going to do -- impulse, popcorn, static, broadband, random, etc --- you can use techniques to reduce it hopefully without reducing the signal.. So fancy noise limiters, signal enhancers, and innovative detectors will work on some types of interference, and not on others. To be a universal S/N improvement, it has to work on "unknown" interference... That's what the ham bands are like. It could be AM splatter, white noise, a welder machine, the "woodpecker", car ignition..... whatever.... That's what restricting the bandpass does, usually. Sometimes it makes the S/N worse, but only with "special" types of interference. There ain't no "magic bullet"..... The subject is a LOT more complicated than just any single simple technique for recovering a signal... However ,our ear/brain, with PRACTICE is an adaptive filter. It's amazing how well it works, after someone has been copying CW for a while. Perhaps a microprocessor controlled adaptive filter can be made to approach it, but ADAPTIVE filtering is the only hope that I can see, given the different types of QRM and QRN that I have encountered. My best bet is that someday an adaptive CW filter would be to do as good as my own ear could do today..... It's like looking at a noisy signal on a scope. Someone with a lot of practice can see a valid signal several db lower in S/N than a novice can do.... Sonar operators can do the same..... "Waterfall" displays simply integrate the signal and noise over time, which is similar mathematically to restricting the bandwidth as far as the S/N "enhancement" properties.... The characteristics of the signal and the characteristics of the noise are known beforehand, and that is used, via the display, to increase the Pd and decrease the Pfa.... End of rant..... I need a beer. Andy W4OAH (retired communications and RADAR systems engineer and ham for about 45 years, or so.... hell, I don't remember any more ) |
CW to FM Remodulator?
A hard limiter decreases the signal to noise by about 5.6 db, and
that's a mathematical fact... ___________________ Andy, Can you point us to a reference document that explains this "mathematical fact"?. Joe W3JDR |
CW to FM Remodulator?
W3JDR wrote: Can you point us to a reference document that explains this "mathematical fact"?. Joe W3JDR Andy writes: No. It was about 25 years ago when I was designing the TI2100 FM Marine Transceiver for Texas Intruments, which was my last commercial Fm unit. ( Some two meter stuff since then as home projects, tho ) It was info gleaned from several technical papers and I don't for the life of me remember which ones. I got a limiter and noise source and checked it in the lab, at the time, and it seemed consistent. I don't remember exactly, but I think I combined noise with a signal and amplified the hell out of it, and then put in an attenuator to get it back down and measured the S/n in a receiver. Then I put a limiter in between the amp and the attenuator, and decreased the atten to get the same level into the receiver, and measured the S/N again. While I didn't get exactly 5.6 db, I remember it was close enough to believe that the mathematical derivation was confirmed ( in my mind ) and that my measurement error was probly due to my own imprecision in the experiment.. Anyway, I moved on..... and it settled the question on whether hard limiting "improves" things..... Sorry, but that's just one of the numbers that stay with a guy, like -174 dbm (God's noise) , and 8.5 db ( tangential sensitivity), and 10Log(bw), and 3.14..... Heck, I forget my phone number from time to time, but numbers that I have used for most of my life stay with me..... And, being in the profession, I have, at some time or another, verified them myself in the lab when the opportunity permitted.. I take that back.... I have never verified PI....... I hope I haven't been too gullible..... :))) So, I regret not having the mental acuity any more to jot down some derivations for you. But , if they are not correct, there's a lot of products on the market which I built whose development was a wild fluke.... If you want to pursue it yourself, I would suggest a few texts that have guided me... Skolnik's Radar Handbook ( the smal one, not the BIG one --- I call it " small Skolnik" ) has a LOT of tech info that is presented in a level only slightly greater than the ARRL handbook. Also "Principles of FM" -- damn, I don't remember the author.....but how many could there be ? :)))) I might have it in my workshop. If I run across it I'll post it here. Well, good luck. Some knowledge can be passed on as a proven fact and one needs look no farther..... like PI, for instance... Other is in conflict with what someone thinks to be "how things work", and doubt is in the air.... No matter -- I was the same way, when I had the energy to pursue it..... Good on ya' , mate, Andy W4OAH in Eureka, Texas |
CW to FM Remodulator?
So it's not a "mathematical fact" in the sense that any of us can look it up
and see how it was derived, it's your recollection of something you heard and vaguely remember convincing yourself it could be true, right? While we're at it, what's the significance of your reference to "8.5 db ( tangential sensitivity)"? Joe W3JDR "AndyS" wrote in message oups.com... W3JDR wrote: Can you point us to a reference document that explains this "mathematical fact"?. Joe W3JDR Andy writes: No. It was about 25 years ago when I was designing the TI2100 FM Marine Transceiver for Texas Intruments, which was my last commercial Fm unit. ( Some two meter stuff since then as home projects, tho ) It was info gleaned from several technical papers and I don't for the life of me remember which ones. I got a limiter and noise source and checked it in the lab, at the time, and it seemed consistent. I don't remember exactly, but I think I combined noise with a signal and amplified the hell out of it, and then put in an attenuator to get it back down and measured the S/n in a receiver. Then I put a limiter in between the amp and the attenuator, and decreased the atten to get the same level into the receiver, and measured the S/N again. While I didn't get exactly 5.6 db, I remember it was close enough to believe that the mathematical derivation was confirmed ( in my mind ) and that my measurement error was probly due to my own imprecision in the experiment.. Anyway, I moved on..... and it settled the question on whether hard limiting "improves" things..... Sorry, but that's just one of the numbers that stay with a guy, like -174 dbm (God's noise) , and 8.5 db ( tangential sensitivity), and 10Log(bw), and 3.14..... Heck, I forget my phone number from time to time, but numbers that I have used for most of my life stay with me..... And, being in the profession, I have, at some time or another, verified them myself in the lab when the opportunity permitted.. I take that back.... I have never verified PI....... I hope I haven't been too gullible..... :))) So, I regret not having the mental acuity any more to jot down some derivations for you. But , if they are not correct, there's a lot of products on the market which I built whose development was a wild fluke.... If you want to pursue it yourself, I would suggest a few texts that have guided me... Skolnik's Radar Handbook ( the smal one, not the BIG one --- I call it " small Skolnik" ) has a LOT of tech info that is presented in a level only slightly greater than the ARRL handbook. Also "Principles of FM" -- damn, I don't remember the author.....but how many could there be ? :)))) I might have it in my workshop. If I run across it I'll post it here. Well, good luck. Some knowledge can be passed on as a proven fact and one needs look no farther..... like PI, for instance... Other is in conflict with what someone thinks to be "how things work", and doubt is in the air.... No matter -- I was the same way, when I had the energy to pursue it..... Good on ya' , mate, Andy W4OAH in Eureka, Texas |
CW to FM Remodulator?
Agilent App Note says: Tangential sensitivity is the lowest input signal power level for which the detector will have an 8 dB signal-to-noise ratio at the output of a test video amplifier. http://www.home.agilent.com/upload/c...orOverview.pdf Bill W0IYH "W3JDR" wrote in message news:UUo3h.2525$qJ6.2375@trndny07... So it's not a "mathematical fact" in the sense that any of us can look it up and see how it was derived, it's your recollection of something you heard and vaguely remember convincing yourself it could be true, right? While we're at it, what's the significance of your reference to "8.5 db ( tangential sensitivity)"? Joe W3JDR "AndyS" wrote in message oups.com... W3JDR wrote: Can you point us to a reference document that explains this "mathematical fact"?. Joe W3JDR Andy writes: No. It was about 25 years ago when I was designing the TI2100 FM Marine Transceiver for Texas Intruments, which was my last commercial Fm unit. ( Some two meter stuff since then as home projects, tho ) It was info gleaned from several technical papers and I don't for the life of me remember which ones. I got a limiter and noise source and checked it in the lab, at the time, and it seemed consistent. I don't remember exactly, but I think I combined noise with a signal and amplified the hell out of it, and then put in an attenuator to get it back down and measured the S/n in a receiver. Then I put a limiter in between the amp and the attenuator, and decreased the atten to get the same level into the receiver, and measured the S/N again. While I didn't get exactly 5.6 db, I remember it was close enough to believe that the mathematical derivation was confirmed ( in my mind ) and that my measurement error was probly due to my own imprecision in the experiment.. Anyway, I moved on..... and it settled the question on whether hard limiting "improves" things..... Sorry, but that's just one of the numbers that stay with a guy, like -174 dbm (God's noise) , and 8.5 db ( tangential sensitivity), and 10Log(bw), and 3.14..... Heck, I forget my phone number from time to time, but numbers that I have used for most of my life stay with me..... And, being in the profession, I have, at some time or another, verified them myself in the lab when the opportunity permitted.. I take that back.... I have never verified PI....... I hope I haven't been too gullible..... :))) So, I regret not having the mental acuity any more to jot down some derivations for you. But , if they are not correct, there's a lot of products on the market which I built whose development was a wild fluke.... If you want to pursue it yourself, I would suggest a few texts that have guided me... Skolnik's Radar Handbook ( the smal one, not the BIG one --- I call it " small Skolnik" ) has a LOT of tech info that is presented in a level only slightly greater than the ARRL handbook. Also "Principles of FM" -- damn, I don't remember the author.....but how many could there be ? :)))) I might have it in my workshop. If I run across it I'll post it here. Well, good luck. Some knowledge can be passed on as a proven fact and one needs look no farther..... like PI, for instance... Other is in conflict with what someone thinks to be "how things work", and doubt is in the air.... No matter -- I was the same way, when I had the energy to pursue it..... Good on ya' , mate, Andy W4OAH in Eureka, Texas |
CW to FM Remodulator?
William E. Sabin wrote: Agilent App Note says: Tangential sensitivity is the lowest input signal power level for which the detector will have an 8 dB signal-to-noise ratio at the output of a test video amplifier. http://www.home.agilent.com/upload/c...orOverview.pdf Bill W0IYH Andy writes: I used 8.5 db, tho , as you know, it has a LOT to do with who is making the measurement and positioning the pulse on the scope.... I'm not sure that I am proficient enough to position an 8db pedestal to within a half db accuracy...... Agilent probly used a math derivation. I have seen it called out at several numbers, tho 8.5 is the one I always used.... I will probly start using 8 db if the Agilent App note says so since there is always somebody wanting to get a "reference", and it's much easier to just give them the App Note source than try to explain.... Once they actually see the scope presentation, they will understand the problem... Andy in Eureka |
CW to FM Remodulator?
W3JDR wrote: So it's not a "mathematical fact" in the sense that any of us can look it up and see how it was derived, it's your recollection of something you heard and vaguely remember convincing yourself it could be true, right? Andy replies: That's right. But just because I don't remember how to derive it, or can give you the name of the paper that I read 25-30 years ago, it doesn't mean it's wrong. So I would suggest you do your own research.... Whether it is true, or not true, IS a mathematical fact, and if you are capable of understanding the math proof, you are probably also capable of proving or disproving it yourself on paper. Me, hell , I always had a good lab where I could try things out if I had doubt..... It is probly easier, if you have access to a good lab, to rig up an experiment to find out..... That way you wouldn't have to bother with newsgroups to learn about this stuff..... I would encourage you to try. If you come up with a good answer you can be proud of, post it back here for us all to see. I am sure many people here would be interested , since lots of time is often wasted coming up with circuits whose purpose is doomed from the start.... such as using "hard limiters" to improve the SNR........ As far as the "tangential sensitivity", you can probly do a google search and learn all about it.... If you haven't run into it yet, you probably don't deal with OOK pulses like radar and stuff. There's no shame in that..... :)))) Andy in Eureka, Texas W4OAH |
CW to FM Remodulator?
Andy writes
Well, I saved you some time by googling it myself. It took about 10 seconds to be able to cut and paste the definition I have been talking about.... Note that in this paper it says TSS is eight db +/- one db .... and depends on several system factors, including the actual detector used... From experience, the detector type, and the absolute signal level it is detecting, is a BIG DEAL. It needs to operate in the "perfect diode" region . Doing the detection in the square law region buggers up the measurement. That +/- one db is due to the accuracy that one operator can set up the scope versus the next operator that comes along. I use 8.5 db, and I DARE anyone to set the same measurement up 10 times in a row and get the same answer to less than +/- one db. Agilent probly uses a math derivation to get 8 db to accomplish some specific criteria, and did not specify a detector type, hence it is a "mathematically perfect " answer. Furthermore, they use a different definition, but ORIGINALLY it was bases on the FIRST SENTENCE in the cut and paste below... The Agilent App Note that Bill Sabin refers to also came up on the first page of hits, and you can go read it for yourself. I hope this satisfies your curiousity. I had nothing better to do today..... Now, I'm outta beer, and Jeff Foxworthy is on TV, so 73s Andy W40AH ***********CUT AND PASTE FROM GOOGLE HIT ************************* TANGENTIAL SENSITIVITY Tangential sensitivity (TSS) is the point where thetop of the noise level with no signal applied is level with thebottom of the noise level on a pulse as shown in Figure 6. Itcan be determined in the laboratory by varying the amplitudeof the input pulse until the stated criterion is reached, or byvarious approximation formulas.The signal power is nominally 8±1 dB above thenoise level at the TSS point. TSS depends on the RFbandwidth, the video bandwidth, the noise figure, and the detector characteristic. TSS is generally a characteristic associated with receivers (or RWRs), however the TSS does not necessarilyprovide a criterion for properly setting the detection threshold. If the threshold is set to TSS, then the false alarm rate israther high. Radars do not operate at TSS. Most require a more positive S/N for track ( 10 dB) to reduce false detectionon noise spikes. SENSITIVITY CONCLUSION When all factors effecting system sensitivity are considered, the designer has little flexibility in the choice ofreceiver parameters. Rather, the performance requirements dictate the limit of sensitivity which can be implemented by theEW receiver.1. Minimum Signal-to-Noise Ratio (S/N) - Set by the accuracy which you want to measure signal parameters and by thefalse alarm requirements.2. Total Receiver Noise Figure (NF) - Set by available technology and system constraints for RF front end performance.3. Equivalent Noise Bandwidth (B ) - Set by minimum pulse width or maximum modulation bandwidth needed toNaccomplish the system requirements. A choice which is available to the designer is the relationship of pre- (B ) and post-IFdetection (B ) bandwidth. The most affordable approach is to set the post-detection filter equal to the reciprocal of theVminimum pulse width, then choose the pre-detection passband to be as wide as the background interference environmentwill allow. Recent studies suggest that pre-detection bandwidths in excess of 100 MHz will allow significant loss of signalsdue to "pulse-on-pulse" conditions. 4. Antenna Gain (G) - Set by the needed instantaneous FOV needed to support the system time to intercept requirements. *****************END OF CUT AND PASTE *********************************** |
CW to FM Remodulator?
There are so many variables and approximations involved that it would seem
difficult to be very precise about 8.0 or 8.5 dB S/N ratio or some other number. The decision probably involves the type of signal involved. 8.0 dB in one application gives the minimum acceptible performance for one kind of signal. A more critical system might want more than 8.0 dB to achieve a better bit error rate, for example. Bill W0IYH "AndyS" wrote in message oups.com... William E. Sabin wrote: Agilent App Note says: Tangential sensitivity is the lowest input signal power level for which the detector will have an 8 dB signal-to-noise ratio at the output of a test video amplifier. http://www.home.agilent.com/upload/c...orOverview.pdf Bill W0IYH Andy writes: I used 8.5 db, tho , as you know, it has a LOT to do with who is making the measurement and positioning the pulse on the scope.... I'm not sure that I am proficient enough to position an 8db pedestal to within a half db accuracy...... Agilent probly used a math derivation. I have seen it called out at several numbers, tho 8.5 is the one I always used.... I will probly start using 8 db if the Agilent App note says so since there is always somebody wanting to get a "reference", and it's much easier to just give them the App Note source than try to explain.... Once they actually see the scope presentation, they will understand the problem... Andy in Eureka |
CW to FM Remodulator?
William E. Sabin wrote: There are so many variables and approximations involved that it would seem difficult to be very precise about 8.0 or 8.5 dB S/N ratio or some other number. The decision probably involves the type of signal involved. 8.0 dB in one application gives the minimum acceptible performance for one kind of signal. A more critical system might want more than 8.0 dB to achieve a better bit error rate, for example. Bill W0IYH Andy comments: Yeah, .... the way I used it was to adjust the level to tangential, then increase the signal level with an attenuator to get the S/n I wanted,,,,, You know how hard it is to put together the stuff to measure the S/N of a pulsed signal ? Well, by setting it up to get 'tangential" then messing with the attenuators to get what I wanted, I could, with reasonable accuracy, set up a measurement for 13 o 14 db S/n , or whatever, to take the Pfa measurement.... ( Those are the S/N levels that reasonable Pfa and Pd numbers occur ) Remember, Tangential Sensitivity was defined 50 years before Agilent was in existence... Maybe more....... It allowed a person with a scope to make reasonably accurate measurements, and refine their systems to take advantage of it, 50 years before the simulators, and math, dealt conclusively with the issue. Hewlitt was using light bulbs to make audio oscillators when RADAR engineers were finding German airplanes.... If my use of the HISTORICAL term has confused these kids that just got their BSEE,...... I don't really care.... Why do we respond to these kids ? Probably because we both are retired and bored.....If they want to prove we are stupid, ... hell,.... it's OK with me..... I admit readily to having only a fraction of the math ability that I would need to understand all of the things I know to be true... ..... Including Pi............. Andy in Eureka, W4OAH, over-the-hill on Lake Richland-Chambers |
CW to FM Remodulator?
) writes:
I have been looking into gizmos that improve CW copy. Most are audio tone detectors that ignore short impulse noise bursts and then regenerate the CW with a keyed tone oscillator. There are several of these designs around and they are all well and good, but I stumbled across something different and was wondering if any of you have had personal experience with it? An October 1971 article in Ham Radio magazine (pg 17) titled "high-performance CW processor for communications receivers", "Frequency modulating the telegraphy signals in your receiver provides an interesting and profitable addition to conventional receiver design". I finally dug out the article. I haven't a clue to its worth, but I don't recall that sort of scenario coming up in other places (while the one about good filtering and using a detected voltage to key an audio oscillator came up a number of times), which may mean nobody found it useful, or nobody else could be bothered replicating the circuitry. What you want to do is check a few issues later, to see if there were any letters related to it in the "Comments" section. It's interesting that the November 1971 issue of Ham Radio had an article entitled "Weak Signal Reception in CW Receivers", which used nothing cutting edge but was a summation of various things one could do to improve reception. Go back a few years, and you'd see an article or two about "under the noise" CW reception, which of course amounted to PLLs driving some indicator, but at the time were pretty out of the ordinary since IC phase locked loops hadn't arrived. I suspect to evaluate this, one really needs to dig through the magazines and look at all the schemes. Ham Radio seemed to have a fair number in the first decade or so. Something about that article you reference reminds me of something in an article about a Hallicrafter's diversity receiver, I forget the issue but it likely was in one of the annual October (or was it November?) receiver issues. About '74 or '75, someone named Hilbert had some scheme that involved active audio filters, but there was more to it than I can think of at the moment. (I seem to recall there was some "stereo" effect, in that different signals were fed to each ear, which in itself may be worth pursuing. Use one of those schemes with the detectors to key the audio oscillator, but also include some of the signal from the receiver output, so you get the noise and the actual signal in it.) Wait, I think it must have been "Hildreth", who also wrote this article you reference. In which case, you can look up what he did later; did he see some fault in this system, or did he just realize it was easier to implement something at audio? Someone mentioned in this thread something that hinted at Coherent CW, which sync'd up the time and frequency at both ends to allow for good filters and fairly deep in the noise CW reception. If you know when and where to look, then it's easier to gather whether there's a signal there or not. By looking at the various schemes people have come up with, one can get a better idea of each one's worth better than looking at each one by itself. Some of the schemes likely panned out to be duds. Others required too much circuitry, at least at the time of the articles, so nobody wanted to replicate them. And then likely they've been forgotten, because otherwise more recent technology advances make the past easier (look at how phasing SSB returned to some level of popularity when ICs and digital audio came along). Others, like Coherent CW had the disadvantage that they were a whole system, rather than a processor, so you needed matching stations at both ends in order for it all to work. You can at least look over the cumulative index of Ham Radio magazine, since someone has put it (or at least some version of it) online at http://webhome.idirect.com/~griffith/hrindex.htm Ham Radio seemed to be the place to look for that sort of out of the ordinary schemes. The idea is to sample the last IF of a receiver after as much IF filtering as you can muster, and then using this as the RF input to a FM modulator. The RF/IF is modulated at the audio frequency you like to hear while copying CW. The next step is to frequency multiply the FM modulated signal to increase the bandwidth and up the modulation index. The following step is to treat it like any normal FM receiver IF and run it through a limiter stripping off any amplitude information. The last step is to put the signal into a normal FM discriminator to recover the modulating tone you used. What this is supposed to do is reduce or eliminate QRN (not QRM) from the CW signal making a "quiet" background to copy the CW. It gets the on/off of the keying, but yes it limits the signals. So widely varying signals will be at about the same level (though that may not always be a feature), and any QRN will be limited too. In thinking about it, I'm not so sure it's all that distant from the schemes that detect the CW and use that to key an audio oscillator. The bulk of the circuitry is not there to improve the CW reception, but to get that needed FM signal, with the incoming CW signal as the "carrier". Again, the more I think about it the more I think his later audio based schemes may implement a similar concept. Michael VE2BVW |
CW to FM Remodulator?
"AndyS" wrote:
My ear copy can still pick those out, and many of the "processors" can't deal with noisy sigs in that region. They tend to fall apart when anything below tangential sensitivity is received. Andy W4OAH in Eureka, Texas Leif Asbrink, SM5BSZ, has some remarkable plots of moonbounce using 25W emitted from single 10 element yagi: http://www.nitehawk.com/sm5bsz/kk7ka/kk7ka.htm He gives his personal experience listening to weak Morse code he http://www.nitehawk.com/sm5bsz/weakcom.htm His entire site is packed with very useful information on receiving weak signals, including his Linrad setup: http://www.nitehawk.com/sm5bsz/weakcom.htm Well worth exploring if you haven't had the chance. Regards, Mike Monett Antiviral, Antibacterial Silver Solution: http://silversol.freewebpage.org/index.htm SPICE Analysis of Crystal Oscillators: http://silversol.freewebpage.org/spice/xtal/clapp.htm Noise-Rejecting Wideband Sampler: http://www3.sympatico.ca/add.automat...pler/intro.htm |
CW to FM Remodulator?
About tone detection in noise with biaural hearing:
In an ancient magazine article (If I recall correctly it was "dubus") there was a scientific reference mentioned. The signal must be delayed AND a difference in power level feeding it to the ears. They mentioned it is possible to get 3dB improvement with this methode. Of course, detecting "submarine" is a good search for Google I think. They do worn stereo head phones. Signal detection in general is how the processing is done: - online or offline. Offline gives the added benefit of knowing all the signal in advance. - bit-speed needed (Here the brain is bounded to limits) - power level achievable (over background) You can't beat the modern detection systems doing DSP algorithms. - Henry |
CW to FM Remodulator?
Mike Monett wrote: "AndyS" wrote: My ear copy can still pick those out, and many of the "processors" can't deal with noisy sigs in that region. They tend to fall apart when anything below tangential sensitivity is received. Andy W4OAH in Eureka, Texas Leif Asbrink, SM5BSZ, has some remarkable plots of moonbounce using 25W emitted from single 10 element yagi: http://www.nitehawk.com/sm5bsz/kk7ka/kk7ka.htm He gives his personal experience listening to weak Morse code he http://www.nitehawk.com/sm5bsz/weakcom.htm His entire site is packed with very useful information on receiving weak signals, including his Linrad setup: http://www.nitehawk.com/sm5bsz/weakcom.htm Well worth exploring if you haven't had the chance. Regards, Mike Monett Antiviral, Antibacterial Silver Solution: http://silversol.freewebpage.org/index.htm SPICE Analysis of Crystal Oscillators: http://silversol.freewebpage.org/spice/xtal/clapp.htm Noise-Rejecting Wideband Sampler: http://www3.sympatico.ca/add.automat...pler/intro.htm Andy comments:\ Mike, The comments you have attributed to me in the above post are in error. You mistakenly copied another's comments and put my name in front of it....... Not a problem for me,but I get into enough trouble on my own without having to catch any hell for other people... (grin) Andy W4OAH |
CW to FM Remodulator?
"AndyS" wrote:
Mike Monett wrote: "AndyS" wrote: My ear copy can still pick those out, and many of the "processors" can't deal with noisy sigs in that region. They tend to fall apart when anything below tangential sensitivity is received. Andy W4OAH in Eureka, Texas [...] Andy comments:\ Mike, The comments you have attributed to me in the above post are in error. You mistakenly copied another's comments and put my name in front of it....... Not a problem for me,but I get into enough trouble on my own without having to catch any hell for other people... (grin) Andy W4OAH Andy, here is your complete post with the original formatting: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~ Subject: CW to FM Remodulator? Date: 3 Nov 2006 04:27:39 -0800 From: "AndyS" Newsgroups: rec.radio.amateur.homebrew wrote: Have any of you ever done this and how did it work out for you? - Jeff Andy writes: Jeff, I haven't tried this method, but one rule of thumb I have always believed in is: "No matter how much you shift, limit, amplify or divide noise, it still ends up as noise" The only effective way I have ever found is to narrow the bandwidth around the signal until the signal starts to get degraded. If done digitally, it can be done by digital processing, but that changes only the technique, not the principle... So, while I would really like to try out some of these "improved methods", I am not confident enough in them to spend a weekend wiring together some hardware.... In my younger years, I probly would've, tho.... Personally, as a CW operator of some 45 years, I have found that my ears/brain does a lot better job of filtering than one would suppose, especially if I am copying some standard message where I sort of know the words the other fellow will send. I only need 2 or 3 letters per word to fill in the pieces with devastating accuracy (grin)..... But, good luck on your efforts. If you do build up something, please come back and post it here. I am sure that there are many experimenters who try something like that if someone thinks it shows promise..... But, please, take some actual measurements. And with S/N ratios of around the 0 db level. My ear copy can still pick those out, and many of the "processors" can't deal with noisy sigs in that region....they tend to fall apart when anything below tangential sensitivity is received.... Andy W4OAH in Eureka, Texas ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~ It appears everything after "Andy writes:" was written by you. If it was written by someone else, you did not indicate so. It also appears the Period key on your keyboard is sticking and producing too many periods. I have gathered the extra ones and put them here in case you run out and need some in the futu ... ... ... .... .... ... ... Regards, Mike Monett Antiviral, Antibacterial Silver Solution: http://silversol.freewebpage.org/index.htm SPICE Analysis of Crystal Oscillators: http://silversol.freewebpage.org/spice/xtal/clapp.htm Noise-Rejecting Wideband Sampler: http://www3.sympatico.ca/add.automat...pler/intro.htm |
CW to FM Remodulator?
"Mike Monett" wrote in message
... Andy writes: Jeff, I haven't tried this method, but one rule of thumb I have always believed in is: "No matter how much you shift, limit, amplify or divide noise, it still ends up as noise" The only effective way I have ever found is to narrow the bandwidth around the signal until the signal starts to get degraded. If done digitally, it can be done by digital processing, but that changes only the technique, not the principle... Actually, digital processing CAN change the principle. FIR filters and similar digital filters do provide a way to reduce the bandwidth digitally, and as you point out, reducing the bandwidth reduces the noise. This helps the same way a crystal filter helps, except perhaps giving a little more flexibility. However, many modern radios have digital noise reduction which is quite a different animal. With digital noise reduction, the incoming signal is analyzed to identify noise components and differentiate them from signal components. The noise is then subtracted from the signal. While this isn't perfect, it can result in quite a substantial reduction in noise without reducing bandwidth. The combination of bandwidth reduction and digital noise reduction can greatly improve readability. I agree that other typical analog techniques don't really affect things all that much, but I'm not convinced that the same techniques that are used for noise reduction digitally couldn't be duplicated with analog components; I've just never seen it done, and without some considerable creativity on the part of the designer it will be quite complex. One analog behavior I have noticed that helps, at least with CW. For passive balanced mixers, there is a diode threshold voltage required for the signal to be detected. If the gain is managed so that the noise level is very close to this threshold, the signal to noise ratio seems to be improved (although I have not personally validated this analytically). Of course, if the signal is at the noise level this doesn't help, and if the signal is barely above the noise level the adjustment is too critical to be a great help, but where the signal has enough headroom, it can pretty dramatically improve the pleasure of listening to a weak signal. ... |
CW to FM Remodulator?
For passive balanced mixers, there is a diode threshold voltage required for
the signal to be detected. ------------------------------------------- This is news to me - can you please elaborate? Joe W3JDR "xpyttl" wrote in message ... "Mike Monett" wrote in message ... Andy writes: Jeff, I haven't tried this method, but one rule of thumb I have always believed in is: "No matter how much you shift, limit, amplify or divide noise, it still ends up as noise" The only effective way I have ever found is to narrow the bandwidth around the signal until the signal starts to get degraded. If done digitally, it can be done by digital processing, but that changes only the technique, not the principle... Actually, digital processing CAN change the principle. FIR filters and similar digital filters do provide a way to reduce the bandwidth digitally, and as you point out, reducing the bandwidth reduces the noise. This helps the same way a crystal filter helps, except perhaps giving a little more flexibility. However, many modern radios have digital noise reduction which is quite a different animal. With digital noise reduction, the incoming signal is analyzed to identify noise components and differentiate them from signal components. The noise is then subtracted from the signal. While this isn't perfect, it can result in quite a substantial reduction in noise without reducing bandwidth. The combination of bandwidth reduction and digital noise reduction can greatly improve readability. I agree that other typical analog techniques don't really affect things all that much, but I'm not convinced that the same techniques that are used for noise reduction digitally couldn't be duplicated with analog components; I've just never seen it done, and without some considerable creativity on the part of the designer it will be quite complex. One analog behavior I have noticed that helps, at least with CW. For passive balanced mixers, there is a diode threshold voltage required for the signal to be detected. If the gain is managed so that the noise level is very close to this threshold, the signal to noise ratio seems to be improved (although I have not personally validated this analytically). Of course, if the signal is at the noise level this doesn't help, and if the signal is barely above the noise level the adjustment is too critical to be a great help, but where the signal has enough headroom, it can pretty dramatically improve the pleasure of listening to a weak signal. .. |
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