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Single Sideband FM
Can anyone direct me to some good understandable references on single
sideband frequency modulation? I have no real practical reason for wanting to know about this. It is interesting to me in a "mathetical" sort of way. Of course, that is dangerous for me because my brain gets very stubborn when I try to do math. Such ideas as "negative frequency" kind of send my mental faculties into total shutdown. But I read schematic very well. It is a visual language I can usually understand. Seems like years ago there was an article on SSB FM in Ham Radio. That would probably be a good start. If anyone can send me a copy of that article I would be much appreciative. Thanks in advance Bruce kk7zz www.elmerdude.com |
No such thing that I have ever heard of.
For an easy explanation of the modulation modes -- see URL: http://www.williamson-labs.com/480_mod.htm Perhaps you were thinking of commercial FM broadcasting -- there is a baseband FM signal, a 19kHz pilot carrier, and a DSB suppressed signal at 38kHz. All this yields stereo FM Baseband is left plus right channel the DSB signal is left minus right -- so after demodulation, adding them gives Left and Right channels -- 73 From The KeyBoard In The Wilderness "Bruce Kizerian" wrote in message om... Can anyone direct me to some good understandable references on single sideband frequency modulation? I have no real practical reason for wanting to know about this. It is interesting to me in a "mathetical" sort of way. Of course, that is dangerous for me because my brain gets very stubborn when I try to do math. Such ideas as "negative frequency" kind of send my mental faculties into total shutdown. But I read schematic very well. It is a visual language I can usually understand. Seems like years ago there was an article on SSB FM in Ham Radio. That would probably be a good start. If anyone can send me a copy of that article I would be much appreciative. Thanks in advance Bruce kk7zz www.elmerdude.com |
No such thing that I have ever heard of.
For an easy explanation of the modulation modes -- see URL: http://www.williamson-labs.com/480_mod.htm Perhaps you were thinking of commercial FM broadcasting -- there is a baseband FM signal, a 19kHz pilot carrier, and a DSB suppressed signal at 38kHz. All this yields stereo FM Baseband is left plus right channel the DSB signal is left minus right -- so after demodulation, adding them gives Left and Right channels -- 73 From The KeyBoard In The Wilderness "Bruce Kizerian" wrote in message om... Can anyone direct me to some good understandable references on single sideband frequency modulation? I have no real practical reason for wanting to know about this. It is interesting to me in a "mathetical" sort of way. Of course, that is dangerous for me because my brain gets very stubborn when I try to do math. Such ideas as "negative frequency" kind of send my mental faculties into total shutdown. But I read schematic very well. It is a visual language I can usually understand. Seems like years ago there was an article on SSB FM in Ham Radio. That would probably be a good start. If anyone can send me a copy of that article I would be much appreciative. Thanks in advance Bruce kk7zz www.elmerdude.com |
W7TI ) writes:
On 21 Oct 2003 10:06:26 -0700, (Bruce Kizerian) wrote: Seems like years ago there was an article on SSB FM in Ham Radio. __________________________________________________ _______ In the April issue? :-) Bill, W7TI No, the January 1977 issue of Ham Radio magazine, it was the cover article. (I knew it was a January issue, and around that time, but I used the online index at http://webhome.idirect.com/~griffith/hrindex.htm to find it, though it took a few tries to figure out which category it was in.) I could never really make sense of the article. My recollection is that it didn't do a good job on conveying the theory to the average ham, or even the purpose of such a mode, and then went into some stuff about implementing using some circuits out of a textbook and not a complete circuit. One or the other would have been fine, but in straddling both theory and practical it did neither well. Michael VE2BVW |
W7TI ) writes:
On 21 Oct 2003 10:06:26 -0700, (Bruce Kizerian) wrote: Seems like years ago there was an article on SSB FM in Ham Radio. __________________________________________________ _______ In the April issue? :-) Bill, W7TI No, the January 1977 issue of Ham Radio magazine, it was the cover article. (I knew it was a January issue, and around that time, but I used the online index at http://webhome.idirect.com/~griffith/hrindex.htm to find it, though it took a few tries to figure out which category it was in.) I could never really make sense of the article. My recollection is that it didn't do a good job on conveying the theory to the average ham, or even the purpose of such a mode, and then went into some stuff about implementing using some circuits out of a textbook and not a complete circuit. One or the other would have been fine, but in straddling both theory and practical it did neither well. Michael VE2BVW |
Michael Black wrote:
No, the January 1977 issue of Ham Radio magazine, it was the cover article. I could never really make sense of the article. My recollection is that it didn't do a good job on conveying the theory to the average ham, or even the purpose of such a mode, I would think it would be for the same reason as SSB for AM? To achieve half the bandwidth utilization for a given signal? (But at the expense of 3dB poorer SNR...) I suspect that going through the math for the 'direct generation' means of SSB-FM would be pretty gnarly, but the 'first generate FM, then add a sharp filter' approach should work (although this will really generate vestigal sideband modulation...). ---Joel Kolstad |
Michael Black wrote:
No, the January 1977 issue of Ham Radio magazine, it was the cover article. I could never really make sense of the article. My recollection is that it didn't do a good job on conveying the theory to the average ham, or even the purpose of such a mode, I would think it would be for the same reason as SSB for AM? To achieve half the bandwidth utilization for a given signal? (But at the expense of 3dB poorer SNR...) I suspect that going through the math for the 'direct generation' means of SSB-FM would be pretty gnarly, but the 'first generate FM, then add a sharp filter' approach should work (although this will really generate vestigal sideband modulation...). ---Joel Kolstad |
Joel Kolstad wrote:
I would think it would be for the same reason as SSB for AM? To achieve half the bandwidth utilization for a given signal? (But at the expense of 3dB poorer SNR...) ^^^^^^^^^^^^^^ It occurs to me that this part of my comment is non-sense given the machinations one has to go through to compute SNR through an FM channel anyway. Sorry. One wonders what the SNR degradation _would_ be, however. ---Joel Kolstad |
Joel Kolstad wrote:
I would think it would be for the same reason as SSB for AM? To achieve half the bandwidth utilization for a given signal? (But at the expense of 3dB poorer SNR...) ^^^^^^^^^^^^^^ It occurs to me that this part of my comment is non-sense given the machinations one has to go through to compute SNR through an FM channel anyway. Sorry. One wonders what the SNR degradation _would_ be, however. ---Joel Kolstad |
"Joel Kolstad" ) writes:
Michael Black wrote: No, the January 1977 issue of Ham Radio magazine, it was the cover article. I could never really make sense of the article. My recollection is that it didn't do a good job on conveying the theory to the average ham, or even the purpose of such a mode, I would think it would be for the same reason as SSB for AM? To achieve half the bandwidth utilization for a given signal? (But at the expense of 3dB poorer SNR...) It's been quite a few years since I looked at the article. There was just something about the article that seemed like I'd been dropped into something. Maybe the style was different from most articles in the magazine, maybe because it didn't really seem to be a practical article. There just seemed to be something missing. Yes, it would take up less space, but then why not go to some other mode? It's the only time I've seen something on the subject, and I think it could have better been handled. Michael VE2BVW |
"Joel Kolstad" ) writes:
Michael Black wrote: No, the January 1977 issue of Ham Radio magazine, it was the cover article. I could never really make sense of the article. My recollection is that it didn't do a good job on conveying the theory to the average ham, or even the purpose of such a mode, I would think it would be for the same reason as SSB for AM? To achieve half the bandwidth utilization for a given signal? (But at the expense of 3dB poorer SNR...) It's been quite a few years since I looked at the article. There was just something about the article that seemed like I'd been dropped into something. Maybe the style was different from most articles in the magazine, maybe because it didn't really seem to be a practical article. There just seemed to be something missing. Yes, it would take up less space, but then why not go to some other mode? It's the only time I've seen something on the subject, and I think it could have better been handled. Michael VE2BVW |
Michael Black wrote:
It's been quite a few years since I looked at the article. There was There just seemed to be something missing. Yes, it would take up less space, but then why not go to some other mode? It's the only time I've seen something on the subject, and I think it could have better been handled. Fair enough. If the idea were to conserve bandwidth, you'd probably be using narrow band FM anyway at which point SSB-NBFM takes no less bandwidth than convention SSB-AM and... it _might_ even have worse SNR, although I don't know calculations off-hand. Hence I suspect that SSB-FM is nothing more than a curiosity... could be fun to implement just for the sake of experimentation once we're all running software defined radios, though! From your description it sounds like someone was excited by the novelty of the idea but ran out of steam before actually implementing the idea! ---Joel Kolstad |
Michael Black wrote:
It's been quite a few years since I looked at the article. There was There just seemed to be something missing. Yes, it would take up less space, but then why not go to some other mode? It's the only time I've seen something on the subject, and I think it could have better been handled. Fair enough. If the idea were to conserve bandwidth, you'd probably be using narrow band FM anyway at which point SSB-NBFM takes no less bandwidth than convention SSB-AM and... it _might_ even have worse SNR, although I don't know calculations off-hand. Hence I suspect that SSB-FM is nothing more than a curiosity... could be fun to implement just for the sake of experimentation once we're all running software defined radios, though! From your description it sounds like someone was excited by the novelty of the idea but ran out of steam before actually implementing the idea! ---Joel Kolstad |
as i understand, the fm signal, due to its nature of changing rate of
phase change generates a number of sidebands. Filtering these sidebands would mean that a band-pass filter is being applied to the fm signal. That would amplitude modulate the signal as well. Amplitude modualting would create some more sidebands but within the filter's band-pass. Finally, we would arrive at a 'least-bandwidth' signal that would resemble SSB. So you might as well expend five crystals (for a ladder filter and an oscillator) and get good ol SSB going. A more intiutive example would be to consider an FM signal being modulated by a single tone. That would waver the carrier back and forth around the center frequency of the carrier. Now, if you passed this through a band-pass filter, you will see the amplitude drop off at the filter's skirts. This will resemble an amplitude modualted signal. depending upon the filter bandwidth, you might see either an AM, or a two-tone (carrier center being one, the modulated tone the other) SSB signal. I may be completely missing the point though, i welcome an explanation. - farhan |
as i understand, the fm signal, due to its nature of changing rate of
phase change generates a number of sidebands. Filtering these sidebands would mean that a band-pass filter is being applied to the fm signal. That would amplitude modulate the signal as well. Amplitude modualting would create some more sidebands but within the filter's band-pass. Finally, we would arrive at a 'least-bandwidth' signal that would resemble SSB. So you might as well expend five crystals (for a ladder filter and an oscillator) and get good ol SSB going. A more intiutive example would be to consider an FM signal being modulated by a single tone. That would waver the carrier back and forth around the center frequency of the carrier. Now, if you passed this through a band-pass filter, you will see the amplitude drop off at the filter's skirts. This will resemble an amplitude modualted signal. depending upon the filter bandwidth, you might see either an AM, or a two-tone (carrier center being one, the modulated tone the other) SSB signal. I may be completely missing the point though, i welcome an explanation. - farhan |
as i understand, the fm signal, due to its nature of changing rate of phase change generates a number of sidebands. Filtering these sidebands would mean that a band-pass filter is being applied to the fm signal. That would amplitude modulate the signal as well. I think you have a point here. Removal of one side of the set of sidebands turns the FM signal into a sort of AM/SSB signal. During transmission, FM's robustness to impulse noise will be lost. The presence of a limiter in the receiver, will turn the signal + noise back into a corrupted DSB FM signal for demodulation. Sverre LA3ZA |
as i understand, the fm signal, due to its nature of changing rate of phase change generates a number of sidebands. Filtering these sidebands would mean that a band-pass filter is being applied to the fm signal. That would amplitude modulate the signal as well. I think you have a point here. Removal of one side of the set of sidebands turns the FM signal into a sort of AM/SSB signal. During transmission, FM's robustness to impulse noise will be lost. The presence of a limiter in the receiver, will turn the signal + noise back into a corrupted DSB FM signal for demodulation. Sverre LA3ZA |
Thanks everyone
Sometimes I really get curious and want to know about something. I haven't seen the Ham Radio article, but I'm thinking if the whole idea had any merit it would be a popular mode by now. Bruce kk7zz www.elmerdude.com |
Thanks everyone
Sometimes I really get curious and want to know about something. I haven't seen the Ham Radio article, but I'm thinking if the whole idea had any merit it would be a popular mode by now. Bruce kk7zz www.elmerdude.com |
Sverre Holm wrote:
I think you have a point here. Removal of one side of the set of sidebands turns the FM signal into a sort of AM/SSB signal. During transmission, FM's robustness to impulse noise will be lost. This would appear to depend on how sharp the skirt of our hypothetical SSB (really VSB, now) filter is? I.e., at low carrier deviations there's some AM and therefore it's not _quite_ as robus, whereas at higher carrier deviations the filter would be nice and flat and look just like regular FM in terms of amplitude. After all... in the presense of some AM on regular double side band FM, most receivers still perform just fine, don't they? ---Joel Kolstad |
Sverre Holm wrote:
I think you have a point here. Removal of one side of the set of sidebands turns the FM signal into a sort of AM/SSB signal. During transmission, FM's robustness to impulse noise will be lost. This would appear to depend on how sharp the skirt of our hypothetical SSB (really VSB, now) filter is? I.e., at low carrier deviations there's some AM and therefore it's not _quite_ as robus, whereas at higher carrier deviations the filter would be nice and flat and look just like regular FM in terms of amplitude. After all... in the presense of some AM on regular double side band FM, most receivers still perform just fine, don't they? ---Joel Kolstad |
W7TI wrote:
In the USA, the FCC used to prohibit simultaneous amplitude and frequency modulation. I did a search of Part 97 rules and I don't see that exact wording now, but I would still tread lightly in this area. Sure, I'm not advocating anybody go out and purposely modulate their FM signals. Provided all the sidebands are confined to a band no wider than conventional AM, you probably won't be bothered by Uncle Charlie but caution is advised. Indeed. One interesting commercial system out there is the Motorola C-QUAM (compatible quadrature amplitude modulation) stereo method, used in the US for commercial AM stereo broadcastings. Since it has to be backwards compatible with envelope detector-based AM receivers, the resultant output is -- I would suggest -- something similar to amplitude modulated phase modulation! (I think it's actually quite clever...) ---Joel Kolstad |
W7TI wrote:
In the USA, the FCC used to prohibit simultaneous amplitude and frequency modulation. I did a search of Part 97 rules and I don't see that exact wording now, but I would still tread lightly in this area. Sure, I'm not advocating anybody go out and purposely modulate their FM signals. Provided all the sidebands are confined to a band no wider than conventional AM, you probably won't be bothered by Uncle Charlie but caution is advised. Indeed. One interesting commercial system out there is the Motorola C-QUAM (compatible quadrature amplitude modulation) stereo method, used in the US for commercial AM stereo broadcastings. Since it has to be backwards compatible with envelope detector-based AM receivers, the resultant output is -- I would suggest -- something similar to amplitude modulated phase modulation! (I think it's actually quite clever...) ---Joel Kolstad |
In article , W7TI
writes: On Wed, 22 Oct 2003 06:52:37 -0700, "Joel Kolstad" wrote: After all... in the presense of some AM on regular double side band FM, most receivers still perform just fine, don't they? _________________________________________________ ________ In the USA, the FCC used to prohibit simultaneous amplitude and frequency modulation. I did a search of Part 97 rules and I don't see that exact wording now, but I would still tread lightly in this area. Provided all the sidebands are confined to a band no wider than conventional AM, you probably won't be bothered by Uncle Charlie but caution is advised. -- Bill, W7TI Bill, I just dug out the 1977 issues of HR from storage and looked the article over. Author Richard Slater (W3EJD) said almost the same thing at the end of the article on page 15 under "closing comments." The nomenclatures for different modulations were formalized by the ITU-R since then but the FCC still doesn't have anything covering this "single-sideband FM" modulation type for U. S. amateur radio. A general problem with understanding the concept is the simplicity of the explanations of AM in today's amateur radio. The mathematical representations of all modulations have been known and distributed in text books for decades...my introduction to that was "Electronic Designer's Handbook by Landee, Davis, Albrecht, McGraw-Hill 1957, Section 5. Those who can follow the series expressions in a summation formula, study it, will understand how a phasing-type SSB modulator and demodulator can work. It is much harder to look at the expressions and "see" FM or PM; Hewlett-Packard's Agilent site has a neat little animated Java display that may help some on that. Filter-type SSB from AM is almost intuitive when the AM spectrum is shown. That is easy to comprehend...once all accept that the content of each AM sideband has the same information. (there are still some long-timers who refuse to accept that the carrier RF energy doesn't change in AM at less than 100% modulation, heh heh) FM and PM sidebands are definitely NOT easy to visualize since their individual amplitudes and phases change depending on modulation index and modulating frequency. There isn't any corresponding similarity of FM and PM to AM for the repetition of sidebands' information when looking at the spectral content. What Slater was discussing in that January 1977 HR article was what a group of researchers had already been doing in the early 1970s to see if there were alternatives to SSB-like frequency multiplexing in multi-channel circuits. Part of that investigation was to get around some of the patents still existing on frequency multiplexing via single sideband techniques (pioneered first on long-distance telephony, by the way). Another part was to simplify (if possible) the circuitry involved when carrying a LOT of channels. Equipent of 3 to 4 decades ago was a lot bulkier than it is now for non-digital multiplexing. The "narrowband" necessities of working in small-bandspace amateur bands was not a prime criteria for that research. Slater explained much of the above in that article and didn't claim any exciting narrowband results of previous art. The (mislabeled in my opinion) "single-sideband FM" technique of combining FM and AM is simply a DIFFERENT way to communicate information. A truly different way of modulation exists in everyone's telephone line modem that can send/receive up to 56 Kilobits/Sec in a bandwidth of only 3 KHz. That is a combination of AM and PM. That isn't intuitive to AM-oriented minds and there still exist arguments in newsgroups that such high rates "aren't possible!" :-) Yet most of us POTS users with computers regularly get 33 to 56 KBPS rates over 2.5 to 3.0 KHz bandwidth telephone circuits. I've not seen much on that "single-sideband FM" stuff in the professional literature after 1980. Based on what was published in the 1970s, it was an interesting technique but did not come up with any advantages for commercial or military adoption or much further work. I think it does show that old paradigms aren't always worth four nickels and that, truly, thinking outside the box might come up with something new and useful. Just some comments from Len Anderson retired (from regular hours) electronic engineer person |
In article , W7TI
writes: On Wed, 22 Oct 2003 06:52:37 -0700, "Joel Kolstad" wrote: After all... in the presense of some AM on regular double side band FM, most receivers still perform just fine, don't they? _________________________________________________ ________ In the USA, the FCC used to prohibit simultaneous amplitude and frequency modulation. I did a search of Part 97 rules and I don't see that exact wording now, but I would still tread lightly in this area. Provided all the sidebands are confined to a band no wider than conventional AM, you probably won't be bothered by Uncle Charlie but caution is advised. -- Bill, W7TI Bill, I just dug out the 1977 issues of HR from storage and looked the article over. Author Richard Slater (W3EJD) said almost the same thing at the end of the article on page 15 under "closing comments." The nomenclatures for different modulations were formalized by the ITU-R since then but the FCC still doesn't have anything covering this "single-sideband FM" modulation type for U. S. amateur radio. A general problem with understanding the concept is the simplicity of the explanations of AM in today's amateur radio. The mathematical representations of all modulations have been known and distributed in text books for decades...my introduction to that was "Electronic Designer's Handbook by Landee, Davis, Albrecht, McGraw-Hill 1957, Section 5. Those who can follow the series expressions in a summation formula, study it, will understand how a phasing-type SSB modulator and demodulator can work. It is much harder to look at the expressions and "see" FM or PM; Hewlett-Packard's Agilent site has a neat little animated Java display that may help some on that. Filter-type SSB from AM is almost intuitive when the AM spectrum is shown. That is easy to comprehend...once all accept that the content of each AM sideband has the same information. (there are still some long-timers who refuse to accept that the carrier RF energy doesn't change in AM at less than 100% modulation, heh heh) FM and PM sidebands are definitely NOT easy to visualize since their individual amplitudes and phases change depending on modulation index and modulating frequency. There isn't any corresponding similarity of FM and PM to AM for the repetition of sidebands' information when looking at the spectral content. What Slater was discussing in that January 1977 HR article was what a group of researchers had already been doing in the early 1970s to see if there were alternatives to SSB-like frequency multiplexing in multi-channel circuits. Part of that investigation was to get around some of the patents still existing on frequency multiplexing via single sideband techniques (pioneered first on long-distance telephony, by the way). Another part was to simplify (if possible) the circuitry involved when carrying a LOT of channels. Equipent of 3 to 4 decades ago was a lot bulkier than it is now for non-digital multiplexing. The "narrowband" necessities of working in small-bandspace amateur bands was not a prime criteria for that research. Slater explained much of the above in that article and didn't claim any exciting narrowband results of previous art. The (mislabeled in my opinion) "single-sideband FM" technique of combining FM and AM is simply a DIFFERENT way to communicate information. A truly different way of modulation exists in everyone's telephone line modem that can send/receive up to 56 Kilobits/Sec in a bandwidth of only 3 KHz. That is a combination of AM and PM. That isn't intuitive to AM-oriented minds and there still exist arguments in newsgroups that such high rates "aren't possible!" :-) Yet most of us POTS users with computers regularly get 33 to 56 KBPS rates over 2.5 to 3.0 KHz bandwidth telephone circuits. I've not seen much on that "single-sideband FM" stuff in the professional literature after 1980. Based on what was published in the 1970s, it was an interesting technique but did not come up with any advantages for commercial or military adoption or much further work. I think it does show that old paradigms aren't always worth four nickels and that, truly, thinking outside the box might come up with something new and useful. Just some comments from Len Anderson retired (from regular hours) electronic engineer person |
Avery Fineman wrote:
There isn't any corresponding similarity of FM and PM to AM for the repetition of sidebands' information when looking at the spectral content. Umm... last I looked the spectrum of FM and PM was symmetrical about the carrier frequency? (Well, the lower sideband is 180 degrees out of phase with the upper, but that's true of AM as well.) Looking at a single sine wave input to an FM or phase modulator, this comes about from the Bessel function expansion of the sidetones and J-n(x)=-Jn(x)? I know you're far more experienced in this area than I am, however, so I'll let you explain what I'm misinterpreting here! ---Joel Kolstad |
Avery Fineman wrote:
There isn't any corresponding similarity of FM and PM to AM for the repetition of sidebands' information when looking at the spectral content. Umm... last I looked the spectrum of FM and PM was symmetrical about the carrier frequency? (Well, the lower sideband is 180 degrees out of phase with the upper, but that's true of AM as well.) Looking at a single sine wave input to an FM or phase modulator, this comes about from the Bessel function expansion of the sidetones and J-n(x)=-Jn(x)? I know you're far more experienced in this area than I am, however, so I'll let you explain what I'm misinterpreting here! ---Joel Kolstad |
In article , "Joel Kolstad"
writes: Avery Fineman wrote: There isn't any corresponding similarity of FM and PM to AM for the repetition of sidebands' information when looking at the spectral content. Umm... last I looked the spectrum of FM and PM was symmetrical about the carrier frequency? (Well, the lower sideband is 180 degrees out of phase with the upper, but that's true of AM as well.) Looking at a single sine wave input to an FM or phase modulator, this comes about from the Bessel function expansion of the sidetones and J-n(x)=-Jn(x)? Yes. More or less. I know you're far more experienced in this area than I am, however, so I'll let you explain what I'm misinterpreting here! Noooo...I'm not going to. About a million subjective years ago I had to slog through a solution and series expansion with the only "help" I got being a suggestion to use Bessel Functions of the First Kind. In doing so - AND thinking about it in the process - I learned quite a bit about the math AND the modulation process. Very useful later on. ALL learning takes place in one's own noggin...doesn't matter whether one is in a formal class or alone being "lectured" by print on paper through the eyeballs. Over on the Agilent website, I would suggest downloading their free Application Note 150-1. That is really a subtle selling thing for their very fine spectrum analyzers but it is also a darn good treatise on modulation and modulation spectra for all the basic types. It should (unless altered there) include that nice little animated display of sidebands versus modulation index. I've always admired those H-P appnotes, valuing most as nice little tutorials on specialized subjects. Richard Slater in the mentioned January '77 HR article was trying to explain a combination of FM and AM. In order to get a proper "feel" for that (in my opinion), one needs the experience of juggling those series terms in the expanded equation form. There IS one hint and that is the not-quite symmetry (in numeric values) of FM and PM spectra as compared to AM spectra. True "single-sideband" has a possibility only on true symmetry. FM and PM spectra, by themselves, don't have that symmetry in the expanded form. I'm not going to discuss that one since it should be apparent. If you want some source code on calculating the numeric values of Bessel Functions of the First Kind, I'll be happy to post it here under some thread. It's short and not complicated and a #$%^!!! faster than slugging through 5-place tables with slide rule and/or four-function mechanical calculator. Been there, done too much of that. Computers aren't just for chat rooms, are very nice for numeric calculations of the large kind. :-) Len Anderson retired (from regular hours) electronic engineer person |
In article , "Joel Kolstad"
writes: Avery Fineman wrote: There isn't any corresponding similarity of FM and PM to AM for the repetition of sidebands' information when looking at the spectral content. Umm... last I looked the spectrum of FM and PM was symmetrical about the carrier frequency? (Well, the lower sideband is 180 degrees out of phase with the upper, but that's true of AM as well.) Looking at a single sine wave input to an FM or phase modulator, this comes about from the Bessel function expansion of the sidetones and J-n(x)=-Jn(x)? Yes. More or less. I know you're far more experienced in this area than I am, however, so I'll let you explain what I'm misinterpreting here! Noooo...I'm not going to. About a million subjective years ago I had to slog through a solution and series expansion with the only "help" I got being a suggestion to use Bessel Functions of the First Kind. In doing so - AND thinking about it in the process - I learned quite a bit about the math AND the modulation process. Very useful later on. ALL learning takes place in one's own noggin...doesn't matter whether one is in a formal class or alone being "lectured" by print on paper through the eyeballs. Over on the Agilent website, I would suggest downloading their free Application Note 150-1. That is really a subtle selling thing for their very fine spectrum analyzers but it is also a darn good treatise on modulation and modulation spectra for all the basic types. It should (unless altered there) include that nice little animated display of sidebands versus modulation index. I've always admired those H-P appnotes, valuing most as nice little tutorials on specialized subjects. Richard Slater in the mentioned January '77 HR article was trying to explain a combination of FM and AM. In order to get a proper "feel" for that (in my opinion), one needs the experience of juggling those series terms in the expanded equation form. There IS one hint and that is the not-quite symmetry (in numeric values) of FM and PM spectra as compared to AM spectra. True "single-sideband" has a possibility only on true symmetry. FM and PM spectra, by themselves, don't have that symmetry in the expanded form. I'm not going to discuss that one since it should be apparent. If you want some source code on calculating the numeric values of Bessel Functions of the First Kind, I'll be happy to post it here under some thread. It's short and not complicated and a #$%^!!! faster than slugging through 5-place tables with slide rule and/or four-function mechanical calculator. Been there, done too much of that. Computers aren't just for chat rooms, are very nice for numeric calculations of the large kind. :-) Len Anderson retired (from regular hours) electronic engineer person |
Sometimes I really get curious and want to know about something.
I haven't seen the Ham Radio article, but I'm thinking if the whole idea had any merit it would be a popular mode by now. Bruce- It has been about 35 years since I had a class in school where SSB-FM was discussed. I recall that if you derive the equations for both AM and FM SSB, they are identical for practical purposes if the FM signal has low deviation (low modulation index?). Looking at Two Meter FM, the deviation typically peaks at about 5 KHz. If you listen to your local repeater with an SSB rig such as the IC-706, it will be obvious that it isn't a clean signal! However, a 3 KHz deviation FM signal on HF (below 29 MHz) will sound much cleaner when tuned as SSB, and you may not notice it isn't AM-SSB. With this in mind, consider that AM-SSB and FM-SSB might just be two ways to generate an SSB signal, assuming you use a filter to eliminate the carrier and other sideband. By the way, an IC-706, especially one with the TCXO, often has a more accurate frequency read-out than a typical Two Meter rig. Therefore you can use it to check a repeater's frequency by tuning it as if it were an SSB station while someone is speaking. It is easy enough to check the IC-706 against WWV on HF. 73, Fred, K4DII |
Sometimes I really get curious and want to know about something.
I haven't seen the Ham Radio article, but I'm thinking if the whole idea had any merit it would be a popular mode by now. Bruce- It has been about 35 years since I had a class in school where SSB-FM was discussed. I recall that if you derive the equations for both AM and FM SSB, they are identical for practical purposes if the FM signal has low deviation (low modulation index?). Looking at Two Meter FM, the deviation typically peaks at about 5 KHz. If you listen to your local repeater with an SSB rig such as the IC-706, it will be obvious that it isn't a clean signal! However, a 3 KHz deviation FM signal on HF (below 29 MHz) will sound much cleaner when tuned as SSB, and you may not notice it isn't AM-SSB. With this in mind, consider that AM-SSB and FM-SSB might just be two ways to generate an SSB signal, assuming you use a filter to eliminate the carrier and other sideband. By the way, an IC-706, especially one with the TCXO, often has a more accurate frequency read-out than a typical Two Meter rig. Therefore you can use it to check a repeater's frequency by tuning it as if it were an SSB station while someone is speaking. It is easy enough to check the IC-706 against WWV on HF. 73, Fred, K4DII |
The amplitudes of the sideband components are symmetrical (at least for
modulation by a single sine wave), but the phases aren't. The phases of all the upper sideband components are in phase with the carrier; in the lower sideband, the odd order components (and only the odd order ones) are reversed in phase. With multitone modulation, things get a whole lot more complex. Unlike AM, FM is nonlinear, so there are sideband components from each tone, plus components from their sum, difference, and harmonics. The inability to use superposition makes analysis of frequency modulation with complex waveforms a great deal more difficult than AM. Note also that unlike AM, whatever fraction of the carrier that's left when transmitting FM also contains part of the modulation information. Of course, at certain modulation indices with pure sine wave modulation, the carrier goes to zero, meaning that all the modulation information is in the sidebands. But this happens only under specific modulation conditions, so you'd certainly have an information-carrying carrier component present when modulating with a voice, for example. Roy Lewallen, W7EL Joel Kolstad wrote: Avery Fineman wrote: There isn't any corresponding similarity of FM and PM to AM for the repetition of sidebands' information when looking at the spectral content. Umm... last I looked the spectrum of FM and PM was symmetrical about the carrier frequency? (Well, the lower sideband is 180 degrees out of phase with the upper, but that's true of AM as well.) Looking at a single sine wave input to an FM or phase modulator, this comes about from the Bessel function expansion of the sidetones and J-n(x)=-Jn(x)? I know you're far more experienced in this area than I am, however, so I'll let you explain what I'm misinterpreting here! ---Joel Kolstad |
The amplitudes of the sideband components are symmetrical (at least for
modulation by a single sine wave), but the phases aren't. The phases of all the upper sideband components are in phase with the carrier; in the lower sideband, the odd order components (and only the odd order ones) are reversed in phase. With multitone modulation, things get a whole lot more complex. Unlike AM, FM is nonlinear, so there are sideband components from each tone, plus components from their sum, difference, and harmonics. The inability to use superposition makes analysis of frequency modulation with complex waveforms a great deal more difficult than AM. Note also that unlike AM, whatever fraction of the carrier that's left when transmitting FM also contains part of the modulation information. Of course, at certain modulation indices with pure sine wave modulation, the carrier goes to zero, meaning that all the modulation information is in the sidebands. But this happens only under specific modulation conditions, so you'd certainly have an information-carrying carrier component present when modulating with a voice, for example. Roy Lewallen, W7EL Joel Kolstad wrote: Avery Fineman wrote: There isn't any corresponding similarity of FM and PM to AM for the repetition of sidebands' information when looking at the spectral content. Umm... last I looked the spectrum of FM and PM was symmetrical about the carrier frequency? (Well, the lower sideband is 180 degrees out of phase with the upper, but that's true of AM as well.) Looking at a single sine wave input to an FM or phase modulator, this comes about from the Bessel function expansion of the sidetones and J-n(x)=-Jn(x)? I know you're far more experienced in this area than I am, however, so I'll let you explain what I'm misinterpreting here! ---Joel Kolstad |
Roy Lewallen wrote:
The amplitudes of the sideband components are symmetrical (at least for modulation by a single sine wave), but the phases aren't. The phases of all the upper sideband components are in phase with the carrier; in the lower sideband, the odd order components (and only the odd order ones) are reversed in phase. I certainly didn't realize that until you pointed it out; I was generalzing from the narrowband FM situation where only the first sideband components are necessarily maintained and incorrectly assuming the same phase differences applied to the general case. However... Say you start with a baseband FM signal. Let's call the two sides of its Fourier transform L and R for the 'left' and 'right' halves. Now we mix up to the desired carrier frequency. At -f_c we have L at even greater negative frequencies and R at smaller negative frequencies. Ditto at f_c. If we now apply a low pass filter to select the lower sideband, we end up with R and L -- No information has been lost! (Likewise, with a high pass filter you have L and R left -- Same deal.) Fundamentally mixing ANY signal followed by SSB filtering shouldn't lose information. Yes, in practice we'll be talking about VSB instead of SSB, but I still think we're OK. Speaking of narrowband FM (NBFM)... and at the risk of splitting this topic... I had a discussion today with someone over the ability to use an envelope detector to recover narrowband FM signals. The output of the envelope detector is approximately 1+0.5*cos^2(2*pi*f*t), where f was the original modulating signal. The '1' will be killed by a capacitor, but that leaves the cosine squared term... which seems impossible to easily change back into cosine, since sqrt(x^2)=abs(x) and therefore it would appear that we've irreversably lost information. Comments? ---Joel Kolstad ....ambitious novice who'll be licensed shortly... ....and I still think C-QUAM AM stereo is quite clever... |
Roy Lewallen wrote:
The amplitudes of the sideband components are symmetrical (at least for modulation by a single sine wave), but the phases aren't. The phases of all the upper sideband components are in phase with the carrier; in the lower sideband, the odd order components (and only the odd order ones) are reversed in phase. I certainly didn't realize that until you pointed it out; I was generalzing from the narrowband FM situation where only the first sideband components are necessarily maintained and incorrectly assuming the same phase differences applied to the general case. However... Say you start with a baseband FM signal. Let's call the two sides of its Fourier transform L and R for the 'left' and 'right' halves. Now we mix up to the desired carrier frequency. At -f_c we have L at even greater negative frequencies and R at smaller negative frequencies. Ditto at f_c. If we now apply a low pass filter to select the lower sideband, we end up with R and L -- No information has been lost! (Likewise, with a high pass filter you have L and R left -- Same deal.) Fundamentally mixing ANY signal followed by SSB filtering shouldn't lose information. Yes, in practice we'll be talking about VSB instead of SSB, but I still think we're OK. Speaking of narrowband FM (NBFM)... and at the risk of splitting this topic... I had a discussion today with someone over the ability to use an envelope detector to recover narrowband FM signals. The output of the envelope detector is approximately 1+0.5*cos^2(2*pi*f*t), where f was the original modulating signal. The '1' will be killed by a capacitor, but that leaves the cosine squared term... which seems impossible to easily change back into cosine, since sqrt(x^2)=abs(x) and therefore it would appear that we've irreversably lost information. Comments? ---Joel Kolstad ....ambitious novice who'll be licensed shortly... ....and I still think C-QUAM AM stereo is quite clever... |
Fred McKenzie wrote:
It has been about 35 years since I had a class in school where SSB-FM was discussed. I recall that if you derive the equations for both AM and FM SSB, they are identical for practical purposes if the FM signal has low deviation (low modulation index?). You're probably thinking of AM vs. narrow band FM. Although the equations look very similar on paper and the MAGNITUDE spectrum is identical, the phase spectrum is different in that -- in the phasor domain -- AM always sits at 0 degrees and just grows and shrinks with modulation (overmodulation pushes it over to 180 degrees, BTW). NBFM, on the other hand, still has the carrier at 0 degrees but grows and shrinks along the imaginary axis. Hence the angle of the phasor is small but time-varying (which implies that the instantaneous frequency is varying as well -- but of course you already knew that since we called this whole mess 'frequency modulation'). The angle is about 15 degrees for a modulation index of 0.3 (what my notes claim as a good cutoff for NBFM) and about 5 degrees at 0.1. See the message I posted earlier tonight for a discussion of whether or not you can recover NBFM with an envelope detector as of course one often does with AM (the difficulty is due to that phasor's wiggling...). I think not, but there's plenty I don't have a clue about... yet! What's the modulation index on two meters anyway? ---Joel Kolstad ....who does know that a wideband FM receiver has no problem whatsoever receiving NBFM... Looking at Two Meter FM, the deviation typically peaks at about 5 KHz. If you listen to your local repeater with an SSB rig such as the IC-706, it will be obvious that it isn't a clean signal! However, a 3 KHz deviation FM signal on HF (below 29 MHz) will sound much cleaner when tuned as SSB, and you may not notice it isn't AM-SSB. |
Fred McKenzie wrote:
It has been about 35 years since I had a class in school where SSB-FM was discussed. I recall that if you derive the equations for both AM and FM SSB, they are identical for practical purposes if the FM signal has low deviation (low modulation index?). You're probably thinking of AM vs. narrow band FM. Although the equations look very similar on paper and the MAGNITUDE spectrum is identical, the phase spectrum is different in that -- in the phasor domain -- AM always sits at 0 degrees and just grows and shrinks with modulation (overmodulation pushes it over to 180 degrees, BTW). NBFM, on the other hand, still has the carrier at 0 degrees but grows and shrinks along the imaginary axis. Hence the angle of the phasor is small but time-varying (which implies that the instantaneous frequency is varying as well -- but of course you already knew that since we called this whole mess 'frequency modulation'). The angle is about 15 degrees for a modulation index of 0.3 (what my notes claim as a good cutoff for NBFM) and about 5 degrees at 0.1. See the message I posted earlier tonight for a discussion of whether or not you can recover NBFM with an envelope detector as of course one often does with AM (the difficulty is due to that phasor's wiggling...). I think not, but there's plenty I don't have a clue about... yet! What's the modulation index on two meters anyway? ---Joel Kolstad ....who does know that a wideband FM receiver has no problem whatsoever receiving NBFM... Looking at Two Meter FM, the deviation typically peaks at about 5 KHz. If you listen to your local repeater with an SSB rig such as the IC-706, it will be obvious that it isn't a clean signal! However, a 3 KHz deviation FM signal on HF (below 29 MHz) will sound much cleaner when tuned as SSB, and you may not notice it isn't AM-SSB. |
Joel Kolstad wrote:
. . . Say you start with a baseband FM signal. Let's call the two sides of its Fourier transform L and R for the 'left' and 'right' halves. Now we mix up to the desired carrier frequency. At -f_c we have L at even greater negative frequencies and R at smaller negative frequencies. Ditto at f_c. If we now apply a low pass filter to select the lower sideband, we end up with R and L -- No information has been lost! (Likewise, with a high pass filter you have L and R left -- Same deal.) Fundamentally mixing ANY signal followed by SSB filtering shouldn't lose information. Yes, in practice we'll be talking about VSB instead of SSB, but I still think we're OK. I'm afraid you lost me with the "baseband" FM signal. Would you provide a carrier frequency, modulation frequency, and deviation or modulation index as an example? The lower and upper sidebands of an FM signal do contain the same information when the modulation is a single sine wave, even though the sidebands aren't identical. But when you modulate with a complex waveform, you might find that some of the information which is adding in one sideband is subtracting in the other, and you might not be able to recover the modulating waveform from only one or the other -- sort of like you can't get two separate stereo channels from just the sum signal. I don't know if that's true, but it wouldn't surprise me. And, as I pointed out in another posting, the entire modulation information isn't even contained in *both* sidebands except under very special conditions -- some is in the carrier. Another question, of course, is whether you can get close enough to be useful. Perhaps with NBFM, at least, you could. Speaking of narrowband FM (NBFM)... and at the risk of splitting this topic... I had a discussion today with someone over the ability to use an envelope detector to recover narrowband FM signals. The output of the envelope detector is approximately 1+0.5*cos^2(2*pi*f*t), where f was the original modulating signal. The '1' will be killed by a capacitor, but that leaves the cosine squared term... which seems impossible to easily change back into cosine, since sqrt(x^2)=abs(x) and therefore it would appear that we've irreversably lost information. Comments? Cos^2(x) = abs(cos(x)) = 1/2 * (1 + cos(2x)). As you've noted, the DC term can be blocked with a capacitor, so you'd end up with a cosine wave at twice the frequency. But I've never heard of trying to detect NBFM directly with an envelope detector like you'd detect AM. The trick we used in ye olden tymes was called "slope detection". You tuned the signal so it was on the edge of the IF filter. The filter slope converted the FM to AM, which was then detected with the normal AM envelope detector. If you tuned directly to the carrier frequency, you didn't hear any modulation to speak of. Roy Lewallen, W7EL |
Joel Kolstad wrote:
. . . Say you start with a baseband FM signal. Let's call the two sides of its Fourier transform L and R for the 'left' and 'right' halves. Now we mix up to the desired carrier frequency. At -f_c we have L at even greater negative frequencies and R at smaller negative frequencies. Ditto at f_c. If we now apply a low pass filter to select the lower sideband, we end up with R and L -- No information has been lost! (Likewise, with a high pass filter you have L and R left -- Same deal.) Fundamentally mixing ANY signal followed by SSB filtering shouldn't lose information. Yes, in practice we'll be talking about VSB instead of SSB, but I still think we're OK. I'm afraid you lost me with the "baseband" FM signal. Would you provide a carrier frequency, modulation frequency, and deviation or modulation index as an example? The lower and upper sidebands of an FM signal do contain the same information when the modulation is a single sine wave, even though the sidebands aren't identical. But when you modulate with a complex waveform, you might find that some of the information which is adding in one sideband is subtracting in the other, and you might not be able to recover the modulating waveform from only one or the other -- sort of like you can't get two separate stereo channels from just the sum signal. I don't know if that's true, but it wouldn't surprise me. And, as I pointed out in another posting, the entire modulation information isn't even contained in *both* sidebands except under very special conditions -- some is in the carrier. Another question, of course, is whether you can get close enough to be useful. Perhaps with NBFM, at least, you could. Speaking of narrowband FM (NBFM)... and at the risk of splitting this topic... I had a discussion today with someone over the ability to use an envelope detector to recover narrowband FM signals. The output of the envelope detector is approximately 1+0.5*cos^2(2*pi*f*t), where f was the original modulating signal. The '1' will be killed by a capacitor, but that leaves the cosine squared term... which seems impossible to easily change back into cosine, since sqrt(x^2)=abs(x) and therefore it would appear that we've irreversably lost information. Comments? Cos^2(x) = abs(cos(x)) = 1/2 * (1 + cos(2x)). As you've noted, the DC term can be blocked with a capacitor, so you'd end up with a cosine wave at twice the frequency. But I've never heard of trying to detect NBFM directly with an envelope detector like you'd detect AM. The trick we used in ye olden tymes was called "slope detection". You tuned the signal so it was on the edge of the IF filter. The filter slope converted the FM to AM, which was then detected with the normal AM envelope detector. If you tuned directly to the carrier frequency, you didn't hear any modulation to speak of. Roy Lewallen, W7EL |
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