On Sep 21, 10:04�am, Jerry Avins <j...@ieee.org> wrote:> Randy Yates wrote: > > HardySpicer <gyansor...@gmail.com> writes: > > >> On Sep 20, 8:01 pm, Jerry Avins <j...@ieee.org> wrote: > >>> HardySpicer wrote: > >>>> On Sep 21, 12:57 pm, Jerry Avins <j...@ieee.org> wrote: > >>>>> PalapaGuy wrote: > >>>>>> Single-sideband FM was a popular topic several years ago but I haven't seen > >>>>>> any articles on it since then. �It looks attractive as a way to transmit > >>>>>> narrowband FM efficiently at VHF over short distances. �Does anyone know of > >>>>>> current uses for this technology? > >>>>> Narrow-band FM (NBFM) is the same as low-modulation-percentage AM with > >>>>> the carrier shifted 90 degrees. If the modulation index is low enough, > >>>>> what you mention is identical to the SSB-SC that is standard on ham and > >>>>> CB bands. I suppose people eventually realized that. Or is there some > >>>>> other meaning to what you write? > >>>>> Jerry > >>>>> -- > >>>>> Engineering is the art of making what you want from things you can get. > >>>>> ����������������������������������������������������������������������� > >>>> Not sure it behaves the same. FM thresholds but AM doesn't. AM (SSB) > >>>> has problems demodulating it I suppose whereas with FM you can easily > >>>> use a PLL. > >>> NBFM doesn't threshold. You confuse the properties of FM with those of > >>> SSB NBFM. As far as I know,n SSB version of high-modulation-index FM > >>> doesn't exist. > > >>> Jerry > >>> -- > >>> Engineering is the art of making what you want from things you can get. > >>> ����������������������������������������������������������������������� > >> NBFM must threshold. In fact it thresholds worse than wide-band since > >> the beta is lower. That anoying sound on every police radio in the UK > >> (as it was of course). > > >> Hardy > > > Mischa Schwartz et al. discuss the FM threshold effect in quite a bit of > > analytical detail (a quad integration is performed at one point on > > four-dimensional probability distribution!) in section 3.6, p134. > > > --Randy > > > @book{schwartzcommtechniques, > > � title = "Communication Systems and Techniques", > > � author = "{Mischa Schwartz and William R. Bennett and Seymour Stein}", > > � publisher = "IEEE Press (reissue, originally New York: McGraw-Hill 1966)", > > � year = "1996"} > > My Schwartz, "Information Transmission, Modulation and Noise", 1st > Edition, 1959; $10.34, says in italics* on p.304, section 6.3, > /"Narrowband FM thus provides no S/N improvement over AM."/ >Yes but Fm thresholds. SSB loses lock of course. Hardy
SSB FM
Started by ●September 20, 2009
Reply by ●September 22, 20092009-09-22
Reply by ●September 22, 20092009-09-22
HardySpicer wrote:> On Sep 21, 10:04 am, Jerry Avins <j...@ieee.org> wrote: >> Randy Yates wrote: >>> HardySpicer <gyansor...@gmail.com> writes: >>>> On Sep 20, 8:01 pm, Jerry Avins <j...@ieee.org> wrote: >>>>> HardySpicer wrote: >>>>>> On Sep 21, 12:57 pm, Jerry Avins <j...@ieee.org> wrote: >>>>>>> PalapaGuy wrote: >>>>>>>> Single-sideband FM was a popular topic several years ago but I haven't seen >>>>>>>> any articles on it since then. It looks attractive as a way to transmit >>>>>>>> narrowband FM efficiently at VHF over short distances. Does anyone know of >>>>>>>> current uses for this technology? >>>>>>> Narrow-band FM (NBFM) is the same as low-modulation-percentage AM with >>>>>>> the carrier shifted 90 degrees. If the modulation index is low enough, >>>>>>> what you mention is identical to the SSB-SC that is standard on ham and >>>>>>> CB bands. I suppose people eventually realized that. Or is there some >>>>>>> other meaning to what you write? >>>>>>> Jerry >>>>>>> -- >>>>>>> Engineering is the art of making what you want from things you can get. >>>>>>> ����������������������������������������������������������������������� >>>>>> Not sure it behaves the same. FM thresholds but AM doesn't. AM (SSB) >>>>>> has problems demodulating it I suppose whereas with FM you can easily >>>>>> use a PLL. >>>>> NBFM doesn't threshold. You confuse the properties of FM with those of >>>>> SSB NBFM. As far as I know,n SSB version of high-modulation-index FM >>>>> doesn't exist. >>>>> Jerry >>>>> -- >>>>> Engineering is the art of making what you want from things you can get. >>>>> ����������������������������������������������������������������������� >>>> NBFM must threshold. In fact it thresholds worse than wide-band since >>>> the beta is lower. That anoying sound on every police radio in the UK >>>> (as it was of course). >>>> Hardy >>> Mischa Schwartz et al. discuss the FM threshold effect in quite a bit of >>> analytical detail (a quad integration is performed at one point on >>> four-dimensional probability distribution!) in section 3.6, p134. >>> --Randy >>> @book{schwartzcommtechniques, >>> title = "Communication Systems and Techniques", >>> author = "{Mischa Schwartz and William R. Bennett and Seymour Stein}", >>> publisher = "IEEE Press (reissue, originally New York: McGraw-Hill 1966)", >>> year = "1996"} >> My Schwartz, "Information Transmission, Modulation and Noise", 1st >> Edition, 1959; $10.34, says in italics* on p.304, section 6.3, >> /"Narrowband FM thus provides no S/N improvement over AM."/ >> > Yes but Fm thresholds. SSB loses lock of course.Don't you get it? Thresholding happens when excess bandwidth is used to improve SNR. When the signal is too low, the excess bandwidth allows more noise into the receiver than the bandwidth-for-SNR trade rejects: that's the threshold. With NBFM, there is no excess bandwidth, hence no SNR improvement, hence no threshold. Unless, of course, your gain control is calibrated up to 11. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●September 22, 20092009-09-22
On Tue, 22 Sep 2009 13:00:35 -0400, Jerry Avins wrote:> HardySpicer wrote: >> On Sep 21, 10:04 am, Jerry Avins <j...@ieee.org> wrote: >>> Randy Yates wrote: >>>> HardySpicer <gyansor...@gmail.com> writes: >>>>> On Sep 20, 8:01 pm, Jerry Avins <j...@ieee.org> wrote: >>>>>> HardySpicer wrote: >>>>>>> On Sep 21, 12:57 pm, Jerry Avins <j...@ieee.org> wrote: >>>>>>>> PalapaGuy wrote: >>>>>>>>> Single-sideband FM was a popular topic several years ago but I >>>>>>>>> haven't seen any articles on it since then. It looks attractive >>>>>>>>> as a way to transmit narrowband FM efficiently at VHF over short >>>>>>>>> distances. Does anyone know of current uses for this >>>>>>>>> technology? >>>>>>>> Narrow-band FM (NBFM) is the same as low-modulation-percentage AM >>>>>>>> with the carrier shifted 90 degrees. If the modulation index is >>>>>>>> low enough, what you mention is identical to the SSB-SC that is >>>>>>>> standard on ham and CB bands. I suppose people eventually >>>>>>>> realized that. Or is there some other meaning to what you write? >>>>>>>> Jerry >>>>>>>> -- >>>>>>>> Engineering is the art of making what you want from things you >>>>>>>> can get. >>>>>>>>¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯>>>>>>> Not sure it behaves the same. FM thresholds but AM doesn't. AM >>>>>>> (SSB) has problems demodulating it I suppose whereas with FM you >>>>>>> can easily use a PLL. >>>>>> NBFM doesn't threshold. You confuse the properties of FM with those >>>>>> of SSB NBFM. As far as I know,n SSB version of >>>>>> high-modulation-index FM doesn't exist. >>>>>> Jerry >>>>>> -- >>>>>> Engineering is the art of making what you want from things you can >>>>>> get. >>>>>>¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯>>>>> NBFM must threshold. In fact it thresholds worse than wide-band >>>>> since the beta is lower. That anoying sound on every police radio in >>>>> the UK (as it was of course). >>>>> Hardy >>>> Mischa Schwartz et al. discuss the FM threshold effect in quite a bit >>>> of analytical detail (a quad integration is performed at one point on >>>> four-dimensional probability distribution!) in section 3.6, p134. >>>> --Randy >>>> @book{schwartzcommtechniques, >>>> title = "Communication Systems and Techniques", author = "{Mischa >>>> Schwartz and William R. Bennett and Seymour Stein}", publisher = >>>> "IEEE Press (reissue, originally New York: McGraw-Hill 1966)", year >>>> = "1996"} >>> My Schwartz, "Information Transmission, Modulation and Noise", 1st >>> Edition, 1959; $10.34, says in italics* on p.304, section 6.3, >>> /"Narrowband FM thus provides no S/N improvement over AM."/ >>> >> Yes but Fm thresholds. SSB loses lock of course. > > Don't you get it? Thresholding happens when excess bandwidth is used to > improve SNR. When the signal is too low, the excess bandwidth allows > more noise into the receiver than the bandwidth-for-SNR trade rejects: > that's the threshold. With NBFM, there is no excess bandwidth, hence no > SNR improvement, hence no threshold. Unless, of course, your gain > control is calibrated up to 11. > > JerryIf you limit the IF signal in NBFM then you do see reduced noise at high signal levels (because you're clipping out the portion of the noise that's in phase with the carrier) and enhanced noise at low signal levels (because you're amplifying the snot out of the noise). So it does do something akin to thresholding -- but it's certainly much softer than the weak signal behavior of broadcast FM. -- www.wescottdesign.com
Reply by ●September 22, 20092009-09-22
On Mon, 21 Sep 2009 23:05:25 -0700, HardySpicer wrote:> On Sep 21, 10:04 am, Jerry Avins <j...@ieee.org> wrote: >> Randy Yates wrote: >> > HardySpicer <gyansor...@gmail.com> writes: >> >> >> On Sep 20, 8:01 pm, Jerry Avins <j...@ieee.org> wrote: >> >>> HardySpicer wrote: >> >>>> On Sep 21, 12:57 pm, Jerry Avins <j...@ieee.org> wrote: >> >>>>> PalapaGuy wrote: >> >>>>>> Single-sideband FM was a popular topic several years ago but I >> >>>>>> haven't seen any articles on it since then. It looks attractive >> >>>>>> as a way to transmit narrowband FM efficiently at VHF over short >> >>>>>> distances. Does anyone know of current uses for this >> >>>>>> technology? >> >>>>> Narrow-band FM (NBFM) is the same as low-modulation-percentage AM >> >>>>> with the carrier shifted 90 degrees. If the modulation index is >> >>>>> low enough, what you mention is identical to the SSB-SC that is >> >>>>> standard on ham and CB bands. I suppose people eventually >> >>>>> realized that. Or is there some other meaning to what you write? >> >>>>> Jerry >> >>>>> -- >> >>>>> Engineering is the art of making what you want from things you >> >>>>> can get. >> >>>>>¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯>> >>>> Not sure it behaves the same. FM thresholds but AM doesn't. AM >> >>>> (SSB) has problems demodulating it I suppose whereas with FM you >> >>>> can easily use a PLL. >> >>> NBFM doesn't threshold. You confuse the properties of FM with those >> >>> of SSB NBFM. As far as I know,n SSB version of >> >>> high-modulation-index FM doesn't exist. >> >> >>> Jerry >> >>> -- >> >>> Engineering is the art of making what you want from things you can >> >>> get. >> >>>¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯>> >> NBFM must threshold. In fact it thresholds worse than wide-band >> >> since the beta is lower. That anoying sound on every police radio in >> >> the UK (as it was of course). >> >> >> Hardy >> >> > Mischa Schwartz et al. discuss the FM threshold effect in quite a bit >> > of analytical detail (a quad integration is performed at one point on >> > four-dimensional probability distribution!) in section 3.6, p134. >> >> > --Randy >> >> > @book{schwartzcommtechniques, >> > title = "Communication Systems and Techniques", author = "{Mischa >> > Schwartz and William R. Bennett and Seymour Stein}", publisher = >> > "IEEE Press (reissue, originally New York: McGraw-Hill 1966)", >> > year = "1996"} >> >> My Schwartz, "Information Transmission, Modulation and Noise", 1st >> Edition, 1959; $10.34, says in italics* on p.304, section 6.3, >> /"Narrowband FM thus provides no S/N improvement over AM."/ >> > Yes but Fm thresholds. SSB loses lock of course.As Jerry has been saying, NBFM doesn't threshold, not really. For that matter, SSB has no lock to lose. So you're wrong on two counts. You _do_ understand the difference between broadcast FM and narrow-band FM, yes? -- www.wescottdesign.com
Reply by ●September 22, 20092009-09-22
On Sep 22, 10:00�am, Jerry Avins <j...@ieee.org> wrote:> HardySpicer wrote: > > On Sep 21, 10:04 am, Jerry Avins <j...@ieee.org> wrote: > >> Randy Yates wrote: > >>> HardySpicer <gyansor...@gmail.com> writes: > >>>> On Sep 20, 8:01 pm, Jerry Avins <j...@ieee.org> wrote: > >>>>> HardySpicer wrote: > >>>>>> On Sep 21, 12:57 pm, Jerry Avins <j...@ieee.org> wrote: > >>>>>>> PalapaGuy wrote: > >>>>>>>> Single-sideband FM was a popular topic several years ago but I haven't seen > >>>>>>>> any articles on it since then. �It looks attractive as a way to transmit > >>>>>>>> narrowband FM efficiently at VHF over short distances. �Does anyone know of > >>>>>>>> current uses for this technology? > >>>>>>> Narrow-band FM (NBFM) is the same as low-modulation-percentage AM with > >>>>>>> the carrier shifted 90 degrees. If the modulation index is low enough, > >>>>>>> what you mention is identical to the SSB-SC that is standard on ham and > >>>>>>> CB bands. I suppose people eventually realized that. Or is there some > >>>>>>> other meaning to what you write? > >>>>>>> Jerry > >>>>>>> -- > >>>>>>> Engineering is the art of making what you want from things you can get. > >>>>>>> ����������������������������������������������������������������������� > >>>>>> Not sure it behaves the same. FM thresholds but AM doesn't. AM (SSB) > >>>>>> has problems demodulating it I suppose whereas with FM you can easily > >>>>>> use a PLL. > >>>>> NBFM doesn't threshold. You confuse the properties of FM with those of > >>>>> SSB NBFM. As far as I know,n SSB version of high-modulation-index FM > >>>>> doesn't exist. > >>>>> Jerry > >>>>> -- > >>>>> Engineering is the art of making what you want from things you can get. > >>>>> ����������������������������������������������������������������������� > >>>> NBFM must threshold. In fact it thresholds worse than wide-band since > >>>> the beta is lower. That anoying sound on every police radio in the UK > >>>> (as it was of course). > >>>> Hardy > >>> Mischa Schwartz et al. discuss the FM threshold effect in quite a bit of > >>> analytical detail (a quad integration is performed at one point on > >>> four-dimensional probability distribution!) in section 3.6, p134. > >>> --Randy > >>> @book{schwartzcommtechniques, > >>> � title = "Communication Systems and Techniques", > >>> � author = "{Mischa Schwartz and William R. Bennett and Seymour Stein}", > >>> � publisher = "IEEE Press (reissue, originally New York: McGraw-Hill 1966)", > >>> � year = "1996"} > >> My Schwartz, "Information Transmission, Modulation and Noise", 1st > >> Edition, 1959; $10.34, says in italics* on p.304, section 6.3, > >> /"Narrowband FM thus provides no S/N improvement over AM."/ > > > Yes but Fm thresholds. SSB loses lock of course. > > Don't you get it? Thresholding happens when excess bandwidth is used to > improve SNR. When the signal is too low, the excess bandwidth allows > more noise into the receiver than the bandwidth-for-SNR trade rejects: > that's the threshold. With NBFM, there is no excess bandwidth, hence no > SNR improvement, hence no threshold. Unless, of course, your gain > control is calibrated up to 11. > > JerryThe excess BW itself is not why threshold happens but it does exacerbate it. The threshold effect is explained in quite a bit of detail in books by Taub and Schilling and Misha Shwartz et al. as it was orignally studied by Rice. To understand it fully, one needs to look at the phasor diagram of the signal and noise. When noise is small (high SNR), it only causes the recieved signal phasor to have small pertubations around the signal phasor and hence only small variations in the received phase. However, as the noise becomes large (below thresold), it can result in the recieved phasor making full encirclements around the origin and each of those encirclements in a 2pi phase shift in the received phase (cycle slip in PM or clicks in FM). The noise contribution from these clicks results in the rapid decrease in the SNR for input CNRs below threshold. The threshold effect is a consequence of these clicks. It is not true that clicks happen only for large modulation indices but it is true that the BW is larger for larger modulation indices and thus more noise enters the receiver increasing the rate of clicks. That is why the threshold effect occurs at lower CNR for higher modulation indices but it occurs for all modulation indices including small ones. Anyone interested in studying the mechanism of threshold effect and the accompanying analysis should check out books by Taub and Schilling and Misha Shwartz et al. As far as the statement "Narrowband FM thus provides no S/N improvement over AM", the gain FM provides over AM is a function of modulation index as one trades off BW, so if the modulation index is low, there will be no S/N improvement but it depends on the modulation index.
Reply by ●September 22, 20092009-09-22
Tim Wescott wrote:> On Tue, 22 Sep 2009 13:00:35 -0400, Jerry Avins wrote: > >> HardySpicer wrote: >>> On Sep 21, 10:04 am, Jerry Avins <j...@ieee.org> wrote: >>>> Randy Yates wrote: >>>>> HardySpicer <gyansor...@gmail.com> writes: >>>>>> On Sep 20, 8:01 pm, Jerry Avins <j...@ieee.org> wrote: >>>>>>> HardySpicer wrote: >>>>>>>> On Sep 21, 12:57 pm, Jerry Avins <j...@ieee.org> wrote: >>>>>>>>> PalapaGuy wrote: >>>>>>>>>> Single-sideband FM was a popular topic several years ago but I >>>>>>>>>> haven't seen any articles on it since then. It looks attractive >>>>>>>>>> as a way to transmit narrowband FM efficiently at VHF over short >>>>>>>>>> distances. Does anyone know of current uses for this >>>>>>>>>> technology? >>>>>>>>> Narrow-band FM (NBFM) is the same as low-modulation-percentage AM >>>>>>>>> with the carrier shifted 90 degrees. If the modulation index is >>>>>>>>> low enough, what you mention is identical to the SSB-SC that is >>>>>>>>> standard on ham and CB bands. I suppose people eventually >>>>>>>>> realized that. Or is there some other meaning to what you write? >>>>>>>>> Jerry >>>>>>>>> -- >>>>>>>>> Engineering is the art of making what you want from things you >>>>>>>>> can get. >>>>>>>>> > ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ >>>>>>>> Not sure it behaves the same. FM thresholds but AM doesn't. AM >>>>>>>> (SSB) has problems demodulating it I suppose whereas with FM you >>>>>>>> can easily use a PLL. >>>>>>> NBFM doesn't threshold. You confuse the properties of FM with those >>>>>>> of SSB NBFM. As far as I know,n SSB version of >>>>>>> high-modulation-index FM doesn't exist. >>>>>>> Jerry >>>>>>> -- >>>>>>> Engineering is the art of making what you want from things you can >>>>>>> get. >>>>>>> > ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ >>>>>> NBFM must threshold. In fact it thresholds worse than wide-band >>>>>> since the beta is lower. That anoying sound on every police radio in >>>>>> the UK (as it was of course). >>>>>> Hardy >>>>> Mischa Schwartz et al. discuss the FM threshold effect in quite a bit >>>>> of analytical detail (a quad integration is performed at one point on >>>>> four-dimensional probability distribution!) in section 3.6, p134. >>>>> --Randy >>>>> @book{schwartzcommtechniques, >>>>> title = "Communication Systems and Techniques", author = "{Mischa >>>>> Schwartz and William R. Bennett and Seymour Stein}", publisher = >>>>> "IEEE Press (reissue, originally New York: McGraw-Hill 1966)", year >>>>> = "1996"} >>>> My Schwartz, "Information Transmission, Modulation and Noise", 1st >>>> Edition, 1959; $10.34, says in italics* on p.304, section 6.3, >>>> /"Narrowband FM thus provides no S/N improvement over AM."/ >>>> >>> Yes but Fm thresholds. SSB loses lock of course. >> Don't you get it? Thresholding happens when excess bandwidth is used to >> improve SNR. When the signal is too low, the excess bandwidth allows >> more noise into the receiver than the bandwidth-for-SNR trade rejects: >> that's the threshold. With NBFM, there is no excess bandwidth, hence no >> SNR improvement, hence no threshold. Unless, of course, your gain >> control is calibrated up to 11. >> >> Jerry > > If you limit the IF signal in NBFM then you do see reduced noise at high > signal levels (because you're clipping out the portion of the noise > that's in phase with the carrier) and enhanced noise at low signal levels > (because you're amplifying the snot out of the noise). > > So it does do something akin to thresholding -- but it's certainly much > softer than the weak signal behavior of broadcast FM.Distortion results if you clip NBFM in the IF. Take low-modulation-%age AM, strip out the carrier, and reintroduce it with quadrature shift. Voila! NBFM!* Clipping either one distorts. You may have sort-of-narrowband FM in mind. Jerry _____________________________ * One of Armstrong's early FM stations had this kind of modulator. The FCC requires crystal control all the way, and the options for crystal-controlled FM modulators are limited. Armstrong used frequency multipliers to increase both the carrier frequency and deviation, then heterodyned that signal to a low carrier frequency, keeping the higher deviation intact. A few passes of that (multiply up, heterodyne down) and he got the standard (50 MHz band) deviation he needed. Clever guy! -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●September 22, 20092009-09-22
Laloo wrote: ...> The excess BW itself is not why threshold happens but it does > exacerbate it. The threshold effect is explained in quite a bit of > detail in books by Taub and Schilling and Misha Shwartz et al. as it > was orignally studied by Rice. To understand it fully, one needs to > look at the phasor diagram of the signal and noise. When noise is > small (high SNR), it only causes the recieved signal phasor to have > small pertubations around the signal phasor and hence only small > variations in the received phase. However, as the noise becomes large > (below thresold), it can result in the recieved phasor making full > encirclements around the origin and each of those encirclements in a > 2pi phase shift in the received phase (cycle slip in PM or clicks in > FM). The noise contribution from these clicks results in the rapid > decrease in the SNR for input CNRs below threshold.Agreed. I soft-pedaled the details. As to whether the excess bandwidth is the cause of quieting, I take no stand. It certainly makes quieting possible, ans allows more noise into the receiver while doing that.> The threshold effect is a consequence of these clicks. It is not true > that clicks happen only for large modulation indices but it is true > that the BW is larger for larger modulation indices and thus more > noise enters the receiver increasing the rate of clicks. That is why > the threshold effect occurs at lower CNR for higher modulation indices > but it occurs for all modulation indices including small ones. > > Anyone interested in studying the mechanism of threshold effect and > the accompanying analysis should check out books by Taub and Schilling > and Misha Shwartz et al. > > As far as the statement "Narrowband FM thus provides no S/N > improvement over AM", the gain FM provides over AM is a function of > modulation index as one trades off BW, so if the modulation index is > low, there will be no S/N improvement but it depends on the modulation > index."Narrow" is merely qualitative. When the modulation index is high enough to permit some noise reduction, "NBFM" doesn't accurately describe the signal. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●September 22, 20092009-09-22
On Sep 23, 5:23�am, Tim Wescott <t...@seemywebsite.com> wrote:> On Mon, 21 Sep 2009 23:05:25 -0700, HardySpicer wrote: > > On Sep 21, 10:04�am, Jerry Avins <j...@ieee.org> wrote: > >> Randy Yates wrote: > >> > HardySpicer <gyansor...@gmail.com> writes: > > >> >> On Sep 20, 8:01 pm, Jerry Avins <j...@ieee.org> wrote: > >> >>> HardySpicer wrote: > >> >>>> On Sep 21, 12:57 pm, Jerry Avins <j...@ieee.org> wrote: > >> >>>>> PalapaGuy wrote: > >> >>>>>> Single-sideband FM was a popular topic several years ago but I > >> >>>>>> haven't seen any articles on it since then. �It looks attractive > >> >>>>>> as a way to transmit narrowband FM efficiently at VHF over short > >> >>>>>> distances. �Does anyone know of current uses for this > >> >>>>>> technology? > >> >>>>> Narrow-band FM (NBFM) is the same as low-modulation-percentage AM > >> >>>>> with the carrier shifted 90 degrees. If the modulation index is > >> >>>>> low enough, what you mention is identical to the SSB-SC that is > >> >>>>> standard on ham and CB bands. I suppose people eventually > >> >>>>> realized that. Or is there some other meaning to what you write? > >> >>>>> Jerry > >> >>>>> -- > >> >>>>> Engineering is the art of making what you want from things you > >> >>>>> can get. > > �����������������������������������������������������������������������>> >>>> Not sure it behaves the same. FM thresholds but AM doesn't. AM > >> >>>> (SSB) has problems demodulating it I suppose whereas with FM you > >> >>>> can easily use a PLL. > >> >>> NBFM doesn't threshold. You confuse the properties of FM with those > >> >>> of SSB NBFM. As far as I know,n SSB version of > >> >>> high-modulation-index FM doesn't exist. > > >> >>> Jerry > >> >>> -- > >> >>> Engineering is the art of making what you want from things you can > >> >>> get. > > ����������������������������������������������������������������������� > > > > >> >> NBFM must threshold. In fact it thresholds worse than wide-band > >> >> since the beta is lower. That anoying sound on every police radio in > >> >> the UK (as it was of course). > > >> >> Hardy > > >> > Mischa Schwartz et al. discuss the FM threshold effect in quite a bit > >> > of analytical detail (a quad integration is performed at one point on > >> > four-dimensional probability distribution!) in section 3.6, p134. > > >> > --Randy > > >> > @book{schwartzcommtechniques, > >> > � title = "Communication Systems and Techniques", author = "{Mischa > >> > � Schwartz and William R. Bennett and Seymour Stein}", publisher = > >> > � "IEEE Press (reissue, originally New York: McGraw-Hill 1966)", > >> > � year = "1996"} > > >> My Schwartz, "Information Transmission, Modulation and Noise", 1st > >> Edition, 1959; $10.34, says in italics* on p.304, section 6.3, > >> /"Narrowband FM thus provides no S/N improvement over AM."/ > > > Yes but Fm thresholds. SSB loses lock of course. > > As Jerry has been saying, NBFM doesn't threshold, not really. �For that > matter, SSB has no lock to lose. > > So you're wrong on two counts. > > You _do_ understand the difference between broadcast FM and narrow-band > FM, yes? > > --www.wescottdesign.comI have heard it thresholding many times on police radios. When there is no signal at all you get thresholding. Not so with AM. (though SSB is a little different). Hardy
Reply by ●September 22, 20092009-09-22
HardySpicer wrote: ...> I have heard it thresholding many times on police radios. When there > is no signal at all you get thresholding.What kind of threshold exists in the absence of a signal? ... Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
