If I have a digital FM system do you have any background on the max bit rate you can achieve for a given depth of modulation? eg for 250kHz depth of modulation I can achieve only 500kb/s. Does this sound right? This must be pretty basic stuff I am sure and related to Shannons law somewhere. Thanks Tom
Max Bit rate question
Started by ●July 22, 2003
Reply by ●July 22, 20032003-07-22
Tom wrote:> > If I have a digital FM system do you have any background on the max > bit rate you can achieve for a given depth of modulation? eg for > 250kHz depth of modulation I can achieve only 500kb/s. Does this sound > right? This must be pretty basic stuff I am sure and related to > Shannons law somewhere. > > Thanks > > TomI might be able to help if I knew what you meant by digital FM. Can you explain it? Where is it used? What kind of signals are sent with it? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●July 22, 20032003-07-22
Jerry Avins <jya@ieee.org> wrote in message news:<3F1DB3DD.2A5B6CB6@ieee.org>...> Tom wrote: > > > > If I have a digital FM system do you have any background on the max > > bit rate you can achieve for a given depth of modulation? eg for > > 250kHz depth of modulation I can achieve only 500kb/s. Does this sound > > right? This must be pretty basic stuff I am sure and related to > > Shannons law somewhere. > > > > Thanks > > > > Tom > > I might be able to help if I knew what you meant by digital FM. Can you > explain it? Where is it used? What kind of signals are sent with it? > > JerryIf he's talking about binary symbols only then I'd assume he means FSK. As far as I remember the maximum rate you can achieve using this methodology is basically determined by the degree to which the two spectral lines for 0 and 1 state become smeared together. That is, if your symbol rate it very slow then the spectrum will look like two spectral lines 500kHz apart. As you increase the bit rate the two peaks begin to widen and at some point they will overlap so much that you won't be able to tell them apart. At this point you'll probably have so much inter-symbol interference that you wont be able to detect what you transmitted. How much you can tolerate will depend upon how you are decoding the signal at the receiver end and how good your signal to noise ratio is. I'd guess that 500kb/s with 250kHz depth is a bit optimistic but I'm not experience in FSK so don't quote me on that. Cheers, Paavo Jumppanen. Author of AtSpec : A 2 channel PC based FFT spectrum analyzer http://www.taquis.com
Reply by ●July 22, 20032003-07-22
Paavo Jumppanen wrote:> Jerry Avins <jya@ieee.org> wrote in message news:<3F1DB3DD.2A5B6CB6@ieee.org>... > > Tom wrote: > > > > > > If I have a digital FM system do you have any background on the max > > > bit rate you can achieve for a given depth of modulation? eg for > > > 250kHz depth of modulation I can achieve only 500kb/s. Does this sound > > > right? This must be pretty basic stuff I am sure and related to > > > Shannons law somewhere. > > > > > > Thanks > > > > > > Tom > > > > I might be able to help if I knew what you meant by digital FM. Can you > > explain it? Where is it used? What kind of signals are sent with it? > > > > Jerry > > If he's talking about binary symbols only then I'd assume he means > FSK. As far as I remember the maximum rate you can achieve using this > methodology is basically determined by the degree to which the two > spectral lines for 0 and 1 state become smeared together. > > That is, if your symbol rate it very slow then the spectrum will look > like two spectral lines 500kHz apart. As you increase the bit rate the > two peaks begin to widen and at some point they will overlap so much > that you won't be able to tell them apart. At this point you'll > probably have so much inter-symbol interference that you wont be able > to detect what you transmitted. > > How much you can tolerate will depend upon how you are decoding the > signal at the receiver end and how good your signal to noise ratio is. > > I'd guess that 500kb/s with 250kHz depth is a bit optimistic but I'm > not experience in FSK so don't quote me on that. > > Cheers, > > Paavo Jumppanen. > Author of AtSpec : A 2 channel PC based FFT spectrum analyzer > http://www.taquis.comSorry Jerry I should have been clearer - yes it is FSK but is there a fundamental formula that I can use to find the theoertical maximum bit rate. Tom
Reply by ●July 23, 20032003-07-23
aberdonian_2000@yahoo.com (Tom) wrote in message news:<e1b1658f.0307221244.575e9f86@posting.google.com>...> If I have a digital FM system do you have any background on the max > bit rate you can achieve for a given depth of modulation? eg for > 250kHz depth of modulation I can achieve only 500kb/s. Does this sound > right? This must be pretty basic stuff I am sure and related to > Shannons law somewhere. > > Thanks > > TomTom, If you are talking about FSK, then there is no limit. The problem is that the bandwidth just keeps getting bigger and bigger. If you had asked what the maximum bitrate is for a given bandwidth, then an answer could be negotiated. --Randy
Reply by ●July 23, 20032003-07-23
"Randy Yates" <yates@ieee.org> wrote in message news:567ce618.0307231342.4637d16f@posting.google.com...> aberdonian_2000@yahoo.com (Tom) wrote in messagenews:<e1b1658f.0307221244.575e9f86@posting.google.com>...> > If I have a digital FM system do you have any background on the max > > bit rate you can achieve for a given depth of modulation? eg for > > 250kHz depth of modulation I can achieve only 500kb/s. Does this sound > > right? This must be pretty basic stuff I am sure and related to > > Shannons law somewhere.> If you are talking about FSK, then there is no limit. The problem > is that the bandwidth just keeps getting bigger and bigger. If > you had asked what the maximum bitrate is for a given bandwidth, > then an answer could be negotiated.Well, I think when you exceed, or even get close to, the carrier frequency you should not call it FSK anymore. It may be that, technically, you can still decode the bits from the sidebands, it isn't because you are demodulating the FM signal. -- glen
Reply by ●July 23, 20032003-07-23
PaavoJumppanen@iname.com (Paavo Jumppanen) wrote in message news:<1a72ed76.0307221814.5e485578@posting.google.com>...> Jerry Avins <jya@ieee.org> wrote in message news:<3F1DB3DD.2A5B6CB6@ieee.org>... > > Tom wrote: > > > > > > If I have a digital FM system do you have any background on the max > > > bit rate you can achieve for a given depth of modulation? eg for > > > 250kHz depth of modulation I can achieve only 500kb/s. Does this sound > > > right? This must be pretty basic stuff I am sure and related to > > > Shannons law somewhere. > > > > > > Thanks > > > > > > Tom > > > > I might be able to help if I knew what you meant by digital FM. Can you > > explain it? Where is it used? What kind of signals are sent with it? > > > > Jerry > > If he's talking about binary symbols only then I'd assume he means > FSK. As far as I remember the maximum rate you can achieve using this > methodology is basically determined by the degree to which the two > spectral lines for 0 and 1 state become smeared together. > > That is, if your symbol rate it very slow then the spectrum will look > like two spectral lines 500kHz apart. As you increase the bit rate the > two peaks begin to widen and at some point they will overlap so much > that you won't be able to tell them apart. At this point you'll > probably have so much inter-symbol interference that you wont be able > to detect what you transmitted. > > How much you can tolerate will depend upon how you are decoding the > signal at the receiver end and how good your signal to noise ratio is. > > I'd guess that 500kb/s with 250kHz depth is a bit optimistic but I'm > not experience in FSK so don't quote me on that. > > Cheers, > > > Paavo Jumppanen. > Author of AtSpec : A 2 channel PC based FFT spectrum analyzer > http://www.taquis.comHello Paavo, I don't believe your analysis is correct. On the other hand, I could be totally off too. Here's how I'm thinking of the situation: If you quadrature downconverted the signal to baseband using a high enough sample rate that any spectrum that is lost is negligible, then you can detect the frequency with a simple d theta / dt, where theta is the instantaneous phase of the complex I/Q data. Now since Tom didn't specify, we can assume a noiseless system ( ;) ). Then the frequency will be perfectly detected with no errors and we will be able to decode the bits. What do you think? As I stated in a parallel response, without any constraints on bandwidth and/or noise, I don't think there is a limit on the bit rate, no matter what the frequency spacing is.
Reply by ●July 24, 20032003-07-24
yates@ieee.org (Randy Yates) wrote in message news:<567ce618.0307231455.4ce300f7@posting.google.com>...> PaavoJumppanen@iname.com (Paavo Jumppanen) wrote in message news:<1a72ed76.0307221814.5e485578@posting.google.com>... > > Jerry Avins <jya@ieee.org> wrote in message news:<3F1DB3DD.2A5B6CB6@ieee.org>... > > > Tom wrote: > > > > > > > > If I have a digital FM system do you have any background on the max > > > > bit rate you can achieve for a given depth of modulation? eg for > > > > 250kHz depth of modulation I can achieve only 500kb/s. Does this sound > > > > right? This must be pretty basic stuff I am sure and related to > > > > Shannons law somewhere. > > > > > > > > Thanks > > > > > > > > Tom > > > > > > I might be able to help if I knew what you meant by digital FM. Can you > > > explain it? Where is it used? What kind of signals are sent with it? > > > > > > Jerry > > > > If he's talking about binary symbols only then I'd assume he means > > FSK. As far as I remember the maximum rate you can achieve using this > > methodology is basically determined by the degree to which the two > > spectral lines for 0 and 1 state become smeared together. > > > > That is, if your symbol rate it very slow then the spectrum will look > > like two spectral lines 500kHz apart. As you increase the bit rate the > > two peaks begin to widen and at some point they will overlap so much > > that you won't be able to tell them apart. At this point you'll > > probably have so much inter-symbol interference that you wont be able > > to detect what you transmitted. > > > > How much you can tolerate will depend upon how you are decoding the > > signal at the receiver end and how good your signal to noise ratio is. > > > > I'd guess that 500kb/s with 250kHz depth is a bit optimistic but I'm > > not experience in FSK so don't quote me on that. > > > > Cheers, > > > > > > Paavo Jumppanen. > > Author of AtSpec : A 2 channel PC based FFT spectrum analyzer > > http://www.taquis.com > > Hello Paavo, > > I don't believe your analysis is correct. On the other hand, I > could be totally off too. > > Here's how I'm thinking of the situation: If you quadrature > downconverted the signal to baseband using a high enough sample > rate that any spectrum that is lost is negligible, then you can > detect the frequency with a simple d theta / dt, where theta is > the instantaneous phase of the complex I/Q data. Now since Tom > didn't specify, we can assume a noiseless system ( ;) ). Then > the frequency will be perfectly detected with no errors and we > will be able to decode the bits. > > What do you think? As I stated in a parallel response, without > any constraints on bandwidth and/or noise, I don't think there > is a limit on the bit rate, no matter what the frequency spacing > is.Hello Randy, I did qualify my assertion with the notion that it will depend upon how you detect it at the receiver end and I was thinking of a real system rather than a noiseless one. Without any noise you're probably correct, though I'm not experienced enough to enough to say. On the other hand I do think that there are real practical limitations on the bit rate. Was Toms question purely theoretical, or did it have a practical stance. I'm assuming that his assertion that he could not achieve more than 500kb/s means it's a practical question. Any comments Tom? I'd also be interested in knowing what sort of Rx topology he's using. Cheers, Paavo Jumppanen, Author of AtSpec : A 2 channel PC based FFT spectrum analyzer http://www.taquis.com
Reply by ●July 24, 20032003-07-24
"glen herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message news:<6hETa.115923$GL4.30071@rwcrnsc53>...> "Randy Yates" <yates@ieee.org> wrote in message > news:567ce618.0307231342.4637d16f@posting.google.com... > > aberdonian_2000@yahoo.com (Tom) wrote in message > news:<e1b1658f.0307221244.575e9f86@posting.google.com>... > > > If I have a digital FM system do you have any background on the max > > > bit rate you can achieve for a given depth of modulation? eg for > > > 250kHz depth of modulation I can achieve only 500kb/s. Does this sound > > > right? This must be pretty basic stuff I am sure and related to > > > Shannons law somewhere. > > > If you are talking about FSK, then there is no limit. The problem > > is that the bandwidth just keeps getting bigger and bigger. If > > you had asked what the maximum bitrate is for a given bandwidth, > > then an answer could be negotiated. > > Well, I think when you exceed, or even get close to, the carrier frequency > you should not call it FSK anymore. It may be that, technically, you can > still decode the bits from the sidebands, it isn't because you are > demodulating the FM signal.Why isn't it? If I quadrature downconvert to baseband and then compute the derivative of the instantaneous phase, isn't that demodulating an FM signal?
Reply by ●July 24, 20032003-07-24
"Randy Yates" <yates@ieee.org> wrote in message news:567ce618.0307241135.76c03856@posting.google.com...> "glen herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in messagenews:<6hETa.115923$GL4.30071@rwcrnsc53>...> > "Randy Yates" <yates@ieee.org> wrote in message > > news:567ce618.0307231342.4637d16f@posting.google.com... > > > aberdonian_2000@yahoo.com (Tom) wrote in message > > news:<e1b1658f.0307221244.575e9f86@posting.google.com>... > > > > If I have a digital FM system do you have any background on the max > > > > bit rate you can achieve for a given depth of modulation? eg for > > > > 250kHz depth of modulation I can achieve only 500kb/s. Does thissound> > > > right? This must be pretty basic stuff I am sure and related to > > > > Shannons law somewhere. > > > > > If you are talking about FSK, then there is no limit. The problem > > > is that the bandwidth just keeps getting bigger and bigger. If > > > you had asked what the maximum bitrate is for a given bandwidth, > > > then an answer could be negotiated. > > > > Well, I think when you exceed, or even get close to, the carrierfrequency> > you should not call it FSK anymore. It may be that, technically, youcan> > still decode the bits from the sidebands, it isn't because you are > > demodulating the FM signal. > > Why isn't it? If I quadrature downconvert to baseband and then > compute the derivative of the instantaneous phase, isn't that > demodulating an FM signal?Say the carrier was 1Hz, FM modulated at 1MHz. Now you want to quadrature downconvert to 0Hz? Hmm. Where do you get the reference signal for the downconverter? How accurate is the reference signal? Does it make sense to call a 1Hz signal modulated at 1MHz FSK? -- glen