Max Bit rate question

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

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
-- 
Engineering is the art of making what you want from things you can get.
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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

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.com

Sorry 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

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
> 
> Tom

Tom,

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

"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.

-- glen

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.

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

"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?

"Randy Yates" <yates@ieee.org> wrote in message
news:567ce618.0307241135.76c03856@posting.google.com...
> "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?

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