On 3 Feb 2004 19:22:04 -0800, jon8spam@yahoo.com (Jon) wrote:
(snipped)
>
>A second question is could anyone explain how to use the hilbert
>transform to demodulate an FM signal. I hear that it is possible but
>I am unsure how to use the hilbert transform to demodulate an FM
>signal.
>
>Thanks,
>
>Jon
Hi,
well, the idea is that you perform the Hilbert
transform on a real sequence, I(n), to obtain a
"quadrature" sequence Q(n). Now Q(n)'s spectral
components are all shifted in phase by 90 deg
relative to I(n)'s spec components.
Creating a complex sequence, C(n) = I(n)+jQ(n)
allows you to measure the instantaneous phase
of C(n) using arctangent. In theory, the instantaneous
frequency (your demodulated FM) is then computed
by taking the derivative of the real-valued
instantaneous phase.
There are all sorts of practical issues to worry
about in this process:
* how to efficiently perform arctangents
* how to accurately perform differentiation
* is auto gain control (AGC) being used
* what happens if the magnitude of
C(n) is fluctuating with time.
Frerking's FM demod (Figure 6.20) in his 1st
edition gets around some of these problems.
His equations contain a couple of
"typos" which I imagine have been corrected
in later editions.
(His scheme has been described many times
here on comp.dsp over recent years.)
I have a chapter on the Hilbert transform, and
discuss FM demodulation, in the 2nd edition of my
DSP book - which I hope'll be available in late
March-early April.
Good Luck,
[-Rick-]
Reply by Peter Haslev●February 4, 20042004-02-04
> How does a frequency offset differ from a DC modulation component?
The whole truth does not lie in the answer to your question! (..neither in
my contribution to the thread)
Adding a message signal to this unintended DC offset often make the
demodulator operate in a non-linear area.
Depending on which demodulation scheme that is used you can experience a
distortion on the either the upper or lower part of the message signal with
respect to your reference. The extend of this problem is ofcourse releated
to the non-linearity of your receiver.
Regards
Peter H
"Jerry Avins" <jya@ieee.org> wrote in message
news:402075a5$0$8381$61fed72c@news.rcn.com...
> Jon wrote:
>
> > Hi all,
> > After seaching the comp.dsp archive for FM demod I did not find
> > anything on the case where there is a frequency offset between the
> > transmitter and receiver. Is there a reference that explains how the
> > frequency offset between the transmitter and reciever effects the
> > performace of the demodulator? I assume that a digital PLL
> > implementation would be required or would the method in the Frerking
> > book work if there is a frequency offset.
> >
> > A second question is could anyone explain how to use the hilbert
> > transform to demodulate an FM signal. I hear that it is possible but
> > I am unsure how to use the hilbert transform to demodulate an FM
> > signal.
> >
> > Thanks,
> >
> > Jon
>
>
> Narrow-band FM is essentially AM with a 80 degree carrier shift.
>
> Jerry
> --
> Engineering is the art of making what you want from things you can get.
> �����������������������������������������������������������������������
>
Reply by Jerry Avins●February 4, 20042004-02-04
Jon wrote:
> Hi all,
> After seaching the comp.dsp archive for FM demod I did not find
> anything on the case where there is a frequency offset between the
> transmitter and receiver. Is there a reference that explains how the
> frequency offset between the transmitter and reciever effects the
> performace of the demodulator? I assume that a digital PLL
> implementation would be required or would the method in the Frerking
> book work if there is a frequency offset.
>
> A second question is could anyone explain how to use the hilbert
> transform to demodulate an FM signal. I hear that it is possible but
> I am unsure how to use the hilbert transform to demodulate an FM
> signal.
>
> Thanks,
>
> Jon
How does a frequency offset differ from a DC modulation component?
Narrow-band FM is essentially AM with a 80 degree carrier shift.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Jon●February 3, 20042004-02-03
Hi all,
After seaching the comp.dsp archive for FM demod I did not find
anything on the case where there is a frequency offset between the
transmitter and receiver. Is there a reference that explains how the
frequency offset between the transmitter and reciever effects the
performace of the demodulator? I assume that a digital PLL
implementation would be required or would the method in the Frerking
book work if there is a frequency offset.
A second question is could anyone explain how to use the hilbert
transform to demodulate an FM signal. I hear that it is possible but
I am unsure how to use the hilbert transform to demodulate an FM
signal.
Thanks,
Jon