| Author |
Message |
jjw265
Guest
|
 Posted:
Sat Dec 03, 2005 5:15 pm Post subject:
FM Demodulator |
|
|
Hello,
I am working on designing an FM demodulator for a software-defined radi
class I am taking. For an earlier lab in the course, I designed an A
demodulator using envelope detection. It is my understanding that it i
possible to create an FM demodulator simply by putting a phase detector i
front of my previously designed envelope detector and adjusting th
bandpass filter and sampling frequencies of my AM design to match th
change in station location. Unfortunately, I can not figure out exactl
how to implement the phase detector so this will work. Does anybody hav
any insight as to whether this method is actually possible and if so, ho
to do it? Any help is much appreciated. I am designing this for
Lyrtech Signalwave board (with a xilinx fpga on it) and programming i
simulink.
Thanks!
John |
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|
 |
Real_McCoy
Guest
|
| Posted:
Sat Dec 03, 2005 11:48 pm Post subject:
Re: FM Demodulator |
|
|
"jjw265" <jjw265@psu.edu> wrote in message
news:dvWdnSIQCIf7GAzenZ2dnUVZ_tOdnZ2d@giganews.com...
| Quote: | Hello,
I am working on designing an FM demodulator for a software-defined radio
class I am taking. For an earlier lab in the course, I designed an AM
demodulator using envelope detection. It is my understanding that it is
possible to create an FM demodulator simply by putting a phase detector in
front of my previously designed envelope detector and adjusting the
bandpass filter and sampling frequencies of my AM design to match the
change in station location. Unfortunately, I can not figure out exactly
how to implement the phase detector so this will work. Does anybody have
any insight as to whether this method is actually possible and if so, how
to do it? Any help is much appreciated. I am designing this for a
Lyrtech Signalwave board (with a xilinx fpga on it) and programming in
simulink.
Thanks!
John
If you are demodulating FM with a processor it has to be done at baseband |
using the I and Q signals. If you have the carrier signal there are other
methods (say a PLL) but processors cannot work at that speed. Once you have
I and Q you find the phase from arctan(I/Q) and differentiate. Practically
you don't need to work out arctan at all - just differentiate the expression
on paper and implment the result. It's 1/(1+(I/Q)^2) X d/dt(I/Q) from which
a simple expression can be derived.
McC |
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Vladimir Vassilevsky
Guest
|
| Posted:
Sun Dec 04, 2005 12:24 am Post subject:
Re: FM Demodulator |
|
|
Real_McCoy wrote:
| Quote: | If you are demodulating FM with a processor it has to be done at baseband
using the I and Q signals. If you have the carrier signal there are other
methods (say a PLL) but processors cannot work at that speed. Once you have
I and Q you find the phase from arctan(I/Q) and differentiate.
|
1. You can implement PLL in the baseband as well.
2. There is a better way for building FM discriminator without arctan:
X = I*(dQ/dt) - Q*(dI/dt);
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com |
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|
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Real_McCoy
Guest
|
| Posted:
Sun Dec 04, 2005 1:07 am Post subject:
Re: FM Demodulator |
|
|
"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message
news:aplkf.26201$7h7.5911@newssvr21.news.prodigy.com...
| Quote: |
Real_McCoy wrote:
If you are demodulating FM with a processor it has to be done at
baseband
using the I and Q signals. If you have the carrier signal there are
other
methods (say a PLL) but processors cannot work at that speed. Once you
have
I and Q you find the phase from arctan(I/Q) and differentiate.
1. You can implement PLL in the baseband as well.
Can you provide a reference for this - sounds interesting. |
| Quote: | 2. There is a better way for building FM discriminator without arctan:
X = I*(dQ/dt) - Q*(dI/dt);
|
That's what I just said in my original post above but I did not work out the
details. When you diferentiate the aarctan function on paper you get the
above result.
McC |
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|
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John E. Hadstate
Guest
|
| Posted:
Sun Dec 04, 2005 1:15 am Post subject:
Re: FM Demodulator |
|
|
"jjw265" <jjw265@psu.edu> wrote in message
news:dvWdnSIQCIf7GAzenZ2dnUVZ_tOdnZ2d@giganews.com...
| Quote: | Hello,
I am working on designing an FM demodulator for a
software-defined radio
class I am taking. For an earlier lab in the course, I
designed an AM
demodulator using envelope detection. It is my
understanding that it is
possible to create an FM demodulator simply by putting a
phase detector in
front of my previously designed envelope detector and
adjusting the
bandpass filter and sampling frequencies of my AM design
to match the
change in station location. Unfortunately, I can not
figure out exactly
how to implement the phase detector so this will work.
Does anybody have
any insight as to whether this method is actually possible
and if so, how
to do it? Any help is much appreciated. I am designing
this for a
Lyrtech Signalwave board (with a xilinx fpga on it) and
programming in
simulink.
|
What you are doing is reproducing the original trick that
was used in analog circuitry to demodulate FM: run the
signal through a filter that converts frequency or phase
variation into amplitude variation, then use an envelope
detector to capture the amplitude variation. The simplest
filter that I know of that converts frequency variation to
amplitude variation is a pure differentiator, which you can
implement by subtracting consecutive samples:
y_n = x_n - x_(n-1)
You will probably be left with a very small signal riding on
a very large DC component, which you can eliminate with an
audio band-pass filter. |
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|
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Vladimir Vassilevsky
Guest
|
| Posted:
Sun Dec 04, 2005 1:15 am Post subject:
Re: FM Demodulator |
|
|
Real_McCoy wrote:
| Quote: | 1. You can implement PLL in the baseband as well.
Can you provide a reference for this - sounds interesting.
|
The processing in the baseband is exactly the same as the processing in
the passband (except it has to be complex). Whatever you do in the
passband can also be done in the baseband. That is just the matter of
the preference.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com |
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john
Guest
|
| Posted:
Sun Dec 04, 2005 1:15 am Post subject:
Re: FM Demodulator |
|
|
Cockney_Wanker wrote:
| Quote: | "Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message
news:XGmkf.26215$7h7.5662@newssvr21.news.prodigy.com...
Real_McCoy wrote:
1. You can implement PLL in the baseband as well.
Can you provide a reference for this - sounds interesting.
The processing in the baseband is exactly the same as the processing in
the passband (except it has to be complex). Whatever you do in the
passband can also be done in the baseband. That is just the matter of
the preference.
Cor blimey mate - not with you there. You see a PLL needs a carrier
frequency to work - to lock on to.You cannot lock on to baseband - how would
you set the VCO ? You need to go back to school mate.
CW
|
In a digital PLL, zero Hertz is a legitimate carrier frequency. It just
means the only input to the NCO is the error term from the loop filter
output.
John |
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|
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Cockney_Wanker
Guest
|
| Posted:
Sun Dec 04, 2005 1:16 am Post subject:
Re: FM Demodulator |
|
|
"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message
news:XGmkf.26215$7h7.5662@newssvr21.news.prodigy.com...
| Quote: |
Real_McCoy wrote:
1. You can implement PLL in the baseband as well.
Can you provide a reference for this - sounds interesting.
The processing in the baseband is exactly the same as the processing in
the passband (except it has to be complex). Whatever you do in the
passband can also be done in the baseband. That is just the matter of
the preference.
Cor blimey mate - not with you there. You see a PLL needs a carrier |
frequency to work - to lock on to.You cannot lock on to baseband - how would
you set the VCO ? You need to go back to school mate.
CW |
|
| Back to top |
|
|
Vladimir Vassilevsky
Guest
|
| Posted:
Sun Dec 04, 2005 1:16 am Post subject:
Re: FM Demodulator |
|
|
Cockney_Wanker wrote:
| Quote: | The processing in the baseband is exactly the same as the processing in
the passband (except it has to be complex). Whatever you do in the
passband can also be done in the baseband. That is just the matter of
the preference.
Cor blimey mate - not with you there. You see a PLL needs a carrier
frequency to work - to lock on to.You cannot lock on to baseband - how would
you set the VCO ? You need to go back to school mate.
|
So, the venerable Matlab & Labview crap does not know how to lock on the
carrier in the baseband. You got to have your own head to figure out
such a simple thing, my dear professional.
VLV |
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|
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Craig.Cockup
Guest
|
| Posted:
Sun Dec 04, 2005 9:15 am Post subject:
Re: FM Demodulator |
|
|
"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message
news:4391FE33.2020200@hotmail.com...
| Quote: |
Cockney_Wanker wrote:
The processing in the baseband is exactly the same as the processing in
the passband (except it has to be complex). Whatever you do in the
passband can also be done in the baseband. That is just the matter of
the preference.
Cor blimey mate - not with you there. You see a PLL needs a carrier
frequency to work - to lock on to.You cannot lock on to baseband - how
would
you set the VCO ? You need to go back to school mate.
So, the venerable Matlab & Labview crap does not know how to lock on the
carrier in the baseband. You got to have your own head to figure out
such a simple thing, my dear professional.
VLV
I expect you mean down-concerving to a lower IF - that's not baseband |
operations.
CC |
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|
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Craig.Cockup
Guest
|
| Posted:
Sun Dec 04, 2005 9:15 am Post subject:
Re: FM Demodulator |
|
|
"john" <johns@xetron.com> wrote in message
news:1133647657.981006.108970@g49g2000cwa.googlegroups.com...
| Quote: |
Cockney_Wanker wrote:
"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message
news:XGmkf.26215$7h7.5662@newssvr21.news.prodigy.com...
Real_McCoy wrote:
1. You can implement PLL in the baseband as well.
Can you provide a reference for this - sounds interesting.
The processing in the baseband is exactly the same as the processing
in
the passband (except it has to be complex). Whatever you do in the
passband can also be done in the baseband. That is just the matter of
the preference.
Cor blimey mate - not with you there. You see a PLL needs a carrier
frequency to work - to lock on to.You cannot lock on to baseband - how
would
you set the VCO ? You need to go back to school mate.
CW
In a digital PLL, zero Hertz is a legitimate carrier frequency. It just
means the only input to the NCO is the error term from the loop filter
output.
John
You mean down-convert so that the FM swings from dc upwards. That much I |
knew but I still consider it carrier operations and not baseband.
CC |
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|
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john
Guest
|
| Posted:
Mon Dec 05, 2005 1:16 am Post subject:
Re: FM Demodulator |
|
|
Craig.Cockup wrote:
| Quote: | "john" <johns@xetron.com> wrote in message
news:1133647657.981006.108970@g49g2000cwa.googlegroups.com...
Cockney_Wanker wrote:
"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message
news:XGmkf.26215$7h7.5662@newssvr21.news.prodigy.com...
Real_McCoy wrote:
1. You can implement PLL in the baseband as well.
Can you provide a reference for this - sounds interesting.
The processing in the baseband is exactly the same as the processing
in
the passband (except it has to be complex). Whatever you do in the
passband can also be done in the baseband. That is just the matter of
the preference.
Cor blimey mate - not with you there. You see a PLL needs a carrier
frequency to work - to lock on to.You cannot lock on to baseband - how
would
you set the VCO ? You need to go back to school mate.
CW
In a digital PLL, zero Hertz is a legitimate carrier frequency. It just
means the only input to the NCO is the error term from the loop filter
output.
John
You mean down-convert so that the FM swings from dc upwards. That much I
knew but I still consider it carrier operations and not baseband.
CC
|
Upwards and downwards. |
|
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|
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Glennaebad
Guest
|
| Posted:
Mon Dec 05, 2005 9:16 am Post subject:
Re: FM Demodulator |
|
|
"John E. Hadstate" <jh113355@hotmail.com> wrote in message
news:O5rkf.107576$xK1.76276@bignews7.bellsouth.net...
| Quote: |
"jjw265" <jjw265@psu.edu> wrote in message
news:dvWdnSIQCIf7GAzenZ2dnUVZ_tOdnZ2d@giganews.com...
Hello,
I am working on designing an FM demodulator for a
software-defined radio
class I am taking. For an earlier lab in the course, I
designed an AM
demodulator using envelope detection. It is my
understanding that it is
possible to create an FM demodulator simply by putting a
phase detector in
front of my previously designed envelope detector and
adjusting the
bandpass filter and sampling frequencies of my AM design
to match the
change in station location. Unfortunately, I can not
figure out exactly
how to implement the phase detector so this will work.
Does anybody have
any insight as to whether this method is actually possible
and if so, how
to do it? Any help is much appreciated. I am designing
this for a
Lyrtech Signalwave board (with a xilinx fpga on it) and
programming in
simulink.
What you are doing is reproducing the original trick that
was used in analog circuitry to demodulate FM: run the
signal through a filter that converts frequency or phase
variation into amplitude variation, then use an envelope
detector to capture the amplitude variation. The simplest
filter that I know of that converts frequency variation to
amplitude variation is a pure differentiator, which you can
implement by subtracting consecutive samples:
y_n = x_n - x_(n-1)
You will probably be left with a very small signal riding on
a very large DC component, which you can eliminate with an
audio band-pass filter.
You won't get as good a SNR as using a PLL though. A PLL will always get you |
at least another 3dB.
Glen |
|
| Back to top |
|
|
Jerry Avins
Guest
|
| Posted:
Mon Dec 05, 2005 5:16 pm Post subject:
Re: FM Demodulator |
|
|
Glennaebad wrote:
| Quote: | "John E. Hadstate" <jh113355@hotmail.com> wrote in message
news:O5rkf.107576$xK1.76276@bignews7.bellsouth.net...
"jjw265" <jjw265@psu.edu> wrote in message
news:dvWdnSIQCIf7GAzenZ2dnUVZ_tOdnZ2d@giganews.com...
Hello,
I am working on designing an FM demodulator for a
software-defined radio
class I am taking. For an earlier lab in the course, I
designed an AM
demodulator using envelope detection. It is my
understanding that it is
possible to create an FM demodulator simply by putting a
phase detector in
front of my previously designed envelope detector and
adjusting the
bandpass filter and sampling frequencies of my AM design
to match the
change in station location. Unfortunately, I can not
figure out exactly
how to implement the phase detector so this will work.
Does anybody have
any insight as to whether this method is actually possible
and if so, how
to do it? Any help is much appreciated. I am designing
this for a
Lyrtech Signalwave board (with a xilinx fpga on it) and
programming in
simulink.
What you are doing is reproducing the original trick that
was used in analog circuitry to demodulate FM: run the
signal through a filter that converts frequency or phase
variation into amplitude variation, then use an envelope
detector to capture the amplitude variation. The simplest
filter that I know of that converts frequency variation to
amplitude variation is a pure differentiator, which you can
implement by subtracting consecutive samples:
y_n = x_n - x_(n-1)
You will probably be left with a very small signal riding on
a very large DC component, which you can eliminate with an
audio band-pass filter.
You won't get as good a SNR as using a PLL though. A PLL will always get you
at least another 3dB.
|
Analog or digital, a reasonably linear and broadband slope detector is
difficult to design. You will usually find one only in NBFM, and even
there, there is no AM rejection without a limiter before it.
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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|
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Vladimir Vassilevsky
Guest
|
| Posted:
Mon Dec 05, 2005 5:16 pm Post subject:
Re: FM Demodulator |
|
|
Jerry Avins wrote:
| Quote: | Analog or digital, a reasonably linear and broadband slope detector is
difficult to design. You will usually find one only in NBFM, and even
there, there is no AM rejection without a limiter before it.
Jerry
|
Trying to build the digital model of the analog discriminator is a
mistake; by all means this is the least efficient way to accomplish the
task.
One time I was thinking on design of the maximum likelihood FM
demodulator for the analog signal. The statistics of the audio signal
can be exploited very well. That would be a fun thing to do, although I
doubt if anyone may need it for any practical purpose...
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com |
|
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