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Jerry Avins
Guest
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 Posted:
Mon Dec 05, 2005 11:24 pm Post subject:
Re: FM Demodulator |
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Vladimir Vassilevsky wrote:
| Quote: |
Jerry Avins wrote:
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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Techniques are like materials in that many can do any particular job,
but differ in how are best used. Imagine a bricklayer who cuts 2x4s into
brick-size pieces and sets them in mortar.
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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Mark
Guest
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| Posted:
Tue Dec 06, 2005 12:50 am Post subject:
Re: FM Demodulator |
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Threshold extension FM demodulators are very interesting.
Mark |
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Jim Thomas
Guest
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| Posted:
Tue Dec 06, 2005 5:16 pm Post subject:
OT: Wooden Rocks (was Re: FM Demodulator) |
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Jerry Avins wrote:
| Quote: | Techniques are like materials in that many can do any particular job,
but differ in how are best used. Imagine a bricklayer who cuts 2x4s into
brick-size pieces and sets them in mortar.
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Sounds a little like George Washington. His house at Mount Vernon has a
wooden exterior, but the planks are beveled to look like stone. He had
them mix sand with the paint so it would have a stone-like texture. It
looks a lot like stone from a distance, and I doubt I'd have noticed if
it hadn't been pointed out.
--
Jim Thomas Principal Applications Engineer Bittware, Inc
jthomas@bittware.com http://www.bittware.com (603) 226-0404 x536
The secret to enjoying your job is to have a hobby that's even worse
- Calvin's Dad |
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Jerry Avins
Guest
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| Posted:
Tue Dec 06, 2005 5:16 pm Post subject:
Re: OT: Wooden Rocks (was Re: FM Demodulator) |
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Jim Thomas wrote:
| Quote: | Jerry Avins wrote:
Techniques are like materials in that many can do any particular job,
but differ in how are best used. Imagine a bricklayer who cuts 2x4s
into brick-size pieces and sets them in mortar.
Sounds a little like George Washington. His house at Mount Vernon has a
wooden exterior, but the planks are beveled to look like stone. He had
them mix sand with the paint so it would have a stone-like texture. It
looks a lot like stone from a distance, and I doubt I'd have noticed if
it hadn't been pointed out.
|
But the planks, however decorated, remain planks. They aren't cut to
stone-size pieces.
I used to be an intellectual-snob kind of purist. I've gotten over it.
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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Stan Pawlukiewicz
Guest
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| Posted:
Tue Dec 06, 2005 5:16 pm Post subject:
Re: OT: Wooden Rocks (was Re: FM Demodulator) |
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Jim Thomas wrote:
| Quote: | Jerry Avins wrote:
Techniques are like materials in that many can do any particular job,
but differ in how are best used. Imagine a bricklayer who cuts 2x4s
into brick-size pieces and sets them in mortar.
Sounds a little like George Washington. His house at Mount Vernon has a
wooden exterior, but the planks are beveled to look like stone. He had
them mix sand with the paint so it would have a stone-like texture. It
looks a lot like stone from a distance, and I doubt I'd have noticed if
it hadn't been pointed out.
|
I noticed it right away. Most new houses in Fairfax county, where Mt
Vernon is, have vinyl siding, i.e. plastic that looks like wood.
> |
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John Herman
Guest
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| Posted:
Wed Dec 07, 2005 1:16 am Post subject:
Re: FM Demodulator |
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A limiter followed by a slope detector would give an amplitude modulated
signal. The AM detector that is already done would do the job.
Couldn't you use the Parks-McClellan program to design a filter to do the job
at that point. Someone pointed out the differentiator (or integrator) is also
a slope detector. This is also an option on PM.
In article <aoCdnak3quJPwgnenZ2dnUVZ_sSdnZ2d@rcn.net>, Jerry Avins
<jya@ieee.org> wrote:
| Quote: | Glennaebad wrote:
"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 |
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Jerry Avins
Guest
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| Posted:
Wed Dec 07, 2005 8:33 am Post subject:
Re: FM Demodulator |
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John Herman wrote:
| Quote: | A limiter followed by a slope detector would give an amplitude modulated
signal. The AM detector that is already done would do the job.
|
...
Not exactly. The limiter removes amplitude variations while retaining
the information in the zero crossings; what remains after filtering is
FM at a standard amplitude. Setting the NBFM signal on the slope of the
IF passband adds frequency-dependent amplitude variation, which the AM
detector converts to audio. Slope detection works without a limiter, but
AM noise rides through.
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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