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From: Vladimir Vassilevsky on 18 May 2010 12:00


jacobfenton wrote:

>>You see the aliases of the input through the S-like frequency response
>>of discriminator.
>>
>>
>
> That definetly makes sense to what I am seeing. I have also implimented
> some strong LPF on I,Q prior to demodulation and the aliasing response gets
> attenuated some, but still shows up. I am using pretty strong LPF, 80-100dB
> at stop band. Any hints as to what else I could try?

Your project has the word "homework" written over it; and the way the
processing goes makes very little sense.

VLV

From: jacobfenton on 18 May 2010 12:17

>Your project has the word "homework" written over it; and the way the
>processing goes makes very little sense.
>
>VLV
>
>
The point of my post is to learn about the problem and ask advice from more
experienced engineers. I don't see the point of you posting anything if you
are not going to post something informative about the problem. I am sure
you didn't learn everything on your own........
From: Ron N. on 18 May 2010 13:36
On May 17, 8:00 pm, "jacobfenton" <jacob.fenton(a)n_o_s_p_a_m.gmail.com>
wrote:
> Tone detection is done with Goertzel Algorithm to check for 7.5kHz
> Block = 240
> threshold = 4000
>
> The simulation works very well, and has been backed up by actual hardware
> system.
>
> The interesting thing I am seeing is that if I sweep the FM carrier (with
> modulation) away from 3MHz, and I look at the output from the Goertzel
> detector, the tone magnitude starts to drop off (which is expected) and
> goes aways but then starts to come back again (with very strong magnitude)
> and continues this pattern (sine wave like).

A Goertzel tone detector acts like a 1-bin DFT with a
rectangular window. A rectangular window in the time domain
convolves a signal with a Sinc function in the frequency
domain. So you could be seeing the humps in the amplitude of
the Sinc function as you sweep the carrier frequency across
the DFT bin center.

Plot the shape vs. frequency offset to test this hypothesis.

And it's not exactly a Sinc function, but a decent enough
approximation. A more detailed description is on my dsp
web page:
http://www.nicholson.com/rhn/dsp.html


IMHO. YMMV.
--
rhn A.T nicholson d.0.t C-o-M
From: Richard Owlett on 18 May 2010 14:37
jacobfenton wrote:
>> Your project has the word "homework" written over it; and the way the
>> processing goes makes very little sense.
>>
>> VLV
>>
>>
> The point of my post is to learn about the problem and ask advice from more
> experienced engineers. I don't see the point of you posting anything if you
> are not going to post something informative about the problem. I am sure
> you didn't learn everything on your own........

Not to worry. He gets cranky at times. I haven't had a problem
set in 40 years and he has made similar comments to me ;/

From: Mark on 18 May 2010 17:20

>
> The interesting thing I am seeing is that if I sweep the FM carrier (with
> modulation) away from 3MHz, and I look at the output from the Goertzel
> detector, the tone magnitude starts to drop off (which is expected) and
> goes aways but then starts to come back again (with very strong magnitude)
> and continues this pattern (sine wave like).

The KEY piece of info that is missing is.....

HOW FAR away from 3 MHz are you "sweeping" the FM carrier to see this
effect?

Mark
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