From: jacobfenton on
Perhaps I need to give some more information.

The I-Q data is coming from an AD9874 IC. I can change the rate at which
is sends me data. But I have a requirement that I need to have a certain IF
bandwidth filter, which is achieved in the AD9874 and is affected by what I
tell the AD9874 to set its decimation factor too, which corresponds to how
fast I get data. This IF bandwidth is rather large, therefore I cannot set
the decimation in the AD9874 to high, so I need to accept the fast data
rate.

Do I make the correct assumption that I must do the FM demodulation at
this high data rate, otherwise if I decimate more inside the DSP before I
demodulate, I effectively reduce my IF bandwidth filter?

It does seem that I can decimate after demodulation for my tone
detection.

I also have bandwidth requirements for my tones I must detect. I put a
matlab file together which does my Goertzel algorithm, therefore the bin
size affected the bandwidth of the tone I could detect along with the
threshold I set. So if N gets too large, my bandwidth for detection does
down it seems. So there is some compromise between N and bandwith of tone I
can detect.

Also is there a faster FM demodulation formula I could use, or is the one
I posted the best one?

Thanks all for your input.

-Jacob Fenton
From: Fred Marshall on
jacobfenton wrote:
> Perhaps I need to give some more information.
>
> The I-Q data is coming from an AD9874 IC. I can change the rate at which
> is sends me data. But I have a requirement that I need to have a certain IF
> bandwidth filter, which is achieved in the AD9874 and is affected by what I
> tell the AD9874 to set its decimation factor too, which corresponds to how
> fast I get data. This IF bandwidth is rather large, therefore I cannot set
> the decimation in the AD9874 to high, so I need to accept the fast data
> rate.
>
> Do I make the correct assumption that I must do the FM demodulation at
> this high data rate, otherwise if I decimate more inside the DSP before I
> demodulate, I effectively reduce my IF bandwidth filter?
>
> It does seem that I can decimate after demodulation for my tone
> detection.
>
> I also have bandwidth requirements for my tones I must detect. I put a
> matlab file together which does my Goertzel algorithm, therefore the bin
> size affected the bandwidth of the tone I could detect along with the
> threshold I set. So if N gets too large, my bandwidth for detection does
> down it seems. So there is some compromise between N and bandwith of tone I
> can detect.
>
> Also is there a faster FM demodulation formula I could use, or is the one
> I posted the best one?
>
> Thanks all for your input.
>
> -Jacob Fenton

I get the impression that you're overconstraining your approach.
For example, why would N affect the bandwidth of the tones you can
detect? If N is too small then I can imagine the filter bandwidth being
too large but not the other way around.

Fred
From: Jerry Avins on
Fred Marshall wrote:
> jacobfenton wrote:
>> Perhaps I need to give some more information.
>>
>> The I-Q data is coming from an AD9874 IC. I can change the rate at which
>> is sends me data. But I have a requirement that I need to have a
>> certain IF
>> bandwidth filter, which is achieved in the AD9874 and is affected by
>> what I
>> tell the AD9874 to set its decimation factor too, which corresponds to
>> how
>> fast I get data. This IF bandwidth is rather large, therefore I cannot
>> set
>> the decimation in the AD9874 to high, so I need to accept the fast data
>> rate.
>>
>> Do I make the correct assumption that I must do the FM demodulation at
>> this high data rate, otherwise if I decimate more inside the DSP
>> before I
>> demodulate, I effectively reduce my IF bandwidth filter?
>>
>> It does seem that I can decimate after demodulation for my tone
>> detection.
>>
>> I also have bandwidth requirements for my tones I must detect. I put a
>> matlab file together which does my Goertzel algorithm, therefore the bin
>> size affected the bandwidth of the tone I could detect along with the
>> threshold I set. So if N gets too large, my bandwidth for detection does
>> down it seems. So there is some compromise between N and bandwith of
>> tone I
>> can detect.
>>
>> Also is there a faster FM demodulation formula I could use, or is the one
>> I posted the best one?
>>
>> Thanks all for your input.
>>
>> -Jacob Fenton
>
> I get the impression that you're overconstraining your approach.
> For example, why would N affect the bandwidth of the tones you can
> detect? If N is too small then I can imagine the filter bandwidth being
> too large but not the other way around.

The distribution of electric power was delayed for years and thought to
be entirely impractical. For maximum power transfer, one matches the
load to the generator, and that allows only 50% efficiency in the ideal
case. Edison realized that making the load's impedance much higher than
generator's avoided that loss.

Jerry
--
Engineering is the art of making what you want from things you can get.
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From: jacobfenton on
>jacobfenton wrote:
>> Perhaps I need to give some more information.
>>
>> The I-Q data is coming from an AD9874 IC. I can change the rate at
which
>> is sends me data. But I have a requirement that I need to have a
certain IF
>> bandwidth filter, which is achieved in the AD9874 and is affected by
what I
>> tell the AD9874 to set its decimation factor too, which corresponds to
how
>> fast I get data. This IF bandwidth is rather large, therefore I cannot
set
>> the decimation in the AD9874 to high, so I need to accept the fast
data
>> rate.
>>
>> Do I make the correct assumption that I must do the FM demodulation at
>> this high data rate, otherwise if I decimate more inside the DSP
before I
>> demodulate, I effectively reduce my IF bandwidth filter?
>>
>> It does seem that I can decimate after demodulation for my tone
>> detection.
>>
>> I also have bandwidth requirements for my tones I must detect. I put a
>> matlab file together which does my Goertzel algorithm, therefore the
bin
>> size affected the bandwidth of the tone I could detect along with the
>> threshold I set. So if N gets too large, my bandwidth for detection
does
>> down it seems. So there is some compromise between N and bandwith of
tone I
>> can detect.
>>
>> Also is there a faster FM demodulation formula I could use, or is the
one
>> I posted the best one?
>>
>> Thanks all for your input.
>>
>> -Jacob Fenton
>
>I get the impression that you're overconstraining your approach.
>For example, why would N affect the bandwidth of the tones you can
>detect? If N is too small then I can imagine the filter bandwidth being

> too large but not the other way around.
>
>Fred
>

I am not trying to constrain my approach any more then the requirements
are set upon me, but perhaps I don't fully understand everything, which is
what I am trying to do, learn.
From: Fred Marshall on
jacobfenton wrote:
>> jacobfenton wrote:
>>> Perhaps I need to give some more information.
>>>
>>> The I-Q data is coming from an AD9874 IC. I can change the rate at
> which
>>> is sends me data. But I have a requirement that I need to have a
> certain IF
>>> bandwidth filter, which is achieved in the AD9874 and is affected by
> what I
>>> tell the AD9874 to set its decimation factor too, which corresponds to
> how
>>> fast I get data. This IF bandwidth is rather large, therefore I cannot
> set
>>> the decimation in the AD9874 to high, so I need to accept the fast
> data
>>> rate.
>>>
>>> Do I make the correct assumption that I must do the FM demodulation at
>>> this high data rate, otherwise if I decimate more inside the DSP
> before I
>>> demodulate, I effectively reduce my IF bandwidth filter?
>>>
>>> It does seem that I can decimate after demodulation for my tone
>>> detection.
>>>
>>> I also have bandwidth requirements for my tones I must detect. I put a
>>> matlab file together which does my Goertzel algorithm, therefore the
> bin
>>> size affected the bandwidth of the tone I could detect along with the
>>> threshold I set. So if N gets too large, my bandwidth for detection
> does
>>> down it seems. So there is some compromise between N and bandwith of
> tone I
>>> can detect.
>>>
>>> Also is there a faster FM demodulation formula I could use, or is the
> one
>>> I posted the best one?
>>>
>>> Thanks all for your input.
>>>
>>> -Jacob Fenton
>> I get the impression that you're overconstraining your approach.
>> For example, why would N affect the bandwidth of the tones you can
>> detect? If N is too small then I can imagine the filter bandwidth being
>
>> too large but not the other way around.
>>
>> Fred
>>
>
> I am not trying to constrain my approach any more then the requirements
> are set upon me, but perhaps I don't fully understand everything, which is
> what I am trying to do, learn.

Jacob,

I'm sorry I wasn't of more help as I don't fully understand your
constraints. But, I trust my gut and that's what it was telling me. I
hope I gave you enough of a hint to be able to explore those woods of
yours on your own.

I *think* I understand the IF issue:

You have a very high carrier frequency or center frequency shall we say?
Pre-sampling, the tones show up there and are quite close to a multiple
of the sampling frequency. In fact, about 7-13 kHz away / close.

Post-sampling IQ, the tones are where they're advertised to be and I'd
say that the "Intermediate Frequency" or IF is intentionally set to be
zero. And, from the information you've given us, the IF bandwidth needs
to be around 16kHz.

I've not yet taken into account that the samples are IQ so there may be
a factor of 2 smaller bandwidth needed as each set of samples only needs
to occupy at most -/+8kHz and the sample frequency 16kHz. I'm not able
to reconcile this with the reported "tone frequencies". If the IQ
sampling were done efficiently then the tones would show up
symmetrically around zero with the lower ones (or upper?) below zero and
the upper ones above zero frequency. But, this isn't something that
I've done so I'd best not go there....

So, now we're really into it.... the AD9874 technical article says:
"For example, with a clock frequency of 24 MHz the signal bandwidth can
be as low as 12.5 kHz or as high as 250 kHz."

Hmmmm .... you say you have an IF bandwidth "requirement" but didn't say
what that bandwidth is. So, is it different than the tones given would
imply? Otherwise I'd be looking for less input bandwidth or Fs coming
into the DSP.

Fred


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