Hi,
I have sampled the  sinosoidal signal of 200Mhz at a rate of 500Mhz and i 
made decimation by factor of 25 on the above samples.The o/p of decimation
is 20 mhz signal..I passed the above signal to a Digital bandpass filter(
198mhz--202Mhz).Later if i take the absolute value of the filter op ..what
will be the output...whether it will be a sine wave  or a constant
DC(video)..pls help me  in this regard...
vengadesh

vengadeshkumar wrote:
> Hi,
> I have sampled the  sinosoidal signal of 200Mhz at a rate of 500Mhz and i 
> made decimation by factor of 25 on the above samples.The o/p of decimation
> is 20 mhz signal..I passed the above signal to a Digital bandpass filter(
> 198mhz--202Mhz).Later if i take the absolute value of the filter op ..what
> will be the output...whether it will be a sine wave  or a constant
> DC(video)..pls help me  in this regard...

It should look like a full-wave rectified 200 MHz si8gnal if all was 
done correctly.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:
> 
> vengadeshkumar wrote:
> > Hi,
> > I have sampled the  sinosoidal signal of 200Mhz at a rate of 500Mhz and i
> > made decimation by factor of 25 on the above samples.The o/p of decimation
> > is 20 mhz signal..I passed the above signal to a Digital bandpass filter(
> > 198mhz--202Mhz).Later if i take the absolute value of the filter op ..what
> > will be the output...whether it will be a sine wave  or a constant
> > DC(video)..pls help me  in this regard...
> 
> It should look like a full-wave rectified 200 MHz si8gnal if all was
> done correctly.

So what's the answer if it's not done correctly:^)

	It looks to me that in his first sentence, at the point where he decimates by
25 the signal should be aliased to DC (assuming the 200 and 500 in the first
part of that sentence are perfectly accurate). After that sentence his
description failed to make sense to me.

-jim


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jim wrote:
> 
> Jerry Avins wrote:
>> vengadeshkumar wrote:
>>> Hi,
>>> I have sampled the  sinosoidal signal of 200Mhz at a rate of 500Mhz and i
>>> made decimation by factor of 25 on the above samples.The o/p of decimation
>>> is 20 mhz signal..I passed the above signal to a Digital bandpass filter(
>>> 198mhz--202Mhz).Later if i take the absolute value of the filter op ..what
>>> will be the output...whether it will be a sine wave  or a constant
>>> DC(video)..pls help me  in this regard...
>> It should look like a full-wave rectified 200 MHz si8gnal if all was
>> done correctly.
> 
> So what's the answer if it's not done correctly:^)
> 
> 	It looks to me that in his first sentence, at the point where he decimates by
> 25 the signal should be aliased to DC (assuming the 200 and 500 in the first
> part of that sentence are perfectly accurate). After that sentence his
> description failed to make sense to me.


It seems to me that it aliases all over the place, and that the bandpass 
filter selects the 200 MHz alias. Then the absolute value .... Aah! I 
assumed analog reconstruction before the ABS. Maybe that doesn't matter.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:
> 
> jim wrote:
> >
> > Jerry Avins wrote:
> >> vengadeshkumar wrote:
> >>> Hi,
> >>> I have sampled the  sinosoidal signal of 200Mhz at a rate of 500Mhz and i
> >>> made decimation by factor of 25 on the above samples.The o/p of decimation
> >>> is 20 mhz signal..I passed the above signal to a Digital bandpass filter(
> >>> 198mhz--202Mhz).Later if i take the absolute value of the filter op ..what
> >>> will be the output...whether it will be a sine wave  or a constant
> >>> DC(video)..pls help me  in this regard...
> >> It should look like a full-wave rectified 200 MHz si8gnal if all was
> >> done correctly.
> >
> > So what's the answer if it's not done correctly:^)
> >
> >       It looks to me that in his first sentence, at the point where he decimates by
> > 25 the signal should be aliased to DC (assuming the 200 and 500 in the first
> > part of that sentence are perfectly accurate). After that sentence his
> > description failed to make sense to me.
> 
> It seems to me that it aliases all over the place, 

No not all over the place - it aliases to just one place. If he has a 2 Hz
sinusoid and samples it at 5 hz the signal repeats every 5 samples (every 2
cycles). Decimating that by 5 (selecting every 5th sample and discarding the
rest) turns the signal to DC. Decimating by 5 again (total decimation = 25)
still is at DC. Assuming whatever he did after that is linear the signal should
stay at DC.

-jim

 


>and that the bandpass
> filter selects the 200 MHz alias. Then the absolute value .... Aah! I
> assumed analog reconstruction before the ABS. Maybe that doesn't matter.
> 
> Jerry
> --
> Engineering is the art of making what you want from things you can get.
> ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯


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On Sep 12, 7:32 pm, "vengadeshkumar" <vengade...@gmail.com> wrote:

> Later if i take the absolute value of the filter op ..what
> will be the output...whether it will be a sine wave  or a constant
> DC(video)..pls help me  in this regard...
> vengadesh

What do you need it to be?

Assorted variations on your idea could result in either, and several
other things as well (recitified sine wave for example as was already
pointed out)

For example, if you want a constant DC level reflecting the signal
level, you should mix it with two 90 degree phase separated local
oscillators to generate I and Q phases (or sample twice with shifted
sampling clocks, which is the same thing) treat them together as a
complex number, and take the magnitude of that.

If you want a sinusoid, mix it down to whatever frequency you want a
sinusoid at.