Basic Signal Processing Question

John, Jerry,
     Thanks for the excellent discussion. I was frustrated in the
beginning when I posted the question becuase I don't seem to explain it
in a very good way and nobody seems to follow it very well what I was
asking.

     Just to clarify the answer I was looking for was what John Moore
gave and later seconded by you.

     I have done my own bit of reading and it is clear that if you
sample a sine wave(Fin) with a clock(+1, 0) signal only then you can
retain the baseband and not when if the clock is (+1, -1). Perhaps it
is called modulation but the point that you guys clarified, which is

"IF THE SAMPLING CLOCK HAS NO DC COMPONENT YOUR OUTPUT DOES NOT HAVE
THE BASEBAND(Fin)"

It is very frustrating that none of the books (I refered atleast 3
books that are very popular in analog and digital singal processing
world). explain this phenomnon and everyone assumes that it is obvious.

Also another thing none of them clearly distinguishes is the fact that
"Sample of Hold" operation is different from Simple sampling process
because Sample and Hold has inband droop (Sinx/x droop as someone
mentioned) due to the hold operation.  I was confused by the fact that
sampling clock  (50% duty cycle clock (+1, 0)) also has a Sinx/x droop
for its harmonics however since the baseband signal (Fin) is sampled by
DC and not by the fundemental clock frequency or its harmonics there is
no inband droop.

Is there a better way of communicating in this forum may be attaching
some pictures or something that I can use to quickly explain what my
question is about instead of so confusing so many people??

Venkat.Vijay.Kumar@gmail.com wrote:
> John, Jerry,
>      Thanks for the excellent discussion. I was frustrated in the
> beginning when I posted the question becuase I don't seem to explain it
> in a very good way and nobody seems to follow it very well what I was
> asking.
> 
>      Just to clarify the answer I was looking for was what John Moore
> gave and later seconded by you.
> 
>      I have done my own bit of reading and it is clear that if you
> sample a sine wave(Fin) with a clock(+1, 0) signal only then you can
> retain the baseband and not when if the clock is (+1, -1). Perhaps it
> is called modulation but the point that you guys clarified, which is
> 
> "IF THE SAMPLING CLOCK HAS NO DC COMPONENT YOUR OUTPUT DOES NOT HAVE
> THE BASEBAND(Fin)"
> 
> It is very frustrating that none of the books (I refered atleast 3
> books that are very popular in analog and digital singal processing
> world). explain this phenomnon and everyone assumes that it is obvious.
> 
> Also another thing none of them clearly distinguishes is the fact that
> "Sample of Hold" operation is different from Simple sampling process
> because Sample and Hold has inband droop (Sinx/x droop as someone
> mentioned) due to the hold operation.  I was confused by the fact that
> sampling clock  (50% duty cycle clock (+1, 0)) also has a Sinx/x droop
> for its harmonics however since the baseband signal (Fin) is sampled by
> DC and not by the fundemental clock frequency or its harmonics there is
> no inband droop.
> 
> Is there a better way of communicating in this forum may be attaching
> some pictures or something that I can use to quickly explain what my
> question is about instead of so confusing so many people??
> 

Vencat,
I suspect the reason the textbooks don't mention the relationship 
between the DC component and the presence of the baseband signal is that 
  this issue only arises when the feasibility of other sampling 
waveforms is investigated.

In analysing the effect of sampling, a description of the sampling 
process: "measure the instantaneous signal value at regular intervals" 
pretty well defines the sampling waveform that is needed.  This gives 
the results we want, so there is no actual need to investigate other 
waveforms. In any case, it is hard to see how any other waveform could 
accomplish this particular task.  Of course it may be an interesting 
exercise to do the investigation, as you have found.

Regarding the sin(x)/x droop, the mere fact that a sample-and -hold 
circuit is present does not introduce sin(x)/x droop.  Once a sample is 
captured it does not matter how long it is held while the DAC does its 
work, as long as the process finishes in time for the next sample. 
Droop does not occur until we translate this instantaneous measurement 
into a broad pulse at the output of the ADC.

Regards,
John

Hi John,
          Thanks for your response. I agree with your comment that we
may not need other waveform to do sampling. But this basic point is
critical if you are trying to undestand the basic sampling and do
further study on non-idealities.

       For example I am currently looking at the impact of jitter on
sampling. If you look at the PLL spectrum in all text books they show
it as phase noise around the clock frequency. And the impact of this
jitter is very small if you use for sampling (Especially if you have
high over sampling).

       But if you go from basic priciple of sampling that it is the DC
that samples the input signal and not the clock frequency and it is the
phase noise around the DC that corrupts the input signal and not the
phase noise around the clock frequency.

      To understand these kind of things I felt it is essential to
understand the basic process and not to have any assumptions..

Just my openion.

    I have a follow up question on jitter that I will post it as a
seperate question.. Thanks for your help.

Venkat.Vijay.Kumar@gmail.com wrote:

   ...

>        But if you go from basic priciple of sampling that it is the DC
> that samples the input signal and not the clock frequency and it is the
> phase noise around the DC that corrupts the input signal and not the
> phase noise around the clock frequency.

   ...

This is an expression of such profound misunderstanding that I have no 
idea how to address it. I will think about it for a while. For one 
thing, DC has no phase, but that's the least of it.

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