coherent sampling and I/Q demod

(how) does coherent sampling (used primarily in A/D testing?) differ from
I/Q demodulation? Say for instance I'm using I/Q to close a PLL. In this
case the 'sine wave' I'm locked to is being sampled at x4 the sine
frequency. This would seem to satisfy the coherent sampling requirement
that regarding the digitizer clock. 

next question - coherent sampling removes the need for windowing in FFT
application. If instead of FFT one wants to run the output of the I/Q
demod thru a lowpass FIR, would the 'coherent sampling' remove the need
for windowing to remove passband ripple?


		
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"petethepop" <cameron@bnl.gov> wrote in message
news:24ednRQgdfwaUpreRVn-rQ@giganews.com...
> (how) does coherent sampling (used primarily in A/D testing?) differ from
> I/Q demodulation? Say for instance I'm using I/Q to close a PLL. In this
> case the 'sine wave' I'm locked to is being sampled at x4 the sine
> frequency. This would seem to satisfy the coherent sampling requirement
> that regarding the digitizer clock.
>
> next question - coherent sampling removes the need for windowing in FFT
> application. If instead of FFT one wants to run the output of the I/Q
> demod thru a lowpass FIR, would the 'coherent sampling' remove the need
> for windowing to remove passband ripple?
>
>
>
> This message was sent using the Comp.DSP web interface on
> www.DSPRelated.com

I'm not with you...I/Q demodulation replaces the PLL
altogether..demodulation being done ta baseband rather than the carrier
frequency.You won't need a PLL...please explain.

Shytot