On 6 Mar 2006 06:09:02 -0800, "mazerom"
wrote:

just wanna confirm.
u were saying we can still get a reliable doppler shift from a
modulated signal be it analog or digital?
thank u so much

Yes.

The Doppler frequency equals the rate of change of the distance
expressed in wavelengths. Thus the Doppler shift is a function of
carrier frequency and relative speed difference. It doesn't matter one
bit (no pun intended) what the modulation, if any, is.

Many tactical Radars for instance use pulse/Doppler to determine both
range and rate of closure. An example that I'm most familiar with was
in the Phoenix Missile. The early variant used a pulsed klystron PA
and all analog signal processing. The pulse repetition frequency
(prf) and duty cycle were fixed and I suppose are still classified,
although the missiles are now retired (sob).

http://www.fas.org/man/dod-101/sys/missile/aim-54.htm

The Doppler is typically determined by having a bank of Doppler
filters (at i-f) and looking in each one for the returned signal.
Either the transmitter TO or the receiver LO, but not both, is then
adjusted to put the returned signal in the center filter. In the old
analog system, the VCO(s) were linearized so that the tuning voltage
was an indicator of the Doppler.

With pulse modulation this can get more complicated because there are
modulation sidebands spaced at the prf above and below the nominal
carrier frequency. If the Doppler is greater than the prf then it's
possible for a carrier sideband +/- Doppler to fall into the Doppler
filter bank and it takes some more manipulation to sort this out.

With the digital processor in the later variants the Doppler filters
were realized in software.

Now, aren't you sorry you asked? :-)