Is this a school project?

I believe this was discussed earlier (a couple of years ago) you should do a search in here.

Search for: RCWL-0516

Modules available on eBay for $1 price class. Reportedly operates at about 3.2 GHz. I'm not sure if the design includes the baseband output (audio of the Doppler tone).

@itsmoked: Its not a school project. I have an idea, but lack the knowledge. Please give relevant answers if you have any. Thank you.

@VE1BLL: Already tried the readily available modules. So we plan to design a circuit from scratch.

Modules from eBay are quite often based on the applicable datasheet's example circuit. SO, at the very least, they provide a useful starting point (after cleaning up the design shortcuts often included) for any chipset or design concept.

Many of the other modules operate on 10.525 or 24.xxx GHz. That PN I mentioned was the only one I've seen that was in the lower frequency range that you mentioned. You'll have to consider the legal ISM bands in your Region and jurisdiction for this "Field Disturbance" application.

For Doppler, typically the LO is the transmitted RF. These days, the oscillator would presumably work directly at the operating frequency (without requiring a Multiplier).

Good luck.