This is a repeat of your previous thread:
Hello all, I'm trying to build a vehicle speed detector using a HB100 radar sensor but I couldn't find a proper code for this. I tried with few codes found on the Internet but I realised that they were not giving a correct output on the serial monitor. As I am a newbie in coding please help me...
www.electro-tech-online.com
A doppler radar module in effect gives a frequency out proportional to the speed of movement of any object that reflects a signal.
The "preamp" circuit you originally link to appears to be a limiter to try and convert the low level AC to logic levels.
Have you built and tried that circuit?
(The noise level etc is irrelevant if it is supposed to be a limiting amplifier; it should have extreme gain).