I am currently working on a project where I have to study a microwave Doppler Radar. Here are some details so that you can better understand it : a local oscillator (LO) at 24GHz is linked to an emitter antenna. A moving target reflects back the emitted wave with some Doppler effect, which is taken back in a receptor antenna. A double-diode mixer follows, taking the LO signal and the received signal, it is linked to an IF filter/amplifier so as to extract the Rice components of the signal.
I have two questions. First on the oscillator, shown here :
24GHz corresponds to WR-42 waveguide format, which we can see on the pic. The two upper pins are the diodes (outputs) + a third pin for ground reference. But what kind of input is shown in red ?
Second on the IF filter/amplifier, shown in block diagram here :
The purpose of a bandpass filter is to extract a frequential component at ω (reflected frequency) minus ω0 (LO frequency), while filtering upper components from the diode mixer and lower componants as DC or 50Hz. But there are two things I don't understand here : why two stages of bandpass filter / amplifier ? And why finally a low pass filter ? Aren't all upper frequency already filtered ?