FIR filter of IQ signal after AD phase discrimination

Recently, I encountered a small problem in the project , Current radar systems have multiple waveforms to choose from , The maximum bandwidth is 300M,AD The module is a module made by the board supplier , For convenience , The other party after the identification IQ The signal passes through a fixed 300M Bandwidth half band filter for low-pass filtering , Output baseband signal .
In practical use , The transmission waveform bandwidth is 300M when , It can achieve the best matched filtering , Have the best signal-to-noise ratio , The bandwidth of the transmitted waveform is less than 300M when , for example 50M, The filter is still pressed 300M Filtering , Because the probability density function of noise is a rectangle in the frequency domain , Other 250M The noise power in the spectrum will also pass through the filter , Cause the power of noise to increase , The signal-to-noise ratio is reduced .
In order to improve the signal-to-noise ratio , You can add another layer to the baseband signal 50M Low pass filter of , In my project , There are two ways to achieve , The first is in the baseband signal IQ Two channels are respectively connected with filter coefficients （ The set of real Numbers ） Convolution ; The second is right IQ The complex signal composed of FFT To the frequency domain , Dot multiply with the frequency domain of the filter coefficients , Then reverse FFT that will do .
In the second implementation method, special attention should be paid , Complex coefficient FIR Filter and real coefficient FIR Filters are slightly different , Suppose the real coefficient is a, Then the corresponding complex coefficient is a+ai, And then we can move on FFT To the frequency domain , You can't simply use real coefficients to transform to the frequency domain .
summary ：
1、AD The post low-pass filter should be designed according to the waveform bandwidth , The low-pass filter in a system with multiple bandwidths must match the waveform , In my case of fixed filter, it can be solved by cascaded filter .
2、 Pay attention to real coefficient and complex coefficient FIR The difference between filters