communication – How do you find Image Rejection Ratio from doing FFT on ADC samples?
I’m trying to understand a particular code written by a former employee for measuring performance of an RX chain in a transceiver. I have to measure IRR(Image Rejection Ratio).
Certain input RF frequency is down converted to a baseband frequency of 76.8MHz. The output is sampled using ADC. I have 512 samples and sampling rate is 983.04 MHz.
My previous employee code roughly says
int Signal_Index= fft.maxPowerIndex();` //**I understand this line**--
it is to bin #40 which makes sense because 40*Fs/512=76.8MHz
int Image_Index=fft.size()-Signal_Index;` // **I don't understand this.**
After that he proceeds to calculate difference between power levels at both index and hence IRR
But I don’t understand why image is at a different bin when in fact both image and signal are both down converted to same frequency.
Please note
- LO freq and input RF freq are both greater than 30GHz
- I understand what an Image is- that both signal and image are equal distance from LO freq. The relation being ω_im = ω_in + 2ω_IF
Have I read the code wrongly?
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