Hi,I created a simple simulink model using DSP builder 9.1 sp1 blocks. The model runs at 120Mhz with 20.02Mhz input signal. After generating 20Mhz cos and sin signals from NCO block, the input signal is multiplied with NCO outputs, so that output signals are located at 0.02Mhz and around 40Mhz. Now I use FIR compilers to generate LPF for getting rid of 40Mhz image signals. However, the LPF output is not what I am expecting. Please review the attached model and share your feedback. Thank you. tim
I don't have your tools but as you said, you get -0.02MHz and 40.02MHz.I checked your filter coefficients and it works and removes 40.02MHz. You are left with -0.02MHz. Your 0.02MHz will need several thousand samples to show one period. May be you are looking at few samples.
Thank you for your comment.I collected many samples to see the pattern, but I couldn't see proper filter output. Also, FFT scope shows incorrect result, too. Could you run this model in your machine if available? Thanks again. Tim
Unfortunately I couldn't open your files.If you want to do fft in Matlab on your vectors then: IQ = complex(I,Q); f = linspace(-60,60,length(IQ)); plot(f,20*log10(abs(fftshift(fft(IQ)))));