LPF filter

Altera_Forum · ‎08-19-2008

Hi everyone,

i have to make an operation of filtering, in particular i need to implement a low pass filter working in a range of frequency from 0 to 50 khz; i tried to use FIR compiler tool to generate the LPF filter (fcut-off=5e4,sr=5e5,window type=hanning) and then i implemented it in a system with a nios and a NCO (generated with NCO generator). I connected the exit of the NCO with the entrance of LPF to check the correct working of filter.

Analyzing exit signal of filter with SIGNALTAP, after downloading system on my Nios II Development Kit Stratix II Edition, i've noticed that operation of filtering doesn't work in the right way; in fact if i vary the frequency of the sine wave generated by NCO (for example 30 khz and 70 khz), the amplitude of the sine wave filtered (exit of LPF) is always the same, in both case, and it doesn't seem to be filtered.

Have i made some errors configuring filter or my system?

Any idea?

Thank you!

Altera_Forum · ‎08-25-2008

Everything seems correct from what you write.

Anyway, just a couple of clues:

1) Have you used 500kHz sampling rate also for the NCO output?

2) Are you using an excessive output truncation factor which may hide the filtering effect?

Altera_Forum · ‎08-26-2008

Thanks for your answer!

You were right, my problem was about sampling rate for the NCO output.

Now it's all ok.

Thank you again!