The lowest frequency!

[Altera_Forum]

everyone for me ask that the lowest frequency when use FIR megacore!

[Altera_Forum]

You should use the same speed as the sampling rate(Fs) of your input signal. 

Your cutoff point is then related to that Fs. For example if a lowpass filter cutoff is 0.25Fs and your signal rate is 12MHz then the cutoff will be 3MHz meaning all frequencies from 0 to 3MHz are passed.   your frequency may be labelled in terms of Fs ratio or in radians(0 ~ pi) where pi means .5Fs   For more advanced designers, you can pass your input signal to a higher or lower data rate(interpolation/decimation)

[Altera_Forum]

Thanks kaz! but i try whith sample rate 10MHz and filter type is bandpass have cutoff are 200Hz and 300Hz, then I see out put equa 0. can you help me?

[Altera_Forum]

Your figures don't sound right to me. You are asking for very unusual cutoff points. You better check your requirements first.

[Altera_Forum]

--- Quote Start ---  but i try whith sample rate 10MHz and filter type is bandpass have cutoff are 200Hz and 300Hz  --- Quote End ---  That's not impossible in theory, but practically, cause you would require several thousand FIR taps in this case. Just consider, that the filter must have at least a length equivalent to several periods of the characteristic filter frequencies. You can either build IIR filters, although they need a rather large coeffecient length and a lot of extra fractional bits, but they are basically feasible for such extreme frequency ratios. Or use a decimation before the filter.

[Altera_Forum]

thanks alot!

[Altera_Forum]

Change your sample rate to something ~10 times your cutoff and you should get decent results. You can change your clock rate by using a clock enable and the multicycle properties in the megawizard for the FIR.