Filter output value is lower than expected

Question

Hi,

I have a fir compiler with the following settings:

Input Bit Width: 16

Coefficient Scaling: Auto with 16 Bits

Filter Type: LP with 37 Coefficients

Sample Rate: 50 MHz

Cut Off Frequency: 10 MHz

Passband Ripple: near 0 dB

A NCO core with a 16 Bit output supplies the filter with a 1 MHz sinus signal. I analyzed the full resolution output (38 Bits) of the filter in Signal Tap II. 6 MSB Bits were identical so I truncated 5 MSB Bits. I also truncated 17 LSB Bits to get 16 Bits output.

But the filter outputs a sinus signal that is attenuated by about -4 dB.

In the Frequency Response Tab in fir compiler parameterize GUI it should be near 0 dB.

Does anyone have an Idea what the problem could be?

altera_forum · Answer

since you are truncating 17 bits,in effect you are dividing the sum of internal products by 2^17. Thus if you scale your coeffs as:

coeff = coeff/sum(coeff) *2^17 then you should get unity dc gain. but what actually happens at 1MHz may be due to your response

altera_forum · Answer

thanks kaz! I scaled the coefficients as you said and now I get the right output signal.

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Filter output value is lower than expected

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