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Altera_Forum
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16 years ago

CIC and FIR megacores

Dear Forum I have designed a narrowband LPF using matlab which comprises a 12 stage CIC decimating filter with a decimation rate of 495. This is followed by a 361 tap Compensating FIR. This co...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
16 years ago

Hi FvM and thepancake

Forget the interpolation issue for the moment as I'm not too sure what the requirements are the demod section of the receiver. I'm assuming that the interpolation would just mean that I get back to the same sample rate as before the CIC, but with a nice clean signal.

Let's just get back to the original question. I don't seem to be able to connect the CIC m/c to the LPF m/c and get sensible results.

I have a 16-bit wide input to the CIC, and am selected the "Hogenauer pruning across all stages" option, to keep my output at 16-bits also. But all I get is a flat line out of the CIC as an impulse response - so nothing to go into the FIR for further processing. Is there some limit to how much 'pruning' the Hogenauer algorithm can handle? The CIC is now 9 stages, with a decimation rate of 4 (just as an exmaple - I'm actually using the variable rate option). With pruning on I get nothing out, with pruning off (and using the full resolution of 97 bits) I seem to get a reasonable impulse response.

Any ideas?

Thanks, Kurt