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filter not converged in Matlab

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senmeis

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Hi,

I have created a downconverter with dsp.DigitalDownConverter System object https://www.mathworks.com/help/dsp/ref/dsp.digitaldownconverter-class.html, but got this warning: The filter design has not converged. Try increasing the passband ripple value, or decreasing the stopband attenuation.

After making DecimationFactor greater this warning disappears. Is it normal for filters? Is the filter unstable if it’s not converged?

Owen
 

zorro

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The filter design method uses an iterative procedure to find the solution.
Sometimes, this iterative procedure fails to converge, especially when the requirements are too stringent.
When that happens, it doesn't mean that you obtained an unstable filter. It simply means that you didn't got a filter at all.
Regards

Z
 

senmeis

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The filter design method uses an iterative procedure to find the solution.
Sometimes, this iterative procedure fails to converge, especially when the requirements are too stringent.
When that happens, it doesn't mean that you obtained an unstable filter. It simply means that you didn't got a filter at all.
Regards

Z

In reality the filter can be created even with this warning. The filter operates well too. I’m just afraid someday an error occurs.

Owen
 

zorro

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Hi Owen,

Does the filter meet the specifications (passband ripple and stopband attenuation)?

Z
 

senmeis

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Hi Owen,

Does the filter meet the specifications (passband ripple and stopband attenuation)?

Z

My parameters look like these:

DecimationFactor = 32
SampleRate = 1e10
Bandwidth = 300e6
StopbandAttenuation = 58
PassbandRipple = 0.2
CenterFrequency = 240e6

This shall down convert a 240MHz passband signal to baseband.

Owen
 

zorro

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Hi Owen,

In reality the filter can be created even with this warning. The filter operates well too.

The question is:
If you plot the transfer function (amplitude vs. frequency) of the created filter, does it have the specified characteristics? i.e.:

Bandwidth = 300e6
StopbandAttenuation = 58
PassbandRipple = 0.2
CenterFrequency = 240e6

Regards

Z
 

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