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Topic: Q and Bandwidth (Read 3766 times) previous topic - next topic
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Q and Bandwidth

Hello,

I have a cetnre frequency, and a bandwidth of a pek filter. I wish to calculate the Q value of the bandwidth which seems to be Q = bandwidth/centre_freq.

The confusion comes in where lets say, Q= 1 at 100Hz meaning the bandwidth is 100Hz, then q=1 at 1000Hz has a bandwidth of 1000hz...that means that as the centre frequency goes higher, the higher the bandwidth? I must be missing something right? 

Thanks,

Aristotel

Q and Bandwidth

Reply #1 –
Q = centre_freq / bandwidth

But yes, that still means that for a constant Q, the BW increases as the frequency increases. Do you find this surprising?

Cheers,
Alan
Cheers,
Alan

Q and Bandwidth

Reply #2 –
Quote
Q = centre_freq / bandwidth

But yes, that still means that for a constant Q, the BW increases as the frequency increases. Do you find this surprising?

Cheers,
Alan
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Hi Alan,
thanks for the reply. According to the DAFX book, Q = Bandwidth / freq. Regarding the constant Q, it just seemed a bit of an odd characteristic of using Q value over bandwidth. When you want a filter to have a bandwidth of 500Hz..then thats what it shall be regardless of the filter's centre Frequency. Then when controlling the centre frequency, that is the only thing that is changing about the filter.  So I guess i was just confused by the frequency dependancy of using a Q value....

thanks!

Q and Bandwidth

Reply #3 –
Quote
thanks for the reply. According to the DAFX book, Q = Bandwidth / freq. Regarding the constant Q, it just seemed a bit of an odd characteristic of using Q value over bandwidth. When you want a filter to have a bandwidth of 500Hz..then thats what it shall be regardless of the filter's centre Frequency.
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Higher Q is narrower bandwidth (intuitive if you consider the sharper filter as higher quality), and higher Q comes from higher quality components (mainly less resistance in your inductors and capacitors).  Wanting a constant bandwith with changing center frequency is one reason they invented radios that mix the signal to an intermediate frequency (IF) so the filters would have a fixed center frequency even while the signal frequency could be changed.