From: Richard Lamont Date: Wed, 10 Jan 2018 20:41:06 +0000 > On 10/01/18 18:53, Andy Walls wrote:
[snip] > > The filters stay within 0.1 dB of each other up through 5000 Hz, > but by > > then your tabular response has noticeably started to bend > upward. The > > difference between the two filters is 3.44 dB at 15000 Hz. > > It bends up because your plot has got one log axis and one linear > one. Ah yes, of course. And on that note, the bilinear transform design method warps the GNURadio FM de-emphasis filter line downward on the log-log graph, which is expected. Regards, Andy > If plotted with a log frequency axis rather than a linear one then > the > slope at the right should tend to a straight line of 6.02 dB/octave > or > 20 dB/decade. Indeed the idea of dB/octave or dB/decade only makes > sense > if you use a log frequency axis. > > Alternatively, you could use a linear voltage axis instead of dB, and > a > linear frequency axis. That should also tend towards a straight line > on > the right hand side.
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