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I've been provided with a Bode plot for an RC filter circuit with a cutoff frequency of 10kHz. Im asked to find the stopband and passband phase from the graph. I'm confused how these can be just a single number, since stopband and passband represent a range of frequencies. (All the frequencies that can pass through the filter and all the frequencies that would be attenuated).

So when giving the stopband or passband phase, should it be the first value of phase in each band? The last value? Maybe a range of values? Or possibly the difference between the first and last values (i.e. how long the band lasts in phase)?

If anyone can help me clear this up I'd appreciate it

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  • \$\begingroup\$ I'm confused how these can be just a single number - I share your confusion - they cannot be a single number. \$\endgroup\$
    – Andy aka
    Apr 23, 2020 at 10:26
  • \$\begingroup\$ @Andyaka so do you think I should write it as a range of values?, e.g. x radians to y radians \$\endgroup\$
    – MendelumS
    Apr 23, 2020 at 10:52
  • \$\begingroup\$ Maybe you need to describe what the phase is doing if it's not constant. The exact numerical values aren't important, they'll change for any given filter. What is important is the overall behaviour. Is the phase approaching any constant value in either of those regions? Is the phase approximately linear in either of those regions? Does it change rapidly anywhere? How about you provide us in your post with the Bode plot you've been provided with? \$\endgroup\$
    – Neil_UK
    Apr 23, 2020 at 10:52
  • \$\begingroup\$ @Neil_UK I don't have a digital copy of the Bode plot unfortunately, but I don't think its asking for what the phase is doing. The actual syntax of the question is "What is the passband phase in radians:" Then it gives a space to answer: "θpass-actual = ......" \$\endgroup\$
    – MendelumS
    Apr 23, 2020 at 10:55

1 Answer 1

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You seem to be overthinking this. This is a basic electronics class usage of how to do simple Bode plots. Just like a Bode amplitude plot is not the exact amplitude plot, but a rough approximate that changes slope only at the calculated 3dB frequency points, the Bode phase plot is similar. So stopband and passband phase are both single numbers that approximate the phase before and after the 3dB points. See Wikipedia article on Bode plots for an example diagram of such Bode phase plot.

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  • \$\begingroup\$ So for that first image on the Wikipedia article (fig. 1a), the passband phase would be 90degrees and the stopband phase would be 0degrees? \$\endgroup\$
    – MendelumS
    Apr 23, 2020 at 12:34
  • \$\begingroup\$ can you confirm? \$\endgroup\$
    – MendelumS
    Apr 25, 2020 at 9:31
  • \$\begingroup\$ Yes, in the first picture yes, and it is so because it is a high pass filter. I don't know whether you have a high pass or a low pass filter. \$\endgroup\$
    – Justme
    Apr 25, 2020 at 10:40
  • \$\begingroup\$ Thanks, appreciate it! My filter is low pass, does that affect the way the values are calculated? My phase plot looks the same (but curved not straight lines) \$\endgroup\$
    – MendelumS
    Apr 25, 2020 at 11:30
  • \$\begingroup\$ Did you see that the second picture in the Wikipedia article is the low-pass example. \$\endgroup\$
    – Justme
    Apr 25, 2020 at 12:47

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