At uni, I carried out some lab observations involving a CR circuit (4V AC power source, capacitor, resistor in series, in than order) with the following parameters:


simulate this circuit – Schematic created using CircuitLab

Here are my observations in graph form:

Graph detailing laboratory observations

I attempted to measure the slope in dB/decade and recieved a value of $$g \approx +28.3dB/dec$$ but I thought values for the slope could not exceed $$g = \pm20dB/dec$$ for any first-order filters.

Any ideas why would this happen? (i.e. anything wrong with my lab setup, unforseen variables, etc.)

  • \$\begingroup\$ Could your 4V AC power source be a function generator? Most function generators have an output impedance, forming a Thevenin equivalent (Vac + series resistor). \$\endgroup\$
    – glen_geek
    Dec 15, 2019 at 19:44
  • \$\begingroup\$ For a linear lumped circuit, the slopes of the gain asymptotes will all be integer multiples of 20dB/decade. If you draw the graph over 4 decades, \$10^2\: \rightarrow \:10^6\:Hz\$, the asymptotes would be more apparent. \$\endgroup\$
    – Chu
    Dec 15, 2019 at 20:59
  • \$\begingroup\$ A bad reading results in app. 18dB/dec (not 28dB) and a correct reading - applying the asymptote correctly in the most left lower corner - gives 20 dB/dec, as expected. \$\endgroup\$
    – LvW
    Dec 16, 2019 at 13:57
  • \$\begingroup\$ @LvW perhaps answer so I can close this thread? \$\endgroup\$
    – buzzysin
    Dec 21, 2019 at 12:39

1 Answer 1


It looks like 6 dB per octave to me: -

enter image description here

And 6 dB / octave = 20 dB / decade.

  • \$\begingroup\$ Red is simulated = expected behaviour, blue is observed = strange behaviour \$\endgroup\$
    – buzzysin
    Dec 21, 2019 at 12:33
  • \$\begingroup\$ Blue looks about 6 dB per decade given such things as measurement errors so what’s the problem? \$\endgroup\$
    – Andy aka
    Dec 21, 2019 at 14:10

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