# Linear Circuits - Erroneous first order gain observation

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: 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.)

• Could your 4V AC power source be a function generator? Most function generators have an output impedance, forming a Thevenin equivalent (Vac + series resistor). Dec 15, 2019 at 19:44
• 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.
– Chu
Dec 15, 2019 at 20:59
• 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.
– LvW
Dec 16, 2019 at 13:57
• @LvW perhaps answer so I can close this thread? Dec 21, 2019 at 12:39 