Electronic – Function generator to test filter capacitors

capacitorfilterfunction generator

I'd like to be able to see the effect that a filtering capacitor has on a voltage source with waves at different frequencies, for learning purposes. I'd like to try to calculate the correct capacitor for a specific wave / frequency, and be able to test it was well.

I was wondering if a function generator connected to a circuit with the capacitor would work for this purpose. I already have an oscilloscope for visualizing the voltage graph.

Also, is there anything I should take into consideration when picking a function generator to buy?

Best Answer

It really depends on the value that your trying to measure and to what accuracy you are trying to measure it. Realize that you are going to have noise in your system and probably the biggest limiting factor will be the resolution of your scope. Buy the function generator with a range based on the desired frequency below.

Here is a great article on how to measure capacitance or inductance. In short you are measuring the reluctance of the circuit.

If you know some algebra and want to use this method for capacitors instead of indicators it works well. You pick a point say 1/2 of your input voltage and then solve the equations for that. I usually do a sweep and then fit the entire equation to a curve so I have multiple points that average but only one point should suffice. $$ \left|\frac{Vscope}{Vgen} \right|= \left|\frac{\frac{j\omega}{C}}{R+\frac{j\omega}{C}} \right| = \left|\frac{\omega^2}{(RC)^2+\omega^2} +j\frac{RC\omega}{(RC)^2+\omega^2} \right| = ...= \frac{\omega }{\sqrt{(CR)^2+\omega^2}}$$

And then solving for \$ \omega\$ which is \$ 2\pi f \$ with R = 50

$$ \frac{1}{2}=\frac{\omega }{\sqrt{(CR)^2+\omega^2}} \Rightarrow C=\frac{\omega\sqrt{3} }{R} $$

Here is the circuit for reference

schematic

simulate this circuit – Schematic created using CircuitLab

Related Topic