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I'm currently conducting an experiment where I need to determine what a black-box component consists of. After measuring input and output voltage for various frequencies I have so far determined that the black box is a first degree passive HP filter (RC), i.e. RC circuit

I've also (based on measuring input / output voltage over various frequencies) learnt that the cutoff frequency is around 3200 Hz (which would mean that the time constant is 49.73591971 µs if I understand it correctly).

But now I'm basically stuck... Based on this information, how would I go about finding the exact values for R and C in this black-box circuit?

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  • \$\begingroup\$ To get R, can't you use an ohmmeter on the output? Then compute C form tau. Or is that just a representation of what it does? PS, you have too many digits in your tau... \$\endgroup\$ – Sredni Vashtar May 10 '15 at 14:20
  • \$\begingroup\$ There is not enough to calculate this but you can measure the resistance directly by applying a DC voltage to the output and measuring the current. Once you have that divide the time constant by the resistance and you have the capacitance. \$\endgroup\$ – Warren Hill May 10 '15 at 14:23
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  1. For passive filters there is a term known as ORDER i.e. first, second, third order etc. It can be found by plotting Frequency Amplitude diagram/graph..including corner frequency and the slope of the amplitude..
  2. for first order one has to have two measuremet of output input at different frequencies.
  3. for higher order more measuremet.more complex..simulatanious equations, matrices etc.

However assuming it isto be simple first order filter: meausre input and output DC resistance..keep the probe firmly connected to output till resistance value stablises. corner frequency= approx. (2*PifC)*R=1 and find out C. vtingole

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Do you know how to measure the input impedance using a periodic signal? Basically, use a function generator to input a signal which is comfortably in the passband. At this frequency, the impedance is determined only by the resistor in the black box. The capacitor is "transparent."

For example, you can add series resistance between the function generator and black box until you find a series resistor that causes the Vout to be half the amplitude of the function generator output. This series resistor will then be the same value as the hidden resistor.

Of course, it would be much easier to measure from Vout to Ground with an Ohmeter. But I am guessing you are not allowed to do that.

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