Hi there,

Could anyone help me with setting up a graph of the frequency dependency of a capacitors value? (so a graph of C as a function of f)
I understand i can simply put a square wave on a capacitor and 'measure' the tau (at 63.2% for charging). But is there a more easy and more accurate way to do this with an oscilloscope and a function generator?

Thanks in advance.

 

If trying to measure Z vs F you could do the following -

http://www.nutsvolts.com/magazine/article/a_low_cost_rf_impedance_analyzer

https://www.google.com/url?sa=t&rct...e_Bridge.pdf&usg=AOvVaw0Tm4IcPZwE9PrK_6BiKLKp

http://www.wb5rvz.com/rlc_meter/05_cal.php

Regards, Dana.

 

You can set up a test circuit using AC on a Wheatstone Bridge.

Replace Z1 and Z2 with fixed resistors.
Replace Z3 with the capacitor under test.
Replace Z4 with a variable resistor.

Note that here you are measuring impedance and not capacitance.

 

Thats so simple with caps: the C graph is strictly horisontal line, or other words, capacity is not changed by the frequency. What are changed - that is impedance, as the inductive component practically unseen at lower frequencies becomes more and more strong until over the resonance frequency of this capacitor, the capacitor becomes a good inductor . Next is tan(fi) what eats more and more energy off of this capacitor. Next is resistive component where mister`s Focault formulae adds a new and new barriers for current flowing aka whirls.And now, with a large tail we may come back to the my first sentence - little bit anyway capacitance really changes, indirectly: - the Focault makes a metallisated layers hot and tan(fi) makes the dielectric even more hot. But hot substances are widdening (surface ~T^2 and the gap ~T^1) thus the capacitance is changing lineary ~T^1. Yet because the most of dielectrics are, at least in some degree, piezoelectrics, they have a Curie temperature, thus with the shift this temperature off one may see the drastic drift of capacitance.
But this was INDIRECT effect.
So, I wanted to bold that there is no big benefit to measure the TKC except the rare cases when one need to balance two caps, one with negative TKC anmd other with positive to get a zero point zero zero. But normally there is very attractive importance to know the IMPEDANCE of capacitor, to prognoze the resonance frequency of it.

 

Thank you guys very much. This all really clarified my viewings on this subject. Gonna start with measurements first in the morning.

Kind regards