Does capacitance changes with frequency?

[fazlina]

Hi!

does capacitance values change with frequency as it sweeps from 0.1 GHz to 100 GHz? why does this happens? why doesn't it stay constant, let say 10 pF all over the frequency? why must it vaties, let say 10 pF and increases up to 50 pF as the frequency increase? can anybody help? Thanks.

 

[bartimaeus]

may be the dielectric materials' properties varies with freq.

at lower frequencies its not there but the effect becomes large enough, at the frequencies you mentioned, for it to cause a change in the capacitance.

thats my guess .

 

[electronics_kumar]

of course it is a frequency dependent component..so that it will vary with respect to frequency

 

[Tinamuline]

normally speaking, capacitance only depends on the physical dimension of the construction. i.e. C = epsilon (d/A). HOwever, if it contains insulating material, then the capacitance of a capacitor will be changed to C' = k(epsilon)(d/A). The k is the dielectric constant of an insulating material which may be affected by internal Electric field (I had better say polarization). Since the polarization of charge distribution of insulating material to some extent may be affected by the input frequency, therefore capacitance changes with frequency provided that the capacitor contains insulating material, otherwise, it doesn't.

 

[Miguel Gaspar]

Well usually we think that all devices are perfect. But that's not correct. If you look up for models of resistors, capacitos and inductors, they are not ideal devices the real models are amix of resistors, capacitors and inductors. So for a certain frequencies capacitances domain but for others frequencies inductances start to domain, and for others inductances domain. That is the razon.
Other factor is the temperature that afects capacitance.

 

[PaulHolland]

Hi, The reason for this is that a capacitor also has some (how small) inductance (leads and internal connections to the plates. This makes the capacitor getting a smaller and smaller value until you get to the resonace point. This is the point at which the induction of the capacitor are the same as the capacitors value XC = XL (XC= 1/(2*PI*f*C) ) XL = 2 * PI * f * L.

Above this point and normally already before this point the deelectric material starts to get high losses and leaking rf currents.

Best capacitors are from ATC that go upto 20 GHz no problem. Normal 0603 from Philips work OK until 1 GHz.

So the main problem is inductance !!. Motorola had a nice formula in an old databook calculating the effective capacitor value if you knew the operating frequency and capacitor value.

regards,

Paul

 

[fazlina]

Hi all!
Thanks for your help...

U guys help me a lot.
Does anyone have any reference which I can refer to?

regards,
Fazlina.

 

[neils_arm_strong]

Capacitance dependss on dilectric constant which in turn is frequency dependent.Therefore capacitance changes with frequency.

 

[RollingEEE]

How does Di-electric constant depend on frequency? Isn't it a fundamental property of a medium?