What L and C values should I choose to determine the resonance?

Hi,


Have a simple question.


When using the equation (attached below) to determine the resonant frequency of a combination of an Inductor L and Capacitor I am a bit confused on what values of L and C to use.

If i want to make a resonant circuit at 10MHz lets say. I have a capacitor that is 100pF at low frequencies (under 1 MHz), but when i check its value at 10MHz it has risen to around 400pF.

So when using the equation below, do i use the C value that is on the packaging (100pF) or do I have to use the C value measured at the frequency of which i want the circuit to resonate (in this case 10MHz).

Same applies to the L value?



Sorry for the crap question Smile


Could someone just please shed some light this Smile


Thanks

Cat

How do you know that the capacitor's value is 400 pF at 10 MHz? What made you draw this conclusion?

I used an impedance analyser to view the C value at different frequencies.

But anyway, what i'm asking is do i use the C value measured at 10MHz in the equation?

You would use the equivalent capacitance measured as close as possible to the resonant frequency. But your measuremetn sounds suspect. Capacitors do not change value by a factor of 4 going from 10 to 400 MHz!

same with inductors?

I also have other capacitors that seem to be inductive reactive at 10MHz, does this make them useless as capacitors?

Can anyone recommend specific types of capacitors or resistors that are good for RF circuits at around 10MHz?




Cat

Where the heck are you buying your capacitors, some 1950's surplus shop? Get with the times dude. Almost any decent ceramic chip capacitor <10,000 pF will be fine up to at least a few GHz, and sometimes up to 15 GHz.

They're silvered mica capacitors which are also meant to be good for RF.

I'm starting to think the impedance analyser setup i have is the culprit :(

Check your impedance analyser setup by measuring a 100 pF chip capacitor...

Can i actually measure the capacitance of a capacitor using a signal generator set at 10MHz feeding a simple circuit with the capacitor in series with a 1 ohm resistor, and then measure the phase difference with an oscilloscope?


Is this an accurate method?

As in resonance with others, where are you buying the capacitors dude? I use Murata components the self resonant frequency is quite high. Also trying to measure capacitance using instruments do not give accurate values in my opinion. And if there are it might be very costly.

try to check with murata components they have special components for high frequency and even in that case 10 MHz is not that high a frequency to worry about.

Prakash.

You may choose smaller packaging of capacitor such as 0402 or 0201 size. When the package dimension of the capacitors that will gain higher SRF (Self-Resonate Frequency) and reduce the parasitic inductance. And the ideal equation (w=1/sqrt(LC)) can be used in the frequency below SRF of the L and C component.

Ya, i think you should be consider on the SRF of the capacitor to make sure that it's operate well within your desire frequency range. Get and study the datasheet of the respective cap before using it in any design.