# LC filter can by as 10KOhm resistance?

#### levnu

Joined Mar 30, 2011
14
Hi to all

is it possible to replace 10KOhm resistor w LC parallel or serial filter or any filter
that will be the same as if this 10 KOhm resistor?

i'm using 10KOhmic res and i would like to achive the same effect w/o using resistance that brings me loses!!!
but using filter at 15MHz freq

Thanks a lot,
Arye

#### MrChips

Joined Oct 2, 2009
20,881
You can calculate the reactance of L and C as follows:

$$X_L= 2\pi fL$$

$$X_C= \frac{-1}{2\pi fC}$$

For a tuned parallel LC circuit, the resonant frequency f is given by:

$$f= \frac{1}{2\pi sqrt{LC}}$$

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#### levnu

Joined Mar 30, 2011
14
Ok this one i know
but when i connecting it instaed of 10KOhm resistance it doesn't have the same stoping effect.. why ??
when i'm in resonance what impedance is equal in such case ?
and is it the same effect as 10KOhm?

Thanks

#### MrChips

Joined Oct 2, 2009
20,881
Looks like you will need 0.56pF and 200uH.

Joined Dec 26, 2010
2,148
The resonant impedance would depend on the reactance of the components, and the Q-factor: information which we do not have.

For instance if C=50pF, at 15MHz Xc = 212Ω, and for a Q of 50 you would expect a parallel equivalent resistance at resonance of 10.6kΩ

But we don't know your circuit values.

Joined Dec 26, 2010
2,148
Looks like you will need 0.56pF and 200uH.
Is that meant to be a joke? The OP may take it literally and go looking for a 560fF capacitor. He may be unaware that a real-world 200μH inductor could have parasitic capacitances greater than this value.

Edit: A few μH would be more realistic, the optimum value depending somewhat on how good a Q could be obtained, and how much parasitic capacitance you have to contend with.

At 15Mhz, don't expect miracles on the Q front. You might get past 10kΩ, just have to try it and see.

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#### levnu

Joined Mar 30, 2011
14
i'm using 0.7uH and 200pF what do you think about that,
it's important for me to use low inductance..

Thanks again,
Arye

Joined Dec 26, 2010
2,148
I think that with an inductance as low as this, the resonant impedance may be fairly low. At 15mHz XL = 2πfL = 66Ω.

Achieving a resonant parallel resistance of 10kΩ would require a Q of 152. I'm not sure if you can get that at 15MHz.

#### MrChips

Joined Oct 2, 2009
20,881
Is that meant to be a joke?
Just checking to see how many folks pay attention to details or if they're even awake when they surf this forum.

#### levnu

Joined Mar 30, 2011
14
does it means there is no way or other filter w higher order that i could achive this impedance? or ohmic resistance that i still dono if its the same in this situation..... 10Kohm resistance or 10 Kohm impedance does it mean the same in my situation?

Arye

#### Papabravo

Joined Feb 24, 2006
13,512
You could only achieve the same impedance as the resistance at a single frequency. The 10K resistor is not dependent on frequency until you get to VHF and above where parasitics start to become significant.