# LC filter can by as 10KOhm resistance?

Thread Starter

#### 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
23,565
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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Thread Starter

#### 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
23,565
Looks like you will need 0.56pF and 200uH.

#### Adjuster

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.

#### Adjuster

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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Thread Starter

#### 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

#### Adjuster

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
23,565
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.

Thread Starter

#### 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
16,182
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.

#### Adjuster

Joined Dec 26, 2010
2,148
I've told you what I can. Others, active radio hams or people who work with RF may have a better idea of what is / is not practicable. In the end it may be as well just to try it and see what you get.

Obviously the resonant impedance of a tuned circuit is not identical to a resistor. That may or may not make a difference in your application.

Unfortunately, it's now after midnight my local time, so that's all I have time for. Good luck.

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