RF filtering question
RF filtering question
(OP)
Hi,
I have to filter RF signals from DC lines. I was thinking using capacitors connected to those lines and ground, shunt capacitors.
I have to filter signals from AM, 150KHz to 2GHz. I was thinking just using small capacitors which SRF is around 2GHz. But looking at some circuits I have seen they use big capacitors, the problem is that its SRF is very low, my question is the following:
Why not use just a small capacitor? as it will behave as a short circuit from very low frequencies to its SFR? unless my approach is not correct and it won´t behave as a short at very low frequencies close to DC.
Thanks.
I have to filter RF signals from DC lines. I was thinking using capacitors connected to those lines and ground, shunt capacitors.
I have to filter signals from AM, 150KHz to 2GHz. I was thinking just using small capacitors which SRF is around 2GHz. But looking at some circuits I have seen they use big capacitors, the problem is that its SRF is very low, my question is the following:
Why not use just a small capacitor? as it will behave as a short circuit from very low frequencies to its SFR? unless my approach is not correct and it won´t behave as a short at very low frequencies close to DC.
Thanks.





RE: RF filtering question
(Is this still the same project?)
RE: RF filtering question
1/(2*pi*f*C)
So for instance a 15 pF cap has an impedance of 70 kohm at 150 kHz.
AVX has a nice free program, "SpiCap 3.0" for looking at decoupling caps.
http://www.avx.com/SpiApps/default.asp
Peter
RE: RF filtering question
Thanks for your commends, they were great, you know it is these kind of things that sometimes I feel
lost because I cannot remember the principle behind and it´s very rewarding to brush it up by someone.
Thanks.
RE: RF filtering question
I'd look for a box to buy and wire into your lines that already has a circuit board on it with those components.
Or, at least make a circuit card and avoid long leads on C's and L's.
If you don't draw much current, make an R C circuit. It helps alot to have a resistor in your lines, then a capacitor between your two lines.
If you have 60 hz. AC fans, you can switch to 28 Vdc to avoid noise. I had to do that way back when and it helped alot.
RE: RF filtering question
In general, you want a low impedance AC short circuit in shunt to ground, and a high impedance series element. If that is not enough, add another shunt AC short, then add another series high impedance element...etc, until you have achieved the desired dB of attenuation.
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