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RF filtering question

RF filtering question

RF filtering question


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.


RE: RF filtering question

It's common to include a series element, such as an inductor. T or Pi filter topology for example. But L might be sufficient.

(Is this still the same project?)


RE: RF filtering question

A small cap won't be a short at low frequencies.  Impedance of an ideal cap is:


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.


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.


RE: RF filtering question

regular circuits, Caps and Inductors are used up to around 750 Mhz, then the leads on each part look like an added inductor.
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

the thing to remember is that a shunt capacitance to ground only will help if "ground" is truly at 0 volts.  If the ground has noise of its own on it, putting a capacitor between the DC line and ground might make the noise on the DC line worse!

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.  



Maguffin Microwave wireless design consulting

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