Electronic – Can you filter out power supply noise by using a capacitor with a resonant frequency that matches the noise frequency

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From my limited understanding, the self-resonant frequency of a capacitor is also where typically impedance is at its minimum, and where Z = ESR.enter image description here

I have a power rail that has some ringing at around 61MHz. What would happen if I place a 10nF capacitor that has a self-resonant frequency of 61MHz, such as the one below, between the rail and ground?

Is it bad to attempt to try to use a capacitor at its self-resonant frequency?

Why don't people just select filtering capacitors based on the least ESR/Z at the frequency of concern (sometimes that means selecting capacitor and value that has a self-resonant frequency at the frequency of concern). What am I missing here?

Best Answer

In the particular example you have in front of you, if ...

a) the power supply has narrowband 61MHz noise and
b) your capacitor has an SRF of 61MHz

then yes, that capacitor in shunt would provide a good reduction in the noise voltage, and probably the best you'd get for a single cheap component

However, the SRF of capacitors, and with that the notch frequency, is not well controlled. As it depends on a residual parameter, the inductance, it may change. As that parameter is Bad, manufacturers are trying to improve it all the time, and may just improve it between batches. Different manufacturers may achieve a different inductance in parts that look the same. This means that SRF is neither consistent across different sources, or even reliable for one source.

Using a notch filter to improve a power supply only works if there's a single frequency of noise, and that's consistent between all examples. If it's broadband, or moves with time, or between supplies, then a fixed frequency notch is no good.

A single shunt component will have only a certain depth of attenuation. If you need more, you will need to build a larger filter anyway.

The thing to do is to design and build a lowpass filter from several components. Choose them with SRFs well above the highest frequency of interest, so that if (when) those SRFs change, they do not affect the performance of the filter significantly. Design the filter to have a stop band covering all of the potential problem frequencies, with a stopband deep enough keep it clean at the load.