Electronic – 100MHz noise filter doesn’t attenuate much

filternoisepassive-filterpower supplyswitch-mode-power-supply

I have a switching regulator that puts 100MHz noise onto the power supply that it shares with a microcontroller, and I'm trying to filter this noise to give the microcontroller a clean DC power source.

Unfortunately my 3 filtering attempts don't attenutate the noise very much:

Inductor circuit

InductorCircuit

  • Part: B82144F2332K000
  • Value: 3.3uH
  • Resonant frequency: 100MHz
  • Link: https://www.mouser.com/productdetail/871-b82144f2332k000

  • Expected NoiseOut/NoiseIn:

    (Based on datasheet, Zinductor = 2000ohms @ 100MHz)

    Rload/Zinductor == 100/2000 == 0.05

  • Measured NoiseOut/NoiseIn:

    248mV/670mV == 0.37

Feedthrough capacitor circuit

FeedthroughCapacitorCircuit

  • Part: YFF31HC2A104MT00
  • Value: 100nF
  • Resonant frequency: 105MHz

  • Link: https://www.mouser.com/productdetail/810-yff31hc2a104mthn

  • Expected NoiseOut/NoiseIn:

    (Based on datasheet, InsertionLoss @ 100MHz == -70dB)

    -70dB == 20Log10[NoiseOut/NoiseIn]

    NoiseOut/NoiseIn == 0.00032

  • Measured NoiseOut/NoiseIn:

    400mV/800mV == 0.5

Ferrite bead circuit

FerriteBeadCircuit

Here's a picture of the ferrite bead circuit and a scope shot (NoiseIn=yellow, NoiseOut=green), for example:

FerriteBeadCircuitPicture

FerriteBeadScopeshot

Why don't these circuits attentuate the noise closer to the expected amount?

Thanks!

Best Answer

Consider preventing noise pollution on the 12V supply in the first place.

schematic

simulate this circuit – Schematic created using CircuitLab
Keep the loop area of the noise as small as possible - ensuring C1 is (a) adequate for U2's HF current demand and (b) as close as possible to its supply pins. And ensure L1 (or ferrite bead and/or feedthrough cap) provides enough impedance at 100MHz to prevent noise reaching your 12V supply.

Of course you can add further decoupling on the 12V supply - L2 and its impedance at 100MHz form a voltage divider, attenuating the noise on U2's 12V input.

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