The ferrite bead at where the supply pin of crystal oscillator is

Question

I have a question about ferrite beads. The DC resistance of the ferrite bead is an important parameter for picking up our design. This value has to be as low as it can because if the current drawn increases where we use it, it causes a voltage drop and it might create a problem in the place where the sensitive voltage need.

When we place a ferrite bead on the supply voltage of a 100MHz crystal oscillator (for example, the ferrite bead used has a 1k ohm resistor value at 100MHz,) does the ferrite bead cause any problems for that crystal oscillator? I know, 100MHz signal produced is in the output pin of oscillator, but does it generate also noise on the supply side while generating this signal?

Answer 1

Based on what you ask, the 1k impedance is at 100 MHz, not at DC.

A ferrite bead would have DC resistance in the order of few tens of milliohms, and the oscillator would consume much less than 50mA, so it is highly unlikely that DC resistance will be a problem.

And you will anyway have to use bypass caps at the oscillator side of the ferrite bead.

There should be very little noise going through the ferrite bead back to power supply.

People use ferrite beads on crystal oscillator power supply pins daily and have no problems.

Perhaps the only points to take care of are not even related to the ferrite bead, but the oscillator having proper bypass caps with proper amount of them and proper frequency range and properly designed to be close to the oscillator.