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Frequently Asked Question

Should I reference the passive filter to ground? or supply?

The passive filter should be referenced according to the oscillator's control pin (tuning voltage). If the oscillator is ground referenced with respect to the control voltage, the passive filter should be grounded. If the oscillator is referenced to the supply, the passive filter should be referenced to the supply side. By doing so, better common-mode rejection is achieved.

For example, if there's noise on the ground to which the passive filter is referenced to, it will couple onto the control voltage input to the oscillator. If the oscillator is ground referenced and the passive filter is grounded, the noise appears as correlated noise across the oscillator input to ground which minimizes the noise impact, else any differential impact between input and ground will degrade performance.

The orientation of the filter can also affect the reference spurs in a similar manner.

Whether the oscillator is referenced to ground or supply is not always available in the data sheets and therefore may require calling the manufacturer.