How do you keep noise out of RF circuit power supplies?

An RF circuit that generates a Radio Frequency (RF) signal, is very sensitive to power supply noise.  If not controlled, the power supply noise can significantly degrade the performance of the system.  So, what are the sources of noise and how can they be addressed?

The first potential source of noise is the power source.  Unless the power source is a stand-alone battery, there is a good chance that there is noise associated with the generation of the power.  To prevent this noise from entering the system, Inductors and Capacitors are used to filter the power prior to it being distributed to active elements in the system.  Electromagnetic Interference (EMI) feed-thru filters can also be used.  These are typically small metal enclosed filters that are mounted onto the external packaging of the system to stop any noise from entering the packaging where it could be radiated as well as conducted.

Another potential source of noise for an RF circuit is the outside environment.  The world is getting more and more congested with RF signals that span the frequency spectrum.  To prevent these signals from causing issues in a system, care must be given to the power cable that is used to connect the system to the power source.  The best protection from external signals is a shielded power cable.  While there are debates about how a power cable should be shielded, we have found that the best approach is to have a metal braid that runs the entire length of the cable and is grounded at both ends.  EMI feed-thru filters will also help by reducing the level of any signals that are picked up on the cable.

 

Inside the system, power supply noise can come from switching noise if a switch mode power supply is used.  This type of power supply is used to efficiently change the power source to a different voltage that may be required by the components of a circuit.  Switching noise can be controlled with the use of proper grounding of the power supply, Inductor and Capacitor filtering, and placing the switch mode power supply under a shield.  In extreme cases, the switch mode power supply could be inside a sealed metal cavity with EMI feed-thru filters isolating the power supply from the rest of the system.  Another method to reduce switching noise is to use a linear regulator after the switch mode power supply to generate the final operating voltage for the circuit.  While this may help in some cases, the isolation provided by linear regulators is usually dependent on frequency which means some signals may not be affected.

A final source of noise is internally generated signals.  Digital circuits, clocks, synthesizers, modulators, and amplifiers are all connected to the internal power supply of the system and each generate noise that could affect the other.  Many techniques are used to mitigate the noise between circuits.  It is good practice to place a range of capacitors on the power supply pins of these circuits since each capacitor value is more effective for different frequencies.  During the layout of the circuit, the capacitors should be arranged so that the smallest value is closest to the power supply pin of the device.  The power supply pins can also be isolated with an inductor or a ferrite chip.  The inductor’s or ferrite chip’s value will vary based on the operating frequency of the circuit.  As with the switch mode power supply, it may be necessary to shield each of these circuits to prevent the noise from radiating.

While these are simple and high-level descriptions of power supply noise sources, NuWaves’ engineers are experienced in designing RF circuit power supplies that integrate into systems and help you meet your mission. Learn about our RF Design Services to find out more and don’t forget to view our wide selection of COTS RF Products.

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