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All About RF Power Supply Noise

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • High supply voltages and frequencies contribute to increased RF emissions, necessitating careful consideration during system design to minimize interference.

  • Depending on the application, using a noise filter can greatly reduce rf power supply noise. 

  • Proper grounding techniques, such as single-point grounding for low frequencies and multipoint grounding for high-frequency digital circuits, are essential to suppress noise and maintain signal integrity.

Representantive image of RF noise

RF noise comes from a variety of sources.

RF switching power supply noise can significantly impact the performance of electronic systems. Switched mode power supplies (SMPS) are both sources and recipients of radio interference, emitting emissions through high-frequency switching. Understanding the sources of RF power supply noise and implementing effective noise reduction techniques is crucial for maintaining signal integrity and minimizing disruptions. 

This article delves into the primary emission sources, the impact of high supply voltages and frequency values, noise generated by current spikes, and the importance of proper grounding techniques. Furthermore, it provides valuable tips and solutions for reducing RF power supply noise and improving signal quality.

RF Power Supply Emission Sources

Tips for Minimizing RF Power Supply Noise
  • Switching devices
  • Bridge rectifier
  • High voltages and frequencies
  • Periodic Signals
  • Current spikes
  • Improper grounding
  • Check antenna system SWR
  • Further space antenna from power supply
  • Insert power line filters before AC input
  • Coat cable conductors with RF-absorbing materials
  • Twist wires coming out of the power supply
  • Single and multi-point ground depending on the frequency

Primary Emission Sources of Supply Noise in RF Switching Power Supplies 

The primary emission sources of supply noise in RF switched-mode power supplies (SMPS) interference stem from the switching devices themselves. These emissions include harmonics of the switching frequency and broadband noise resulting from under-damped oscillations in the switching circuit. 

Additionally, the bridge rectifier in the SMPS contributes to the interference, further generating rectifier noise and diode recovery noise. In the front end of switching power supplies (excluding those with power factor correction), the AC input rectifier/capacitor is notorious for generating power supply harmonics due to the non-linear input current waveform. Consequently, the noise generated can be both conducted and radiated through the input power cord and the DC output wiring, ultimately affecting nearby radios.

RF Switching Power Supplies: Sources and Solutions for Radio Interference

Switching power supplies are not only a source of radio interference but also recipients. The normal operation of a power supply can be disturbed when RF noise couples into it, leading to excessive RF noise and output voltage regulation issues. Excessive transmitter energy coupling through the AC/DC lines into the power supply's regulator feedback path can occur when the antenna is positioned too closely or when the antenna or feed system fails to radiate properly. 

Several steps can be taken to address these issues, such as checking the antenna system's standing wave ratio (SWR), examining the coaxial connector for looseness, and ensuring that the ground connection is intact. Increasing the separation between the power supply and receiving antenna can mitigate the impact of these noises, even though they are generally below the background or "band" noise.

Additional Tips for RF Power Supply Noise Reduction

When dealing with RF switching power supply noise, implementing effective strategies for noise reduction is crucial. Below are some additional tips for reducing interference and improving RF signal quality, including employing power line filters, utilizing ferrite-coated cables, and implementing noise filters for stable DC voltage, along with wire twisting techniques for cleaner power transmission.

Employing Power Line Filters and Ferrite-Coated Cables

To further address AC radio frequency interference (RFI) in the power supply, consider implementing the following tips:

Tips for RF Power Supply Noise Reduction

1. Use an appropriate power line filter before the AC input to address AC radio frequency interference (RFI). Filtered cord sets with an integral line interference filter are effective options. These cord sets reduce common and differential mode interferences across a wide frequency range and offer shielding against radiated interferences.

2. Consider using cable conductors coated with an RF-absorbing ferrite compound. This provides additional attenuation at high frequencies, helping prevent resonances and further reducing conducted and radiated RF noises.

Noise Filters for Stable DC Voltage and Wire Twisting

For improved RF signal quality and reduced interference during transmission, consider the following steps:

Tips for Improved RF Signal Quality

1. For power amplifiers or components requiring stable DC voltage, insert a noise filter (high-frequency coil or chip ferrite beads) immediately behind the power inductor and capacitor at the output of the DC-DC converter. This effectively attenuates unwanted noise and interference, leading to cleaner and more reliable RF signals.

2. Twist the positive and negative wires coming from the power supply's output to minimize interference during transmission to the radio. Twisting the wires reduces electromagnetic interference and ensures cleaner power transmission, improving signal quality.

Noise Generation Sources in RF Power Supplies

Beyond the switching devices and bridge rectifiers already mentioned, RF power supply noise can stem from various other sources. Firstly, high supply voltages result in greater voltage swings, increasing emissions. Therefore, it is crucial to consider the impact of supply voltage when designing a system to minimize emissions and maintain signal integrity.

Additionally, signals' frequency value and periodicity play a significant role in generating emissions. Higher frequency values, as well as periodic signals, can contribute to higher levels of emissions. This is particularly important to consider when dealing with sensitive electronic systems that require precise and clean signal transmission.

Furthermore, noise can be created due to current spikes, which are commonly observed in digital systems during the switching ON/OFF of transistors. It is essential to address and minimize these current spikes to reduce unwanted noise in the system.

Improper grounding can also be a significant source of noise. Different types of grounding techniques are used depending on the circuit type. Single-point grounding is suitable for frequencies below 1 MHz, multipoint grounding is recommended for high-frequency digital circuits, and hybrid grounding combines single-point and multipoint grounding for low and high-frequency components. Proper ground and power plane layout should be implemented on the PCB after segregating digital, analog, and high-frequency noisy parts to ensure effective noise suppression and maintain signal integrity throughout the system.

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