How to select the best power solution to optimise RF signal chain phase noise performance
Author : Mitchell Sternberg, Erkan Acar & Sydney Wells (AEs) + David Ng, Power Systems Manager | ADI
01 June 2022
ADI tutorial_How to select the best power solution to optimise RF signal chain phase noise performance
Today’s RF (radio frequency) systems are becoming more & more complex. This added complexity requires the best performance across all system metrics, such as stringent link & noise budgets. Ensuring proper design of the entire signal chain is critical. An often-overlooked section of this signal chain is DC power. It plays an important role in the system, but can also introduce unwanted effects.
This full version of this tutorial was originally featured in the June 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
One important measurement for RF systems is phase noise, a metric that can be degraded depending upon the choice of power solution. Here, Applications Engineers, Mitchell Sternberg, Erkan Acar & Sydney Wells, and Power Systems Manager, David Ng at semiconductor firm specialising in sensing, data conversion, signal processing, RF & power management, Analog Devices, Inc. (ADI) investigate the effect that power designs have on the phase noise of RF amplifiers. From the data collected, they conclude that proper choice of power modules can result in up to a 10 dB improvement in phase noise, making it crucial for optimising RF signal chain performance.
Phase noise is the noise that is present in a signal that comes from an unexpected lead or lag when the signal arrives at the receive side of a system. Just as amplitude noise is a shift or deviation from the signal’s nominal amplitude, phase noise is a shift or deviation from the signal’s nominal phase...
Cause & contribution of phase noise
An important and often-overlooked cause of phase noise is the DC power solution of the signal chain. Any noise or ripple on the power rails that supply the signal chain can couple internally. This can lead to an increase in phase noise, which may hide critical frequency components in the bandwidth transmitted, or could induce spurs offset from the carrier. These spurs can be particularly hard to deal with because they are close to the carrier, and would pose a challenge to filters due to the stringent transition-band requirements.
Many different factors can contribute to phase noise. There are three main sources, known as white floor, shot, and 1/f, or flicker noise. White floor noise is from the random thermal movement of free electrons when current is passing. It is similar to shot noise, which is from the random nature of current flow. Unlike white floor and shot, flicker noise changes over frequency. Arising from defects in the semiconductor lattice structure, it is random in nature as well. Flicker noise does decrease with frequency; therefore, a low 1/f corner frequency is highly desirable...
Power solutions
Figure 1. Phase noise of a non-ideal sine wave
Ensuring proper biasing and supply of power to amplifiers in an RF signal chain can be a challenge, especially with drain voltages also used as the output port. There are numerous types of power solutions and topologies on the market. Which power solution you may need will depend on your application and system requirements...
Test set-up
This experiment utilised three different Analog Devices power products: LTM8063, LTM4626 and LT3045. Table 1 summarises some of the datasheet specifications of the power solutions used...
Conclusion
It is important to consider all sources of noise when performing a signal chain analysis. One commonly overlooked source is the DC power solution, which can couple to and severely degrade the performance of the signal chain. Our results show that proper choice of power modules is paramount and can lead to as much as a 10 dB improvement of phase noise at 10 kHz offset... ...knowing the correct power solution to choose is very important for optimising your RF signal chain.
For the complete tutorial, including diagrams, read the full article in EPDT's June 2022 issue...
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