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overview
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Today's radio frequency (RF) systems are becoming increasingly complex. This high level of complexity demands optimal performance across all system metrics such as tight link and noise budgets. Ensuring proper design of the entire signal chain is critical. One part of the signal chain that is often overlooked is the DC power supply. It holds an important place in the system, but it can also have negative effects. An important metric for RF systems is phase noise, which can be degraded depending on the power solution chosen.
overall architecture process
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This article investigates the effect of power supply design on the phase noise of an RF amplifier. Our test data proves that choosing the right power module can improve the phase noise by 10 dB, which is the key to optimizing the performance of the RF signal chain.
Explanation of technical terms
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Phase noise is the noise in a signal that occurs due to an unexpected lead or lag when the signal reaches the receiving end of the system. Just as amplitude noise is an offset or deviation from the nominal amplitude of a signal, phase noise is an offset or deviation from the nominal phase of a signal.
technical details
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An ideal oscillator outputs a sine wave, as shown in Equation 1:
This sine wave exhibits perfect periodicity, and the Fourier transform of Videal(t) is expressed as an impulse function of the frequency of the output waveform. A more realistic representation of the oscillator output includes random fluctuations in phase (and amplitude), as shown in Equation 2:
This waveform includes some random process ϕ(t), which will cause the signal phase shift to some extent. This phase shift causes the Fourier transform of the non-ideal clock output to look more similar.
The DC power solution of the signal chain is an important but often overlooked contributor to phase noise. Any noise or ripple on the power rails feeding the signal chain can be coupled internally. This results in increased phase noise, which can hide critical frequency components in the transmitted bandwidth, or introduce spurious offsets from the carrier. These spurs are especially difficult to deal with because they are close to the carrier, and pose a great challenge to the filter because of the strict transition band requirements
Many different factors can contribute to phase noise. There are three main sources, white noise, shot noise, and 1/f, or flicker noise. The white background noise is caused by the random thermal motion of free electrons when the current is passed. It is similar to shot noise, which is caused by the random nature of electric currents. Unlike white noise and shot noise, flicker noise varies with frequency. It arises from defects in the semiconductor lattice structure and is also random in nature. Flicker noise decreases with frequency; therefore, a low 1/f corner frequency is useful. A typical phase noise curve can be approximated as regions with a slope of 1/fX, where x = 0 corresponds to the white noise region (slope = 0 dB/decade) and x = 1 corresponds to Flickering phase noise region (slope = -20 dB/decade). The region of x = 2, 3, 4 is closer to the carrier frequency.
In an RF signal chain, ensuring proper biasing and powering the amplifier can be challenging, especially when the drain voltage is also used as the output port. There are many types of power solutions and topologies on the market. Which power solution you need depends on your application and system requirements. In this experiment, a low-dropout (LDO) linear regulator and a step-down switching regulator are used to collect data, as shown in Figure 3. A step-down switching regulator is a typical solution for large voltage drops with high efficiency and low operating temperature. Switching power supplies can step down higher voltages such as 12 V to more commonly used chip-level voltages such as 3.3 V and 1.8 V. However, they may introduce severe switching to the output voltage.
noise or ripple, resulting in significantly degraded performance. LDO regulators can also step down these voltages and are less noisy; however, their power dissipation is mostly in the form of heat. When the difference between input voltage and output voltage is small, using LDO regulator is a good choice, but when the connection ambient thermal resistance θJA exceeds 30°C/W, the large current drawn from FPGA and ASIC will cause The performance of the LDO regulator degrades rapidly.
summary
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For example:
, the phase noise response of the power amplifier. It can be seen that the performance of the power amplifier degrades slightly beyond the 1/f frequency. The power amplifier draws more supply current, and the observed phase noise increases by about 2 dB to 4 dB. '
When performing signal chain analysis, it is important to account for all sources of noise. DC power solutions are an often overlooked source of noise that can impact and severely degrade signal chain performance. Experimental results show that choosing the right power block is critical and can improve phase noise by as much as 10 dB at 10 kHz offset. In this application, the LTM8063 gives the best results. While an LTM4626 cascaded with an LT3045 can provide equivalent phase noise performance, it should be understood that selecting the correct power supply solution is important for optimizing the RF signal chain