本文是能源管理Series:Optimizing Power Systems for the Signal Chain
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What you'll learn:
- How to quantify the power-supply noise sensitivity of signal-processing chain loads.
- How to calculate the maximum acceptable power-supply noise.
- 满足功率域灵敏度的策略，并具有现实的功率供应噪声要求。
从5G到工业应用程序中收集，通信和存储的数据量的增加已扩大了模拟信号处理设备的性能限制，其中一些是每秒的gigasmples。由于创新的速度永远不会减慢，下一代电子解决方案将导致溶液量进一步缩小，提高功率效率以及对更好噪声性能的需求更大。
人们可能会假设在各种功率域中产生的噪声（Analog，数字，串行数字和数字输入输出（I/O））可以简单地最小化或隔离，以实现最佳的动态性能。不过，追逐噪声的绝对最小值可能是对回报减少的研究。
How does a designer know when noise performance of a supply or supplies is sufficient? A good start is to quantify the sensitivity of devices so that the power-supply spectral output can be matched to the domain. Knowledge is power: It can greatly help in design by, namely, avoiding over-engineering and thus saving time.
本文概述了如何量化信号处理链中负载的功率供应噪声灵敏度，以及如何计算最大可接受的功率供应噪声。也讨论了测量设置。我们通过探讨一些策略来满足功率域灵敏度，并具有现实的功率供应噪声要求。本系列中的随后文章将更深入地研究为ADC，DAC和RF收发器优化功率分布网络（PDN）的细节。
Understanding and Quantifying Signal-Processing Load Sensitivity to Power-Supply Noise
The first step in power-supply optimization is to investigate the true sensitivity of analog signal-processing devices to power-supply noise. This includes understanding the effects of power-supply noise to key dynamic performance specifications, and characterization of power-supply noise sensitivity—namely, the power-supply modulation ratio (PSMR) and power-supply rejection ratio (PSRR).
PSMR and PSRR are good supply rejection characteristics, but alone they’re insufficient to determine how low the ripple should be. This article demonstrates how to establish a ripple tolerance threshold or maximum allowable power-supply noise using PSMR and PSRR. Matching this threshold to the power-supply spectral output is the basis in designing an optimized power-system design. An优化power supply will not degrade the dynamic performance of each analog signal-processing device if power-supply noise remains below its maximum specification.
功率供应噪声对模拟信号处理设备的影响
The effects of power-supply noise on signal-processing devices should be understood. These effects can be quantified by three measured parameters:
- Spurious-free dynamic range (SFDR)
- Signal-to-noise ratio (SNR)
- Phase noise (PN)
Understanding the effects of power-supply noise on these parameters is the first step to optimizing the power-supply noise specification.
无虚拟动态范围（SFDR）
力量-supply noise can be coupled into the carrier signal of any analog signal processing system. The effect of power supply noise depends on its strength relative to that of the carrier signal in the frequency domain. One measure is SFDR, which represents the smallest signal that can be distinguished from a large interfering signal—specifically, the ratio of the amplitude of the carrier signal to the amplitude of the highest spurious signal, regardless of where it falls in the frequency spectrum, such that:
sfdr = 20×log（载波信号/虚假信号）（1）
where SFDR = spurious-free dynamic range (dB); carrier signal = rms value of the carrier signal amplitude (peak or full scale); and spurious signal = rms value of the highest spur amplitude in the frequency spectrum.
SFDR can be specified with respect to full scale (dBFS) or with respect to the carrier signal (dBc). Power-supply ripple may produce unwanted spurs by coupling into the carrier signal, which degrades SFDR.
图1compares the SFDR performance of theAD9208high-speed analog-to-digital converter (ADC) when powered by a clean vs. noisy power supply. In this case, power-supply noise degrades the SFDR about 10 dB when a 1-MHz power-supply ripple appears as modulated spurs beside the carrier frequency in the fast Fourier transform (FFT) spectrum output of the ADC.
Signal-to-Noise Ratio (SNR)
尽管SFDR取决于频谱中最高的刺激，但SNR取决于频谱中的总噪声。SNR限制了模拟信号处理系统看到低振幅信号的能力，并且从理论上讲，该系统受到转换器在系统中的分辨率的限制。SNR在数学上定义为载体信号水平与所有噪声光谱成分的总和的比率，除了前五个谐波和DC，其中：
SNR = 20 × log(Carrier Signal/Spectral Noise) (2)
信噪比=信噪比(dB);航空公司如果gnal = rms value of the carrier signal (peak or full scale); and spectral noise = rms sum of all noise spectral components excluding the first five harmonics.
嘈杂的电源可以通过在载体信号处耦合并在输出频谱中添加噪声频谱成分来导致SNR的减少。如图所示图2, the SNR of the AD9208 high-speed ADC decreases from 56.8 dBFS to 51.7 dBFS when a 1-MHz power-supply ripple produces spectral noise components in the FFT output spectrum.
Phase Noise (PN)
Phase noise is a measure of the frequency stability of a signal. Ideally, an oscillator should be able to produce a specific set of stable frequencies over a specific time period. However, in the real world, small, unwanted amplitude and phase fluctuations are always present on the signal. These phase fluctuations, or jitter, can be seen spreading out on either side of the signal in the frequency spectrum.
Phase noise can be defined in several ways. For the purposes of this article, phase noise is defined as single-sideband (SSB) phase noise—a commonly used definition—that uses the ratio of the power density of an offset frequency from the carrier signal to the total power of the carrier signal where:
SSB PN = 10 × log (Sideband Power Density/Carrier Power) (3)
where SSB PN = single-sideband phase noise (dBc/Hz); sideband power density = noise power per 1-Hz bandwidth at an offset frequency from the carrier signal (W/Hz); and carrier power = total carrier power (W).
In the case of analog signal-processing devices, voltage noise coupled to the device clock through the clock supply voltage produces phase noise, which in turn affects the frequency stability of the internal local oscillator (LO). This widens the scope of LO frequency in the frequency spectrum, increasing the power density at the corresponding offset frequency from the carrier, in turn increasing phase noise.
图3显示了比较相位的噪声性能ADRV9009transceiver when powered by two different power supplies.图3a显示两个供应的噪声光谱，并显示图3b显示结果相噪声。
Both power supplies are based on theLTM8063µModule regulator with spread-spectrum frequency modulation (SSFM) on. The advantage of SSFM is that it improves noise performance at the converter’s fundamental switching frequency and its harmonics by spreading the fundamental over a range of frequencies. This can be seen in图3a—note the relatively wide noise peaks at 1 MHz and its harmonics. The tradeoff is that the frequency of SSFM’s triangular wave modulation produces noise below 100 kHz—note the peaks starting around 2 kHz.
The alternate power supply adds a low-pass filter to suppress noise above 1 MHz, and anADP1764low-dropout (LDO) post regulator to reduce the overall noise floor, particularly below 10 kHz (mostly SSFM-induced noise). The overall improvement in power-supply noise due to the additional filtering results in enhanced phase-noise performance below the 10-kHz offset frequency(Fig. 3b)。
模拟信号处理设备的功率供应噪声灵敏度
The sensitivity of the load to power-supply ripple can be quantified by two parameters:
- 力量-supply rejection ratio (PSRR)
- 力量-supply modulation ratio (PSMR)
力量-Supply Rejection Ratio (PSRR)
PSRR表示设备在一系列频率上降低功率供应引脚上的噪声的能力。通常，PSRR有两种类型：静态（DC）PSRR和动态（AC）PSRR。DC PSRR用作输出偏移的变化的量度，是由直流电压电压的变化引起的。这是一个最小的关注点，因为电力供应系统应为负载提供良好调节的直流电压。另一方面，AC PSRR表示设备在多个频率范围内拒绝直流电源中的交流信号的能力。
Determining ac PSRR involves injecting a sine-wave signal at the power-supply pin of the device and observing the error spur that appears on the noise floor of the data converter/transceiver output spectrum at the injection frequency(Fig. 4)。
It’s defined as the ratio of the measured amplitude of the injected signal to the corresponding amplitude of the error spur on the output spectrum where:
ACPSRR(dB) = 20 log(Injected Ripple/Error Spur) (4)
在输入电源引脚处注入纹波=正弦波振幅并测量；and error spur = spur amplitude seen in the output spectrum due to the injected ripple.
图5shows the block diagram of a typical PSRR setup. Using theAD921310-GS/s high-speed ADC as an example, a 1-MHz, 13.3-mV p-p sine wave is actively coupled at the 1.0-V analog supply rail. A corresponding 1-MHz digitized spur appears above the –108-dBFS FFT spectrum noise floor of the ADC. The 1-MHz digitized spur is –81 dBFS, corresponding to a peak-to-peak voltage of 124.8 μV in reference to the analog input full-scale range of 1.4 V p-p.
Calculating the ac PSRR at 1 MHz using Equation 4 yields an ac PSRR of 40.5 dB at 1 MHz.图6shows the ac PSRR of AD9213 for the 1.0-V AVDD rail.
力量-Supply Modulation Ratio (PSMR)
diff PSMR影响模拟信号处理设备erently than PSRR. PSMR shows the sensitivity of a device to power-supply noise when it modulates with an RF carrier signal. The effect can be seen as a modulated spur around the carrier frequency applied to the device and appears as the carrier sideband.
力量-supply modulation is achieved by combining the input ripple signal with a clean dc voltage using a line injector/coupling circuit. Supply ripple is injected as a sine-wave signal from the signal generator to the power-supply pin. The sine wave modulated into the RF carrier creates sideband spurs with offset frequency equal to the sine-wave frequency. The level of the spurs is affected by both the sine-wave amplitude and the sensitivity of the device.
A simplified PSMR test setup is the same as that of PSRR（图5，再次）, but the output display is focused on the carrier frequency and its sideband spurs(Fig. 7)。
PSMR is defined as the ratio of the injected ripple amplitude of the power supply to the modulated sideband spur amplitude around the carrier where:
PSMR（db）= 20 log（注入纹波/调制旋转）（5）
在输入电源引脚处注入纹波=正弦波振幅并测量；和调制的Spur =由于注入的纹波，在载体频率的边带处的旋转幅度。
Consider theAD917512.6-GS/s high-speed DAC operating with a 100-MHz carrier, and a 10-MHz supply ripple of about 3.05 mV p-p actively coupled at the 1.0-V AVDD rail. A corresponding 24.6-μV p-p modulated spur appears in the sideband of the carrier signal with offset equal to the frequency of the supply ripple of about 10 MHz. Calculating the PSMR at 10 MHz using Equation 5 yields 41.9 dB.图8shows the AD9175 1.0-V AVDD rail PSMR for channel DAC0 at various carrier frequencies.
确定最大允许的功率供应纹波
PSMR can be combined with a powered device’s reference threshold to determine the maximum allowable voltage ripple on each of the power-supply domains of an analog signal-processing device. The reference threshold itself can be one of several values representing the allowable spur level (as caused by power-supply ripple) that the device can tolerate without significantly affecting its dynamic performance. This spur level can be the spurious-free dynamic range (SFDR), a percentage of least significant bit (LSB) or output spectrum noise floor.
等式6显示了最大允许输入纹波（VR_MAX）作为PSMR的函数和每个设备的测量噪声层，其中：
VR_MAX= [10（PSMR/20）]×阈值（6）
where VR_MAX= the maximum allowable voltage ripple on each of the power-supply rails before producing spur in the output spectrum noise floor; PSMR = the noise sensitivity of the power-supply rail of interest (in dB); and threshold = a predefined reference threshold (for the purposes of this article, the output spectrum noise floor).
例如，AD9175的输出频谱噪声底部约为1μVP-P。1800-MHz载体的10 MHz涟漪的PSMR约为20.9 dB。使用等式6，设备电源引脚中的最大允许纹波在不降解其动态性能的情况下可以耐受性能为11.1μVP-P。
图9shows the combined results of the spectral output of theLT8650Sstep-down Silent Switcher regulator (with and without an output LC filter) and the maximum allowable ripple of AD9175 for the 1.0-V AVDD rail. The regulator spectral output contains spurs at the fundamental switching frequency and its harmonics.
The LT8650S directly powering the AD9175 produces a fundamental exceeding the maximum allowable threshold, resulting in modulated sideband spurs in the output spectrum（图10）。只需添加LC滤波器就可以减少最大允许纹波以下的开关刺激（图11）。
Conclusion
高速安娜的优越的动态性能log signal-processing devices can easily be undercut by power-supply noise. A thorough understanding of the sensitivity of the signal chain to power-supply noise is necessary to avoid performance degradation of the system. This can be determined by establishing a maximum allowable ripple—vital to designing the power distribution network. When the maximum allowable ripple threshold is known, various approaches in designing an optimized power supply can be applied. A good margin from the maximum allowable ripple is an indication that the PDN will not degrade the dynamic performance of high-speed analog signal-processing devices.
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References
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