对比功率计和功率分析仪

射频百花潭2019-07-07 03:12:24
点击标题下「射频百花潭」可快速关注公众号!


这是一篇2005年原发表于EDN的文章,现重新挖掘出来,以展示功率计和功率分析仪的不同。
http://www.mwrf.com/Articles/Index.cfm?ArticleID=9573&pg=1 

Robert Green, Yunli He Compare Analyzers And Meters For Power Tests 




对比功率计和功率分析仪 


Programmable power analyzers offer certain advantages in accuracy and speed compared to conventional power meters when evaluating wireless handsets. 
当评估无线手持设备时,在精度和速度方面可编程功率分析仪比传统的功率计有更多优势。(注:下文中提到的功率分析仪和功率计军事指射频范围)   
Power meters are often considered for mobile-telephone production testing, because they can quickly measure peak power, average power, and peak-to-average ratio. But an RF power meter has certain deficiencies that hinder accurate wireless handset testing. For example, an RF power meter cannot identify where in the frequency spectrum a device is transmitting its power. Because of this, a handset may pass a power test with a power meter, but transmit at the wrong frequency or suffer excessive spurious transmissions. Fortunately, frequency-domain analysis performed on spectrum analyzers and RF power analyzers can overcome these limitations. An RF power analyzer provides both power and transmission frequency information, and can also overcome false readings associated with an RF power meters limited video bandwidth. 
功率计经常用于手机生产测试,因为它能快速的测量峰值功率,平均功率,峰均比。但是功率计的天生缺陷将限制其在手持设备测试方面的精度。比方说,由于这个原因功率计无法识别一个设备在其频谱的何处发射功率,用功率计测试的话,手机可以通过功率测试,但手机可能在错误的频率上发射或承受过大的杂散辐射而。幸运的是,在频谱分析仪和RF功率分析仪上执行的频域分析可以克服这些局限。功率分析仪提供了功率和发射频率的信息,也可以克服由于功率计有限视频带宽而引起的的错误读数。   


In production, every mobile telephone is checked for carrier frequency, primary-channel power, adjacent-channel power, and alternate-channel power. Typically, such instruments as RF power meters, spectrum analyzers, and RF power analyzers are used for such testing, but the capabilities of and results from these tools can vary widely. To maximize measurement throughput and accuracy, the trade-offs of these measurement instruments should be considered. 
在生产中,每部手机都要检查载波频率,主信道功率,ACP,交替信道功率。一般用到的仪器有功率计,频谱分析仪,功率分析仪,但是这些仪器的功能和测试结果区别却非常大。为了最大化吞吐量和精度,要考虑这些仪器之间的平衡。   


RF power meters are used not only for measuring power levels in two-way radio systems, broadcast systems, and radar/satellite systems, but for calibrating other measurement instruments and probes. Unfortunately, an RF power meter is designed for broadband frequency coverage and cannot determine the carrier frequency associated with a power reading or whether the measured power is within the proper bandwidth. Normally, a spectrum analyzer is also needed to make these determinations. 
功率计不仅用来测试无线电系统,广播系统,雷达/卫星系统的功率级别,还用来校正其它测试设备和探头。不幸的是,功率计被设计成宽带仪器,不能确定功率读数的载波频率或者在或其是否正确的带宽内。通常频谱分析仪被用来做出这些判断。  

 
An RF power meter is basically an untuned RF sensor or detector followed by signal-processing circuitry consisting of DC or AC-DC amplification stages and analog-to-digital-conversion (ADC) stages (Fig. 1). It is designed to measure RF power, including peak power, average power, and peak-to-average power ratio. It does not measure frequency, since all frequency information is lost before signal processing begins. While this is a drawback in some ways, no tuning is required for a measurement. If the input signal is anywhere in the frequency range of the detector, the meter will make a valid power measurement. 
一般功率计的信号链路如图1所示,由一个未调谐的射频传感器或检测器,后面是包括DC或者AC-DC放大环节和ADC环节组成的信号处理电路。它被设计用于功率测量,包括峰值功率,平均功率,峰均功率比。不能用来测量频率,在信号处理之前所有的频率信息都将丢失。但这也有一些好处,测量无需调谐。只要输入信号落在检测器的频率范围内,则功率计就能给出准确的的功率。  



An RF power meter measures input signals in the time domain. An incoming carrier of constant amplitude will produce a DC output, while any amplitude variation will be reproduced in the output, as the instrument responds to the envelope of the incoming signal. Because of this, the instrument's video bandwidth is an important characteristic when peak detection measurements are required. 
功率计是时域测量仪器,输入信号的恒定幅值会产生一个DC输出,任何幅值的变化都会产生输出。功率计只对信号的包络产生响应,为此功率计的视频带宽对于峰值测量是非常重要的。 

  
The video bandwidth of a test instrument, in simplest terms, is an indication of how fast it can track signal variations for peak power envelope measurement, and can be considered the range of frequencies occupied by all the modulating signals. For example, a wideband-code-division-multiple-access (WCDMA) signal consists of a 1950-MHz carrier modulated by a 3.84-MHz modulating signal; after detection, the signal that remains extends from 0 to 3.84 MHz. For an instrument to process that signal correctly, all circuits downstream of the detector must have a bandwidth greater than 3.84 MHz (and preferably 5 MHz). An instrument with a narrower bandwidth cannot follow or capture the peaks of the 3.84-MHz WCDMA signal envelope. Video bandwidth is not the frequency range over which the instrument itself can accept signals; it is an indication of how well the power meter can capture the peaks of a modulated waveform. 
视频带宽,简单来讲,反映了对于峰值功率包络测量来说,追踪信号变化是如何的快,并要考虑所有调制信号的占用频率范围。举个例子,WCDMA信号由1950MHz的载波信号和3.84MHz的调制信号组成;经过检测器后,只保留了从0到3.84MHz的信号。对于可以正确处理那些信号的仪器而言,所有检测器之后的电路必须拥有超过3.84MHz的带宽(最好是5MHz)。一个窄带仪器不能跟随或捕捉3.84MHz的WCDMA信号包络的峰值。视频带宽不同于仪器本身接受信号的频率范围;它只是功率计有效地捕捉调制波形峰值的指示器。   


Time variations in the detected signal can come from several sources. The obvious source is pulse or amplitude modulation. For example, if the incoming signal is pulse-modulated at 1 MHz, the bandwidth of the circuits following the sensor/detector would have to be greater than 1 MHz to accurately measure the peak value, although average value measurement would still be accurate with a smaller video bandwidth. 
被检测信号的时间变化可以来自于几个因素。显著的因素是脉冲或幅度调制。举个例子,如果输入信号是1MHz的脉冲调制,传感器/检测器之后的电路带宽则必须大于1MHz才能确保峰值测量的准确性,既使带有小视频带宽的平均值测量依然是准确的。   


How video bandwidth affects peak power measurements and time-gated average power measurementsa but not average power is illustrated in the following example. Two RF sinewave signals (in the 800-MHz range) whose frequencies differ by 1 MHz and have magnitudes equal to one are represented in Fig. 2: 




Assuming the impedance is 1 for the convenience of power calculations, the power versus time (real-time power) of these two signals is represented as Eq. 1: 
为了便于功率计算,假定阻抗为1,这两个信号相对于时间的功率(实时功率)为公式1:  


Substituting v1 and v2 by their sinewave equivalents, power versus time, P(t), is derived in Eq. 2, which indicates that P(t) consists of the power of two individual signals (v1, v2) and twice the amplitude product of the two signals: 
用同等的正弦波替代v1和v2,相对于时间的功率P(t),则为公式2,它表明了P(t)包含了v1,v2两个独立信号和两倍于两个信号幅度。 


Equation 3 is inferred by applying trigonometric identitiesb into Eq. 2. Equivalently, P(t) consists of one DC signal and four frequency components, which are 2f1, 2f2, f1 + f2, and f1 − f2, respectively: 
公式3是由三角运算导入公式2得出,同理,P(t)包好了直流信号和4个频率成分,分别是2f1,2f2,f1+f2,f1-f2。 



The average power is defined by and calculated from 
平均功率可以由下式定义和计算得出 


which is the DC component of Eq. 3. 
这是公式3 的直流成分。   


From Eqs. 3 and 4, the peak power depends on all components while the average power depends on the DC component only. If a power meter and power sensor has a 1-MHz video bandwidth, it will act as a typical low pass filter with a 3-dB rolloff point at 1 MHz. With regard to the power envelope, components 2f1, 2f2, f1 + f2 are filtered, leaving the DC offset and the |f1 − f2| = 1 MHz, periodic signal as seen in Eq. 3. If the video bandwidth is greater than 1 MHz, the peak value is 2. However, the |f1 − f2| term is attenuated in half (3 dB) due to the 3-dB rolloff at 1 MHz. This causes a 1.24-dB error for peak-to-average ratio.c Video bandwidth affects the measurement of the peak of the power envelope but has no effect on average power measurement. 
由公式3和4可以看出,当平均功率仅取决于直流成分时峰值功率取决于所有成分。如果功率计和功率传感器拥有1MHz的视频带宽,他将扮演一个-3dB截止点为1MHz的典型低通滤波器。考虑到功率包络,成分2f1,2f2,f1+f2被滤掉,只保留了直流成分和|f1 − f2| = 1 MHz的周期信号,如公式3所示。如果视频带宽大于1MHz,峰值则为2。然而|f1 − f2|部分将由于-3dB截至点为1MHz而衰减一半(3dB)。这将引起1.24dB 的峰均比测量误差。视频带宽影响到功率包络的峰值测量但不会影响平均功率测量。   


This might lead to the impression that a wider video bandwidth is always preferable to a narrower one, but that is not the case. For one thing, the greater the instrument's video bandwidth, the smaller its dynamic range and the greater the variation in linearity. For example, a video bandwidth of 5 MHz will give a dynamic range of −32 to +20 dBm, while a 300-kHz bandwidth will give a dynamic range of −42 to +20 dBm. 
这将会导致宽的视频带宽总是比窄的要好的印象,但事实并非如此。首先,越大的视频带宽,动态范围就越小和线性误差则越大。举个例子,5MHz的视频带宽的动态范围是-32~20dBm,而300kHz带宽给出的动态范围为-42~20dBm。  

 
In general, the instrument's video bandwidth should be equal tod or greater than the input signal's modulation bandwidth. Table 1 lists recommended video bandwidths for different wireless standards. For example, a cdmaOne signal has a 1.23-MHz modulation bandwidth, and the recommended video bandwidth is 1.5 MHz. For multiple signal types, it may be necessary to consider multiple sensors. 
通常,视频带宽应该等于或大于输入信号调制带宽。表1列出了对于不同无线标准推荐的视频带宽。举个例子,CDMAone信号拥有1.23MHz的调制带宽,推荐的视频带宽则为1.5MHz。对于多种信号类型,应考虑多种传感器。 


 Table 1: Video bandwidth and wireless standards 
VIDEO BANDWIDTH          WIRELESS STANDARDS 
300 kHz                              NADC, GSM, GPRS, EDGE 
1.5 MHz                              cdmaOne, CDMA2000 1x 
5 MHz                                 WCDMA, CDMA2000 3x   
An RF power meter has one major drawback: it cannot measure frequency, so it may give an "approved" rating to a defective mobile telephone, as in the following two examples. 
功率计的一个主要缺点是:不能测量频率,因此它会对一个有缺陷的手机给出“通过”,就像下面的两个例子。   


In the first example, two mobile phones are transmitting the same RF power, with the spectrums shown in Fig. 3. The good telephone has its power within the transmission channel, while the output of the defective telephone is half in the transmission channel and half outside, probably because of a bad modulator or power amplifier. A power meter will give the same reading for both telephones. 
例1,2个手机发射相同的功率,频谱如图3所示。好手机的功率在发射通道内,而坏手机的发射功率一半在通道内一半在通道外,可能是由于坏的调制器或功放引起。但功率计会给出相同的读数。  




In the second example (Fig. 4), the defective phone is transmitting a signal with the proper power level and with the proper frequency spectrum but at an incorrect frequency, due to a defective oscillator circuit. Once again, the RF power meter cannot detect the error. 
例2(图4),由于坏的振荡器电路,坏手机在功率和频谱正确而频率错误下发射一个信号,。功率计再一次的没检测到错误。  




No matter where the RF signal is centered, the power meter reads the same total average power value (Table 2). The RF power analyzer is programmed to read the power only in the channel in which the mobile device is supposed to be transmitting. As the signal is shifted from the original center frequency, the RF power analyzer reads lower and lower power levels as more and more of the signal shifts out of the intended transmission channel. 
无论射频信号的中心在哪里,功率计都会得到相同的平均功率读数(表2)。功率分析仪可控制只读取在信道内的发射功率,假定该手机要发射信号。当信号从原中心频率偏移,信号越来越从设定的发射通道偏移,功率分析仪的功率电平会越来越低。 


Both an RF power analyzer and a spectrum analyzer are essentially specialized superheterodyne receivers. They have mixers, local oscillators, and IF chains. The essential difference is that the spectrum analyzer, being designed for laboratory use, is designed to cover an extremely wide frequency range with widely adjustable tuning range, bandwidth, etc. It uses one or more sweeping local oscillators (Fig. 5), while the RF power analyzer uses one or more fixed-frequency local oscillators (LOs), an analog-to-digital converter (ADC), and a digital signal processor (DSP) to extract information on the primary channel, the upper and lower adjacent channels, and the upper and lower alternate channels (Fig. 6). 
功率分析仪和频谱分析仪都本质上都是超外差接收机。由混频器,本振,中频组成。区别在于频谱分析仪被设计成是实验室使用,设计成是带有很大可调范围和带宽等并覆盖非常大频率范围的仪器。频谱分析仪使用了1个或多个扫频本振(图5),而功率分析仪带有1个或多个频点的本振,一个ADC,和一个DSP去分析主信道,上下邻近信道,上下交替信道的信息。   


Unlike a spectrum analyzer, an RF power analyzer is optimized for high-volume production testing and features a wide dynamic power measurement range. As a production test instrument, it is intended for one job: mobile-telephone testing during production. 
不同于频谱分析仪,功率分析仪被优化成适用于高产量测试和带有非常大的动态功率测量范围。作为生产测试仪器,它适用于一种工作:手机的生产测试。

 

 The ability of the RF power analyzer to measure power within specified frequency bands enables it to spot defects that an RF power meter cannot. In Fig. 3, the RF power analyzer would give a reading one-half as great as that of the RF power meter, because it would measure only the power within the intended channel. In Fig. 4, the RF power analyzer would again give a reading one-half that of the RF power meter, because one-half of the transmitted power is outside the designated band. In addition, the RF power analyzer has a carrier-frequency-measurement function, and would immediately detect the off-frequency situation in Fig. 4. 
功率分析仪可在指定频段测量的能力使其能发现功率计发现不了的错误。图3中,功率分析仪给出了功率计一半大的读数,因为它仅测试在指定信道的功率。图4中,功率分析仪再次给出一半的读数,因为一半的发射功率已处于设计频段之外。另外功率分析仪拥有载波频率测量功能,它能立即检测到频率空闲的情况,如图4。   


A high-end spectrum analyzer can make the measurements of an RF power analyzer, with similar accuracy. Unfortunately, the spectrum analyzer costs more than the RF power analyzer (as much as $43,000 versus about $16,000 for an RF power analyzer). The spectrum analyzer is also more complicated to set up, is physically larger, and takes much more time to make a measurement. For development work, a spectrum analyzer is indispensable. But for production testing, it offers more measurement power (and expense) than needed. 
高端的频谱分析仪能以相似的精度代替功率分析仪。不过,频谱仪价格要比功率分析仪贵的多(大约是$43,000,而功率分析仪大约$16,000的)。频谱仪在设置上也要复杂的多,体形很大,获得测量结果要花很多时间。对于研发工作,频谱仪是不可缺少的,但对于生产测试,其测量能力远远超出了需要,造价也更高。   


When total RF power transmitted is the key parameter—in a two-way radio system or an antenna system, for example—an RF power meter is an excellent solution. But when information is also needed about the transmit frequency, as in mobile-telephone production testing, the RF power analyzer is a better choice. 

在双路无线电系统或天线系统中,发射的总射频功率是关键参数,功率计就是一个非常好的解决方法。但当需要知道发射频率时,比如在手机生产测试中,则功率分析仪是最佳选择。  FOOTNOTES 


a Although time-gated average power is a type of average power measurement, it requires quick response from a power sensor. How fast the response will be is limited by the video bandwidth of a power sensor. 
虽然时闸平均功率是平均功率测量的一种类型,但它需要功率传感器的快速反应。其反应速度受限于功率传感器的视频带宽。   


b Note that sinA · sinB = 0.5[cos(A − B) - cos(A+B)], with cos2A = 1 − 2sin2A implying that sin2A = (1 − cos2A)/2.   
cThere is no attenuation at 1 MHz, since peak power = 10log(Ppeak/Paverage) = 2/1 = 3.0 dB. However, there is a 3-dB rolloff at 1 MHz, with peak power = log(Ppeak/Paverage) = 1.5/1 = 1.76 dB. 
由于峰值功率= 10log(Ppeak/Paverage) = 2/1 = 3.0 dB,因此在1 MHz时无衰减。然而,当峰值功率= log(Ppeak/Paverage) = 1.5/1 = 1.76 dB,在1MHz时具有3dB的下滑。   


The video bandwidth of some power sensors is not the traditional 3-dB bandwidth. 
有些功率传感器的视频带宽不是传统的3dB带宽。


关于出处:本公众号尊重知识产权,转载的文章都有注明作者和出处,不正确或侵权行为请联系射频百花潭更正或删除!




更多精彩请加入“中国射频微波微信第一群”, 先加徐老师微信号:xuxingfu2016,注明“公司+方向”,通过验证后加入。(注:本群属纯技术交流群,销售代理等非射频技术人员勿加)

1. 本群7000人,成员涵盖了国内所有射频方向企业,高校,研究所。其中IEEE fellow,教授,总监,总经理,主任专家,千人计划,长江学者,首席科学家,博士1000+人。

2. 本微信群/公众号由资深射频专家,兴森快捷射频实验室主任,《ADS2008/2011射频电路设计与仿真实例》《HFSS射频仿真设计实例大全》(电子工业出版社)主编徐兴福建立。



Copyright © 深圳证券交易所交流群@2017