标签: noise

晶振是个大家族，除了SPXO外，更有压控晶体振荡器（VCXO）、温补晶体振荡器（TCXO）、恒温晶体振荡器（OCXO），以及数字补偿晶体振荡器（MCXO或DTCXO），每种类型都有独特的性能，例如相位噪声和抖动(jitter)这两个指标。 什么是相位噪声和抖动？ 简单讲，抖动(jitter)是某一事件的时程与理想时程的时间偏差，单位以fs（微微秒、飞秒，即10-15秒），或者ps（皮秒，1ps = 1000fs = 10-12秒）表示。 如果用仪器测量，呈现出的是信号的频域特性，称作“相位噪声（Phase Noise）”。本质上，这两者是一样的，只是表述方式不同而已。 （1）抖动 抖动分确定性抖动（Deterministic jitter，DJ）和随机性抖动（Random jitter，RJ）两种。DJ通常幅度有限，以单位时间表示；RJ为高斯分布，以RMS均方根值表示，RMS Jitter值大小与振荡输出频率成反比。 晶体振荡器的RMS Jitter值与输出频率成反比 晶振的抖动通常由噪声引起，并导致频率不稳定。对于精密电子仪器、无线电定位、高速目标跟踪和宇航通信等应用领域，选择低噪声晶振十分重要。 （2）相位噪声 相位噪声（Phase Noise）是抖动在测量仪器上的表现，通常定义为一个振荡器在某一偏移频率fm处1Hz宽带内的单边信号功率和信号总功率比值，单位是dBc/Hz，通常表示为dBc/Hz@fm。 若没有相位噪声，振荡器的整个功率都集中在f0（10MHz为例），功率频谱就是一条以f0为中心的直线，且信号为纯正的正弦波。但是任何信号都有不稳定性，从而产生了边带sideband。 相位噪声的来源主要有三方面： （1）晶体品质Q值。高频晶体有很高的近载波相位噪声（Close-in Phase Noise）, 因为他们有低的Q值和更宽的边带。 （2）晶体外围电路：包括包括IC、RC元件、引脚等。 （3）信号输出(白噪声)。 高速系统对晶振相位噪声的要求 在通信网络、无线传输、ATM和SONET等高速系统中，时钟或振荡器波形的时序误差会限制一个数字I/O接口的最大速率。不仅如此，它还会导致通信链路的误码率增大，甚至限制A/D转换器的动态范围。有资料表明，在3GHz以上的系统中，时间抖动（jitter）会导致码间干扰（ISI），造成传输误码率上升，这就要求晶振选型必须满足严格的抖动指标。 如果需要设备即开即用，就必须选用VCXO或温补晶振；如果要求稳定度在0.5ppm以上，则需选择数字温补晶振（MCXO）。模拟温补晶振适用于稳定度要求在5ppm～0.5ppm之间的需求。VCXO只适合于稳定度要求在5ppm以下的产品。在不需要即开即用的环境下，如果需要信号稳定度超过0.1ppm的，可选用OCXO。 面向高速通讯应用的高频低噪声晶体振荡器 一般来说，晶体振荡器的相位噪声在远离中心频率的频率下有所改善。TCXO和OCXO振荡器以及其它利用基波或谐波方式的晶体振荡器具有最好的相位噪声性能。采用锁相环合成器产生输出频率的振荡器比采用非锁相环技术的振荡器一般呈现较差的相位噪声性能。例如，对于需要低噪声、稳定和精确时钟源的工业级设备（比如收发器模块或数据中心），可选择150fs小型塑封石英PLL振荡器；而通讯、导航、雷达应用领域的要求会有更高如50fs，这需要将100MHz以上基波起振的高频石英晶体单元与噪音特性优越的振荡IC相组合。

Hmmm, it appears that the title of this column has the potential to be a tad confusing. I'm not talking about music suppressors that are really annoying -- I'm more concerned with finding something that can help me suppress really annoying music. Let me elucidate (don't try this at home. I'm a professional)...   Before we start, it might be worth taking a moment to set the scene. My wife (Gina the Gorgeous) has a humongous family on her mother's side. (She can actually trace the family back to Nova Scotia and then to Europe before the Pilgrim Fathers landed, but that's a tale for another day.) Gina's mother was one of seven siblings. Her mother's mother was one of ten. Most of the family lives in Louisiana.   About ten years ago we had a big get-together featuring just two branches of the family (branches at Gina's grandmother's level). It took the camping grounds at a really large state park to accommodate all of us. Did I ever mention that I'm really bad at remembering names and faces? But we digress …   Last week, Gina, our son Joseph, and I went down to Louisiana to celebrate Gina's birthday. During the day, Gina and Joseph visited with her mother and aunts and uncles and cousins while I worked out of our hotel room, then I met up with them all in the evenings. The reason for my mentioning this here will become apparent in a moment.   Gina loves music. She plays it all the time. She's also amazingly knowledgeable. If a tune comes on the radio and I say, "Who's that?" (or, possibly, "What's that rubbish?"), she will immediately respond with the group, album, track, and the year in which it appeared on the scene, and she's invariably right on all counts. (I can do the same sort of thing, but in my case I'm just making things up.)   The problem (for me) is that Gina likes her music really loud. She’s also keen on listening to Christian rock when she’s driving. I think both of these things are wonderful … just so long as I'm not in the car with her. The downside for me comes when we happen to be taking a long drive together -- say an 8.5 hour drive down to Louisiana (just to pick a hypothetical example out of thin air, you understand).   Now, I'm the proud owner of some amazing MDR-NC500D digital noise-cancelling headphones from Sony. I picked these up on a trip three years ago when I went to give a talk to a bunch of engineers at Microsoft in Seattle, Washington (see I've fallen in love! ).       These little beauties bring tears of joy to my eyes whenever I'm flying somewhere on a plane. The difference when you activate the noise cancellation is truly amazing. Since I had my backpack with me on our trip to Louisiana, I surreptitiously donned these headphones. Do you know, I hadn’t realized just how much ground noise there is when you are travelling in a car -- even a really nice car like Gina's? When I turned on the noise cancellation, the rumble from the ground almost completely disappeared. Unfortunately, the noise cancellation doesn’t work on things like music, so the end result was to make the songs Gina was playing sound clearer and -- paradoxically -- louder.   It's amazing how we humans can forget what pain feels like. Once we'd arrived in Louisiana, I was swept away by the fun and frivolity, and the horror that is Christian rock became naught but a distant memory … until the time came for our 8.5 hour return trip. All I can say is that there was much metaphorical gnashing of teeth and rending of garb on the way home. Thus it was that I decided something had to be done to save me from similar anguish in the future. Upon our return, I started to Google furiously. Initially, I honed in on a pair of 3M Peltor H10A Optime 105 Earmuffs from Amazon. These weigh in at 12.3 ounces, have an NRR (noise reduction rating) of 30dB, and are an absolute bargain at only $21.61.     However, I then ran across a pair of 3M Peltor X5A Earmuffs as illustrated below. On the one hand, these are a little more boring to look at. They are also a tad more expensive at $25.92. But their main claim to fame is that they boast an NRR of 31dB.     Of course, you might be tempted to ask, "What's the big deal -- what's 1dB between friends?" All I can say in response is that -- when it comes to my listening to Christian rock -- every extra dB of noise reduction is worth its weight in gold.   These little scamps arrived at my office yesterday. I immediately put them on and took them for a stroll around the building. It was wonderful -- I couldn’t hear a thing. While I was perambulating along, my chum Paul saw me scampering past his office, and he came out to see what all the fuss was about.   When I explained about my noise-reducing earmuffs, Paul begged to try them on. He explained that he has a large ride-on mower. (He and his wife own a pretty big property out in the county.) His usual modus operandi while mowing the yard is to crank his Bluetooth earbuds up to full volume, which obviously does not bode well for his long-term hearing.     As you can see in the image above, Paul actually has his earbuds on under my earmuffs. He says that he can now run these at a low volume and that the quality of the sound is incredible. The last I saw of Paul, he was bounding into his office crying, "I've got to get me some of these!"   But wait, there's more … I took my earmuffs home. Yesterday evening, while Gina was watching some dross called Dance Moms on TV, I whipped out my trusty little beauties and slapped them on my head and … it was like being transported to another world. (The strange writhing that ensued in my seat can be attributed to my performing my happy dance while remaining in a reclined position.)   In conclusion, I'm delighted to report that all is now "peachy" in the Land of Max (where the colors are brighter, the butterflies are bigger, the birds sing sweeter, and the beer is plentiful and cold). How about you -- do you think you might be tempted to invest in a pair of these little rascals?  

I'm not sure why, but I am having a picture of a Discworld-esque scene that takes place in the wee hours of the morning. A figure makes his furtive way down an alleyway and taps on a door. A flap opens and the following exchange takes place between the doorkeeper and the furtive figure: * Furtive Figure: " A gibbous moon hangs pendulously in the night sky." * Doorkeeper: "Monk fish croon their soothing songs of snooze." * Furtive Figure: "Yet still the dog doth grunt and snore." And back and forth they go with evermore obscure passwords. Sometimes my imagination runs away with me. I really should go back to taking my dried frog pills. But we digress... The point is that my wife (Gina the Gorgeous) and I have two stupid dogs that spend the night in our room under the bed. Henri (the French Pronunciation) is something of a glutton – he eats anything he can find – including cat food, bird food, and things I don't care to mention here. As a result, Henri is overweight, and he now grunts and snores his way through the night. It's gotten to the stage where it's hard for us to get a good night's sleep, so we decided to purchase one of those relaxing noise machine thingies. You know the sort of thing – they generate the sound of rain, or wind, or surf on the beach. You used to be able to get them from almost any store as standalone units or integrated into an alarm clock. We went to numerous stores including Best Buy, Walmart, and Target ... nothing. Even worse, when we tried to explain what we were looking for to the shop assistants (who are growing younger by the day), they looked at us like we were from another planet (my impression is that technology is now changing so fast that they had simply never heard of these devices, which is pretty amazing when you come to think about it). And then, while we were on our way out of the last store, Gina said "You don't suppose that there's an iPhone app for this, do you?" Well, put me in a dress and call me Laura! I felt like a fool (but where were we going to find a fool at that time of the day? Ah, the old jokes are the best ones). I really should have thought about this myself. As you may recall, when I built my Ukulele I found a bunch of Ukulele tuning apps in the iTunes Store, so I have absolutely no idea why I didn't think of looking there first. When we returned home, Gina looked online and found an amazing free app. The sound, when Gina's iPhone is played through our docking station, is amazing. As opposed to electronically generated versions, this uses real-world recorded sounds. The people who created this app have obviously done their research, because they have sounds in there that I would never have thought of, including a fan, an air conditioner, the sound of a city, the sound you hear while sitting in the back seat of a car, the sound of being on an airplane, the sound of the tracks when riding a train, and all sorts of other things. Of course they also have "White Noise" and "Pink Noise," plus they have "Blue Noise," which I – for one – had never heard about. But I just had a quick Google (it's OK, no one was looking) and found a Colors of Noise entry on the Wikipedia. Wow – in addition to White and Pink Noise, it turns out that there's also Brown, Blue, Violet, and Gray Noise. And these are just the ones with technical definitions – it seems that there are also "unofficial colors" such as Red, Orange, Green, and Black Noise. I learn something new every day...

I don't know why, but I am imagining a Discworld-esque scene that takes place in the wee hours of the morning. A figure makes his furtive way down an alleyway and taps on a door. A flap opens and the following exchange takes place between the doorkeeper and the furtive figure: * Furtive Figure: " A gibbous moon hangs pendulously in the night sky." * Doorkeeper: "Monk fish croon their soothing songs of snooze." * Furtive Figure: "Yet still the dog doth grunt and snore." And back and forth they go with evermore obscure passwords. Sometimes my imagination runs away with me. I really should go back to taking my dried frog pills. But we digress... The point is that my wife (Gina the Gorgeous) and I have two stupid dogs that spend the night in our room under the bed. Henri (the French Pronunciation) is something of a glutton – he eats anything he can find – including cat food, bird food, and things I don't care to mention here. As a result, Henri is overweight, and he now grunts and snores his way through the night. It's gotten to the stage where it's hard for us to get a good night's sleep, so we decided to purchase one of those relaxing noise machine thingies. You know the sort of thing – they generate the sound of rain, or wind, or surf on the beach. You used to be able to get them from almost any store as standalone units or integrated into an alarm clock. We went to numerous stores including Best Buy, Walmart, and Target ... nothing. Even worse, when we tried to explain what we were looking for to the shop assistants (who are growing younger by the day), they looked at us like we were from another planet (my impression is that technology is now changing so fast that they had simply never heard of these devices, which is pretty amazing when you come to think about it). And then, while we were on our way out of the last store, Gina said "You don't suppose that there's an iPhone app for this, do you?" Well, put me in a dress and call me Laura! I felt like a fool (but where were we going to find a fool at that time of the day? Ah, the old jokes are the best ones). I really should have thought about this myself. As you may recall, when I built my Ukulele I found a bunch of Ukulele tuning apps in the iTunes Store, so I have absolutely no idea why I didn't think of looking there first. When we returned home, Gina looked online and found an amazing free app. The sound, when Gina's iPhone is played through our docking station, is amazing. As opposed to electronically generated versions, this uses real-world recorded sounds. The people who created this app have obviously done their research, because they have sounds in there that I would never have thought of, including a fan, an air conditioner, the sound of a city, the sound you hear while sitting in the back seat of a car, the sound of being on an airplane, the sound of the tracks when riding a train, and all sorts of other things. Of course they also have "White Noise" and "Pink Noise," plus they have "Blue Noise," which I – for one – had never heard about. But I just had a quick Google (it's OK, no one was looking) and found a Colors of Noise entry on the Wikipedia. Wow – in addition to White and Pink Noise, it turns out that there's also Brown, Blue, Violet, and Gray Noise. And these are just the ones with technical definitions – it seems that there are also "unofficial colors" such as Red, Orange, Green, and Black Noise. I learn something new every day...  

Back in the mid to late '90s, I worked for a small engineering firm that makes automated testing equipment for the fluid power industry; test stands for things like hydraulic pumps, motors and valves. In combination pump/motor test stands, we typically used Variable Frequency AC drives for the prime mover and loading device. On one particular machine, we selected a new and new-to-us VFAC drive based on its price, small size and advanced capabilities. This one was rated for 100 HP (74 kW) and ran on 480VAC, three phase power. This drive was working out nicely, but for one BIG problem, noise. Our single-ended (long story) analogue signal conditioning for flows, pressures and torque were picking up noise in a big way. Intermittent, irregular and relatively infrequent, HUGE spikes were swinging the proper signal either high or low by as much as 25% full scale. It didn't take long to realise that the VFAC drive was the source. Stop the drive and everything became peaceful again. Though I studied EE some at school, I'm primarily a software guy. I tried ferrite chokes, filters, and variations on signal cable and shield termination. Nothing seemed to help much and I had pretty much used up my knowledge on the topic. Troubleshooting this problem quickly became a collaborative effort between me, the technicians I worked with, the North American drive reseller and the Japanese drive manufacturer. By this time, the test stand was late and our customer was in a bind. They already had an assembly line piling up parts that, by their certified ISO process, had to be tested before they could be sold. The reseller sent their resident EE to our plant to have a look. Our effort at chokes and such weren't working because the very air was filled with high frequency pulses. One could hold a scope probe in the air and see a classic ringing waveform on the scale of tens of nanoseconds. Every wire that wasn't transmitting this was sure as heck receiving it. The drive OEM finally sent their senior design engineer from Japan to our little factory here in Nowhere, Oklahoma. After much probing and brow-wiping, we were told that the IGBTs used in this drive were 5th generation, the newest at the time, and offered great benefits from the efficiency gained by their high switching speed. Well at 600V on the DC buss, there's a lot of potential being modulated at a very high speed. Every time the IGBTs fired (about 14,000 times per second per IGBT), the airwaves lit up. We only saw irregular and infrequent spikes in our analog signals because our sample rate was so low and the duration of the pulses so short. But when we did see a spike, it swamped our signal. A few tweaks were made to the drive's internal settings, to lower the modulation frequency and stretch the switching time a little. We even added a filter between the drive output and the motor leads. In the end, however, optical isolation was the real winner. Every interface wire that connected to the drive was acting as an antenna to transmit that noise throughout the entire test stand. Full credit goes to the reseller's EE who tried optically isolating the digital interfaces to the drive and the result was dramatic; we had a solution. We wound up designing an optical isolator circuit board of our own that handled all digital and analog interfaces to the drive, which we used in all relevant machine designs going forward. - Reagan Thomas Reagan Thomas studied EE and EET in school, but wound up becoming a software guy who dabbles with electrons. He worked 18 years designing software (and some hardware) for automated testing equipment. He now deals with software for radar video distribution equipment.