标签: doppler

多普勒频移效应对车载电台的影响 2016-01-29 InterCar InterCar InterCar   InterCar 微信号 CarRadio_Engineering 功能介绍 1.车载电台（对讲机），车载收音机相关技术问题探讨，涵盖发射，接收，噪声，天线，干扰，通信加密，EMC/EMI，ESD等。2.车载音响，音效，音质问题技术探讨。3.车联网(IOV)行业发展和趋势资讯。4.经济与生活，读书，旅游，投资等话题。   什么是多普勒频移效应   生活中我们往往会有这样的印象，迎面而来正在鸣笛的火车，或者警车，或者救护车，逐渐在靠近我们时，我们听到的鸣笛声是越来越刺耳，一旦鸣笛的火车，警车或者救护车远离我们越来越远后，我们听到的鸣笛声却越来越柔和，变得不是那么刺耳。我们知道，声音是一种具有某种振动频率的波动，声音刺耳，意味着声波的频率越来越高，声音逐渐柔和，意味着声波的频率越来越低。这个现象的背后，其实就是多普勒频移效应作用的结果。   多普勒频移效应模型和频率偏移量的简单计算   Case 1 ：   当移动台向波源方向以速度 V 移动时，波源发射出来的波的频率 fo 固定，近似于移动台在单位时间内看到的波动的振动周期增加，相当于感受到的波动的频率增加了。所有的运动都是相对运动，如果移动台固定，波源运动，其结果也是一样的。如果设定波动的速度为 C ，那么多普勒频移量可以按照如下方式计算：   Case 2:   当移动台移动方向和波源传播方向有一角度 α 时，这个其实是我们经常碰到的情形，根据矢量的运算合成和分解法则，可以得到如下多普勒频移偏移量的计算方式：   多普勒频移对车载电台的影响   从上述两种情况下的多普勒频移的计算公式可以看到，波源的工作频率 fo 对多普勒频移的影响是直接正相关的，移动台的速度 V 也是直接正相关的，可以设想，移动台速度为 0 ，那么多普勒效应就不会发生，波速 C 的影响是负相关，一般对于电磁波而言，波速是固定的光速， 300k 公里 / 秒，所以对于移动台受多普勒频移的影响，我们就用简单的常识性数值做一个计算，设定汽车的速度为 30m/s ，对应 108 公里 / 时的时速，对于车载 VHF 电台，我们用 300MHz 的载波频率以简化计算，对于车载 FM 电台，我们用 100MHz 的载波频率以简化计算，最终可以得到如下结果：   从上述结果可知，相对于300MHz和100MHz的载波频率，30Hz和10Hz的多普勒频移量对于车载移动电台的影响是很小的，可以忽略不计。 版权声明 ： 任何个人或者媒体转载本文必须征得本文作者同意，或者注明文章出处。 如果您对本订阅号内容感兴趣，请长按或者扫描下面二维码关注本订阅号，也可直接点击微信订阅号搜索： InterCar 微信号：CarRadio_Engineering

The United States Army Signal Corps' first attempt to "touch" another celestial body was on January 10, 1946, when it bounced radio signals off the moon and received the reflected signals. Dubbed "Project Diana" for the Roman moon goddess, the effort led to what is today known as EME (Earth-Moon-Earth) communications, used for ham radio. Project Diana is often noted as the birth of the US space program, as well as that of radar astronomy. The project was the first demonstration that artificially created signals could penetrate the ionosphere, opening the possibility of radio communications beyond the Earth for space probes and human explorers. Project Diana also established the practice of naming space projects after Roman gods and goddesses, like Mercury and Apollo. Project Diana's first successful echo detection came on at 11:58 am by John H DeWitt and his chief scientist E King Stodola from a lab at Camp Evans, in Wall Township, NJ. A large transmitter, receiver, and antenna array were constructed at the lab for the project. The transmitter, a highly modified World War II SCR-271 radar set, provided 3,000 watts at 111.5MHz in quarter-second pulses, while the "bedspring" dipole array antenna provided 24 dB of gain. Reflected signals were received about 2.5 seconds later, with the receiver compensating for Doppler modulation of the reflected signal. Attempts could be made only as the moon passed through the 15-degree-wide beam at moonrise and moonset, as the antenna's elevation angle was horizontal. About 40 minutes of observation was available on each pass as the moon transited the various lobes of the antenna pattern. The Project Diana site is today maintained by the Infoage Science/History Learning Center.   Suzanne Deffree is EDN's executive editor. She is an award-winning journalist who manages several blogs and sections of EDN.com and EDN’s e-newsletters including its daily newsletter, EDN Today, and EDN Fun Friday. She also heads EDN’s social media and community efforts.  

2012年7月的一份日记   昨天重新学习了一下doppler频移的相关知识。Wikipedia上面有很详细的解释。简单的说，就是当信号源与接收器之间有相对运动时，观察到的信号频率会随着速度发生变化。   这种频率上的变化，在无线信号里面，很容易理解的是，RF接收到的信号往往会留有一个低频率的包络。在频谱上看，就是有个频偏存在。   比较容易忽视的一点是，这个频移还会体现在基带信号的频率上。例如CDMA信号的码片速度上。例如在1GHz的RF接收到信号上有1khz的频偏，在1MHz的基带信号上，对应的其实还有1hz的码片速度的变化。   通常，移动通信系统的终端，处理RF残留的那1khz的频偏，不处理那1hz的码片速度。   这个原因，主要是移动通信系统设计，往往都是基于无线帧结构来做。而一个帧的长度，由于考虑心道的时变，都比较短，在几百us到几个ms的数量级。在每个无线帧中都会设计参考信号。这些参考信号会用于对信道变化的估计，也会用于对基带时间同步的估计。于是，终端处理这些参考信号，对接收的信号进行均衡，同时也调整本地接收时钟的帧同步点。于是，在频域的同步上能看到对doppler频移的处理，而在接收时间上没有看到明显的doppler处理。但事实上，他是存在的。   另外，接收信号残留的频差也好，接收时钟帧同步的偏差也好，其实都包含了doppler频移和两个钟本身的偏差两个方面。但在处理中，不需要区别这两者。按照同样的方法对接收的信号进行修复就好。   而如果在GPS接收机算法设计中，这个doppler频率对码片速度的影响就是不可忽略的了。因为GPS接收机的原理就是没完没了的积分，一定要锁定码片和频率的相位。积分时间一长，这些doppler频移因为的码片相位的变化就不是按照理论上的1.023M来走的了。进一步，如果要用doppler频移来计算速度，就需要考虑如何将时钟本身的偏差怎么扣除掉。  

  In the case of the Doppler panel, imagine the sound a train's horn makes as it approaches you and then passes you and recedes into the distance. As the train approaches, the sound waves are compressed (as shown to the right of the sculpture); as the train recedes, the sound waves are stretched out (as shown to the left of this image). With regard to the Orion's belt panel, this is a visualization of the 42 strongest stars in a 6x7 degree section of the constellation of Orion, showing Orion's belt and sword as imaged through a hole the size of a pencil-tip (0.1 mm), where the central 99% of the hole is blocked. The ring patterns from the 42 different stars will then interfere with each other producing a very complex result. This panel was produced by convolving a point spread function with the 42 point sources from a star catalog. When we returned inside the building and started to walk around, I was surprised to see pictures of yours truly plastered everywhere announcing my talk.     My presentation was scheduled to start at 12:15pm. So, at around 11:55am we strolled up to the lecture theater to make sure that I could use their equipment. The theater itself was huge (much bigger than it appears in the photo below). We entered at the top to see tier-after-tier of chairs arranged like a movie theater.       You can only imagine my surprise to find that the lecture theater was jam-packed full. "Oh wow," I thought to myself, "they must really want to hear what I have to say." And then the bell rang and everyone got up and left (grin). The image above shows the state of play about 20 seconds after the lecture had just ended. Happily, my audience soon began to arrive, and we ended up with quite a crowd. The image below shows the scene after I had just finished my presentation and folks were coming down to chat with me. I'm the guy facing the crowd and furthest away from the camera (Joseph took this picture, which may explain the blurred effect :-)   And how did my talk go? Well, it would be immodest of me to "blow my own horn" as they say, but I did receive quite a number of complements at the time and also via email afterwards. For example, I received one email that read as follows: Dear Max. I found your talk today both exciting, inspiring, and entertaining. Three of my favorites. ;-) The writer then goes on to say that when he was young he hung out with some friends who built a computer. He didn't have any money to buy electronic components so he instead went into the software side of things. He finished as follows: Now that you have some of my background, I will get to the real point of what I wanted to say... Thank you for your talk and converzation today. You brought some of that old glow, optimism and happiness of the early years back to me. Don't misunderstand, I have not been unhappy. It is just that you brought to the top of my stack of thoughts that there is so much more cool stuff to enjoy. Well, what can I say? This is high praise indeed. I have to tell you that receiving feedback like this is what makes all of the time it takes to pull one of these presentations together so worthwhile. Last but not least (for this column), the following image shows me after my talk. We were walking past the library portion of the building when we spotted yet another poster advertising my presentation.   The white box in my hands is a present from the university. It contains a miniature model in cast aluminum of one of the "Equations in Stainless Steel" panels that adorn the outside of the building. The one I have represents the interference patterns caused by someone singing a pure high C (C6 = 1046.5Hz) while walking toward one wall – and away from another – at a brisk stroll of 10m per second. (Just in case you were wondering, a temperature of 15C is assumed ). If you are ever passing by my office, please feel free to drop in and I would be delighted to show it to you. Following my talk we had a wonderful lunch with the attendees, and then we toured the department to see all of the research they are doing. This includes some incredible work with FPGA reconfiguration, self-learning robots, interactive music systems, and... but I'm afraid this will all have to wait until the next installment of my Norwegian Odyssey...