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Return Loss and VSWR
Hi all. I am an EE student currently working for a company that
manufactures RF and Microwave components. I find this technology fascinating, and I beleive that RF may be something that I want to do with my life. Unfortunately for me, my job is in production; testing to be specific. Although I have learned a lot, I feel limited for the time being. After all, now that I have learned how to program the network analyzer, everything else is just numbers on a screen with a PASS/FAIL sign. But anyway I digress... I have done research on my own about what I am doing at work. I feel that I am finally having a grasp on what the concepts are - coupling, return loss, VSWR, etc - as in having a feel for what is actually going on rather than just comparing readings to limit lines. So I guess I will say how I understand it and I would greatly appreciate it if anyone could correct me where I am wrong (because I probably am in plenty of areas!). (-: VSWR Voltage STANDING WAVE Ratio tells me that the nodes and antinodes of the wave are stationary, so the signal varies in amplitude until it reaches the end of the rail, the mainline. If it is terminated with ideal impedance matching (50-ohms), then the signal is completely absorbed. However if the impadnaces are unmatched, say 50 to 49.9 ohms, some of the signal is reflected back, 180° out-of-phase. It is 180° because the reflected signal is now going in the opposite direction (like making a U turn?). This then is algebraically added to the original. So it follows that if there is no termination (or short to ground?) the entire signal will be reflected back 180° out-of-phase and cancel out the original. This is how I understand reflection. I am still stuck on what the VSWR ratios actually mean. I know 1:1 is ideal but how should I interpret 1:1.15 for example? I have also learned that the 1/4 wavelength is important in power dividers for determining trace length... is this because the voltage (or power, for that matter) amplitude is at its greatest (90°)? This then cooresponds to 1/4 length of the physical wavelength? I also leanred that the trace width has different impedances (thinner traces are higher imp.). It is starting to go over my head at this point but as I go back and forth I pick up more and more of this. Lastly I will present a short gripe. I am taking classes that supposedly teach this stuff but we don't do like anything in the class... I guess that's community college for you. At any rate, I am not satisfied to just look at pass/fail because electronics theory is my passion and I want to feel it, know it, and live it. Thanks to anyone who reads all this for any help in advance! |
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