EMRFD Message Archive 9239

9239 2013-10-12 15:05:33 Kerry Return-Loss Bridge Operation Message Date From Subject A "How Does It Work" question was posted on the VNWA Group; I've asked myself the same question over the years and have never found the answer. In my pre-VNA days I built RLBs and used a HP 8405A vector voltmeter across the detector; this gave magnitude and angle between the two points but I never worked-out what to do with this information. The only explanation of RLB operation that I can remember seeing is in Wes' "Introduction to RF Design"; Wes explains very clearly the operation of the device in scalar terms but doesn't go-on to a detailed description in vector terms although he mentions the use of a vector voltmeter or a two-channel 'scope. I'd like to understand the operation of an RLB in a vector sense. Suppose I attach a signal generator and do an SOL calibration of an RLB with a vector voltmeter across the 50-ohm detector points. I will see a magnitude and a phase (+ & -) for the S and O steps and a magnitude with, ideally, no phase difference for the L step. Now I attach a complex impedance to the DUT port; I see a magnitude and a phase difference which might be, say, 1.27 volts with a phase difference of 27.6 degrees (these numbers are just made-up for illustration). The phase at the DUT end of the detector lags with respect to the other end of the detector. The 1.27 volts may be converted to power in dBm as the detector impedance is 50 ohms. Now I have all this information about the DUT; how do I convert it to the impedance, R +/- jX, of the DUT? I suspect that this is a Smith chart operation; I can do Smith charts but am certainly no expert so a description of how to do it would be most helpful. Otherwise, answers, for me at least, should not include more than mildly-advanced maths; I am not skilled in that regard. Kerry VK2TIL. Thanks Russell; a very clear explanation. I will work though it in detail at my leisure but I can clearly see how it works. I will have to learn how to solve ZL = Zo(1 + rho)/(1 - rho) in complex form; do you have any internet references that might help me? Kerry VK2TIL. Get Hayt's Engineering Electromagnetics. Lucid, yet complete. - f On 10/13/13, Russell Shaw wrote: > Thanks Farhan; I will look for it. Thanks Russell. It's a co-incidence that you mention pulses & TDR; that's just what I've been learning about in the last couple of weeks. I built a pulse generator; http://i44.tinypic.com/vd1sw2.jpg It works very well with my TDS-320 'scope which has digital-readout cursors so gives results to 0.1ns precision. I'd forgotten that I do have books that may help me; I will look in the bookshelves. Kerry VK2TIL.