EMRFD Message Archive 9239

Message Date From Subject
9239 2013-10-12 15:05:33 Kerry Return-Loss Bridge Operation
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

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

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.
9240 2013-10-12 17:09:26 Russell Shaw Re: Return-Loss Bridge Operation
9241 2013-10-12 17:39:59 Kerry Re: Return-Loss Bridge Operation
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.
9242 2013-10-12 22:37:09 Russell Shaw Re: Return-Loss Bridge Operation
9243 2013-10-13 06:41:18 Ashhar Farhan Re: Return-Loss Bridge Operation
Get Hayt's Engineering Electromagnetics. Lucid, yet complete.
- f

On 10/13/13, Russell Shaw <rjshaw@netspace.net.au> wrote:
9246 2013-10-13 14:11:43 Kerry Re: Return-Loss Bridge Operation
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;


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.