measure SWR with Scope and TG (scaler)

[cncjerry]

OK, I always thought you needed to first calibrate using a RLB or DC and a 50 ohm load.  So in doing so, I can see where the resonant points are.  no problem, this works, except the trace is above the RL, not below as expected.  So in scanning around, they say you should calibrate with the load open instead of with a 50 ohm load.  Then measure and you can see the trace drop below the RL.  You then measure the dB drop and can calculate the SWR at that point.  The numbers I get from each are the same.  In my case, about 21dB except one is positive and the other negative.

I think I am missing something here as don't you want the most accurate load possible and a 50ohm load is more accurate than an open circuit, no? I mean, your levels of infinite depend on many factors whereas a 50ohm load is just that.

What am I missing?

thanks.

Jerry

[technogeeky]

You mentioned two scenarios:

* calibrate w/ 50 ohm, measure above RL
* calibrate w/ open, measure below RL

It would seem there is one option you left out:
* calibrate w/ short, measure below RL

In this video of an early The Signal Path episode, Shahriar uses a directional coupler and a Rigol spectrum analyzer with a tracking generator. He does this in a few steps:

* He calibrates against a short, explaining that this represents a perfect reflection of all incident waves at all frequencies.
* He then compares this to a crude open (crude because he leaves the calibrated open in a box, and just disconnects the short). The open appears to be roughly at the same level as the short, but has very slight bumps over and under the normalized reference level.
* And lastly, to verify the setup before connecting the device under test, he connects the 50 ohm load which represents a perfect absorber. The measurement suddenly drops about 40dB below the reference level.

I am happy to blindly trust an expert like Shahriar, so I would say this is the right way to do it. Thinking about it logically, too, it makes some sense:

* the 50 ohm load is the most expensive and difficult to perfectly match component. Its imperfections, if they exist, will limit our measurement in unforeseeable ways if we calibrate against it as a normalized reference.
* the short is the least expensive and easiest to perfectly match (and make, DIY) component. It's just a short. If it has any imperfections, they will be slight and they will probably be shared by the physical dimensions of the other devices. In any case, it's seems tremendously easier to make something 0 ohms for all frequencies than to make something 50 ohms for all frequencies.
* the open is, in a sense, the least well defined of these. I suppose it's the impedance of free space, or the environment it's in. It can pick up noise. In theory, it should be nearly the same thing as the short, just the opposite direction in some space. I'm not sure about this, but it seems wrong to calibrate against an open by itself.

Overall, I'm not entirely sure which is exactly, right, and thinking about this I believe there are more variables, including:

* what is the range exactly (such as: if we are making the 50 ohm, the open, or the short into the zero calibration, then what is the opposite condition? above or below RL? and what does it represent?)
* what about the other measurements you can make with a TG and a RLB; with the opens/short/load in the reflected port?
* what about using 50 ohm feed-through loads instead of terminating loads?
* what about all variants on these devices, the RF bridge, directional coupler, etc...

I would love to collect enough discussion and information so that we (or I) can write up an article and/or chart about what's appropriate, what to think about, and what all of the variations are in this simple measurement setup.

[rfeecs]

OK, I always thought you needed to first calibrate using a RLB or DC and a 50 ohm load.  So in doing so, I can see where the resonant points are.  no problem, this works, except the trace is above the RL, not below as expected.  So in scanning around, they say you should calibrate with the load open instead of with a 50 ohm load.  Then measure and you can see the trace drop below the RL.  You then measure the dB drop and can calculate the SWR at that point.  The numbers I get from each are the same.  In my case, about 21dB except one is positive and the other negative.

I think I am missing something here as don't you want the most accurate load possible and a 50ohm load is more accurate than an open circuit, no? I mean, your levels of infinite depend on many factors whereas a 50ohm load is just that.

What am I missing?

thanks.

Jerry

By calibrating, you are setting the measurement of the calibration standard to zero dB.  So you need to use a standard that has zero dB return loss.  You must calibrate with an open or a short.

[rfeecs]

* what is the range exactly (such as: if we are making the 50 ohm, the open, or the short into the zero calibration, then what is the opposite condition? above or below RL? and what does it represent?)
* what about the other measurements you can make with a TG and a RLB; with the opens/short/load in the reflected port?
* what about using 50 ohm feed-through loads instead of terminating loads?
* what about all variants on these devices, the RF bridge, directional coupler, etc...

I would love to collect enough discussion and information so that we (or I) can write up an article and/or chart about what's appropriate, what to think about, and what all of the variations are in this simple measurement setup.

There is plenty of literature out there.  Here's a good reference:
"Keysight Technologies Applying Error Correction to Vector Network Analyzer Measurements"
http://literature.cdn.keysight.com/litweb/pdf/5965-7709E.pdf

By calibrating, you are doing some error correction.

For a return loss measurement, there are three error terms:  reflection tracking, directivity, and source match.

For a VNA, you can correct for all three terms by doing a vector measurement of 3 cal standards: short, open and load.
You get three equations and three unknowns.  Note that the unknown error terms are vectors (with magnitude and phase).

For a scalar network analyzer, you can't do full error correction because you can't measure the phase.

So you can only correct for the magnitude of the reflection tracking error by measuring an open or a short. 
You will still get ripple if you cal with a short and measure an open because of the uncorrected source mismatch.
You will still be limited in how large a return loss you can measure because of the uncorrected directivity of your coupler or bridge.

So for the best accuracy in a scalar measurement of return loss, you need a high directivity bridge with a very good port match.

[cncjerry]

thanks, all makes sense.  By the way, with the bridge I am using, there is only about a 1db delta at 30Mhz using a short vs an open.