Do a regular VNA calibration, preferable with a short coaxial cable added during calibration that is around same length as the unknown transmission line, if it is a longer transmission line, for best precision if only open or short can be calibrated for.
It is then relative easy to find transmission line impedance or deembed it, setting up any complex impedance curve as reference and much more in AnTune.
Here is auto port forwarding time and attenuation performed for a transmission line:
and it certainly has multiple parallel branches
The R&S ZNL already has this feature (for $100k, you'd hope so).
I was asking if there was a way to:
1) Estimate the equivalent lumped model.
2) What the best way to calibrate/deembed a test fixture out of a 1 port measurement, where I can only leave the other end open.
rebuild it with a bunch of RF relays and digital control?
A possibility to identify correct electric length is if short can be found by manually short-cutting with a blank PCB placed in the socket.
You can estimate short to have about same reflection attenuation and similar phase delay as for open but if high amount of precision is needed must also short parameters be measured.
A one-port measurement is especially easy and involves placing loads with three known impedances at the terminals in the fixture, usually a short, an open, and some resistance of a particular value, often the termination resistance of a transmission line, but it doesn't need to be that. The chicken-and-egg problem is to characterize your loads, which is why often the calibration standards are difficult to make.
I can explain the math of how VNA calibration works. I did it for my VNA project
...
And that is how you deembed one port.
Dan
Herein lies the problem. It is impossible to place a known load across the terminals. Only an open or a short. I have resorted to utilizing TDR to estimate the impedance seen by the port. Time-domain methods have assisted me where frequency-domain methods have failed.
If they are two adjacent balls in a BGA, you might be able to bridge them temporarily with a SMD resistor. Or you could fab a special PCB with pads the same spacing as the BGA and then solder the resistor or load to it temporarily.
What is the frequency range of interest, and what is (roughly) the distance on the PCB between RF connector and chip? Just to get a feeling about the magnitudes we are talking about.
What about time gating or impedance peeling?
I wonder, though, whether your VNA is fast enough in order to provide the necessary time resulution. Depends of course on the frequency range of interest.
Herein lies the problem. It is impossible to place a known load across the terminals. Only an open or a short.
Herein lies the problem. It is impossible to place a known load across the terminals. Only an open or a short.
I found yet another de-embedding method which requires only a single known impedance at the DUT location (which can be short). It makes use of time-gating.
Herein lies the problem. It is impossible to place a known load across the terminals. Only an open or a short.
I found yet another de-embedding method which requires only a single known impedance at the DUT location (which can be short). It makes use of time-gating.
I believe this is what R&S does with their "direct compensation" de-embedding setting in their VNA's. You can either perform "auto length" or "auto length and loss" compensation, these both assume that you simply have a TL attached and by using TDR with an open and short load, it figured out the delay and loss.
The VNA also has a feature called "Direct Compensation", which I'm fairly certain implements something similar to what you have linked above, which is what I have been utilising this whole time, and seems to be doing the job really well (I'll post some pictures when I get the chance).
"Direct Compensation" provides a frequency-dependent transmission factor. The phase of the transmission factor is calculated from the square root of the measured reflection factor, assuming a reciprocal test fixture. The sign ambiguity of this calcula-ted transmission factor is resolved by a comparison with the phase obtained in an Auto Length calculation.
I do not think so. I downloaded the ZNL handbook and the text readsQuote"Direct Compensation" provides a frequency-dependent transmission factor. The phase of the transmission factor is calculated from the square root of the measured reflection factor, assuming a reciprocal test fixture. The sign ambiguity of this calcula-ted transmission factor is resolved by a comparison with the phase obtained in an Auto Length calculation.which does not indicate that any time domain processing were involved in "Direct Compensation". Unfortunately they don't specify the underlying maths in detail. My guess were rather some kind of "frequency-dependent transmission line model", but I may be completely wrong.
I do not think so. I downloaded the ZNL handbook and the text reads
I haven't used that function in a while, but from what I remember the "Direct Compensation" function simply works by measuring S_nn at the calibration plane (i.e., the fixture connector) at each port, with the fixture contacts open or shorted (this can be selected). Assuming reciprocity (i.e., the path from the connector to the fixture contact is the same in both directions), the square root of the measured S_nn describes the influence of one pass trough the path from connector to fixture contact. This quantity can then be applied at each port to correct for phase and loss.
This correction is applied at each frequency individually. There is no transmission line model involved, in contrast to Auto Length and Loss.
For a single port, Direct Compensation is equivalent to simple trace normalization. I.e., measure S_11^ref at the cal plane with the fixture shorted. Then the corrected measurement, up to an overall phase, is given by S_11^meas/S_11^ref.
You can do Auto Length and Loss as well as Direct Compensation with open, short, and both open and short. Due to the fringing capacity of an open a short is probably closer to ideal.
My really silly (maybe not so) approach:
Can't I just calibrate so that the plane is at the end of my cable, connect it to my test PCB, and take "reference" S_11 measurements. After inserting my chip I can then take the S_11 measurement again, and then simply divide the measurement by the "reference". (i.e. abs(S_11/S_11,ref))*.
This has been done commercially for a few years using Automatic Fixture Removal (AFR for short, paid for app). It does just what you want to do. The fixture can be characterized with just an open, or just a short, or open/short combo; or just a thru. Used all the time by RF chip makers to remove their fixture effects.
The basic principle is discussed in chapter 11 of my book (2nd edition www.tinyurl.com/joelsmicrowavebook ). Basically uses some sophisticated time domain gating techniques and little special sauce. But that chapter also details Automatic Port Extension which might do the trick for you. Its a free feature on Keysight VNAs.
But the chapter also tells you how to make your own calibration kits and suss out kit coefficients and use them for in-fixture cal.
Or, just for fun, I can do can AFR for you; calibrate carefully at the end of a cable using your best SMA cal kit, post an S1P file of your fixture with the port open, and I'll send you back the S2P S-parameter file of the fixture, but the measured open-data needs to go to 20 GHz, 10 MHz steps (2000 points).
*and also phase(S11_/S_11,ref).
So I guess really not so silly after all...
*and also phase(S11_/S_11,ref).
So I guess really not so silly after all...
In the general case, you would need to measure the fixture terminated with three known impedances in order to obtain an unambiguous solution.
But if you have some a priori knowlege about the fixture, then this knownledge may help to constrain the set of possible solutions, so that fewer than three measurements may suffice.
A simple S11 response normalization resolves the ambiguity by making the a priori assumption that the fixture can be modeled by an equivalent 2-port network whose S11=S22=0. Only if this assumption happens to apply to your fixture, then the result is correct for any DUT connected to the fixture, otherwise it is not.
my e5100 has a 'thru' calibration option.