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| Kirkby calibration kit alternatives? |
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| hendorog:
--- Quote from: G0HZU on July 24, 2018, 10:15:31 pm ---I think you also have to correct for the time delay in the SMA load I'm not sure how muddy things get if you were to try and do some precision work with a full 2 port VNA with an uncorrected 51.1R SMA load as the calibration reference. I wouldn't want to use it like that even if it would be OK for many casual tasks. I'm not sure I want to waste the time working out how much it will degrade certain types of measurement. I'd rather source a decent load and keep it simple :) --- End quote --- Yes - but consider the inverse case, which is what I was trying to do: Can you make your measurements _more_ accurate by entering the exact resistance of the load? I believe that the assumption that a load is exactly 50 ohms is a bit of an historical relic. So taking the argument one step further, why not do away with the model altogether? Since a better model of the load is an S-param measurement of the load, then we could just use that instead and calibrate on the PC. |
| G0HZU:
--- Quote ---Can you make your measurements _more_ accurate by entering the exact resistance of the load? --- End quote --- I guess so, assuming the VNA supports this feature. However, I'm not sure how it handles both the resistance and the Zo of the delay in the standard. It just seems to me to be a bad idea to have to resort to this. Pretty much all of my VNA experience is with official cal kits or my own homemade ones and they are probably all within 0.05R of 50R at LF so I don't have much experience of fudging the 50R standard. I chimed in because DrKirkby seemed to imply (to his customer) that the DC/LF resistance is irrelevant and can be ignored in the cal kit file. But I think the real/imaginary parts of the load at lower frequencies needs to be very close to a pure/accurate 50R resistor if you want to exploit the potential of a decent VNA when measuring impedance or if making a decent 2 port model at these lower frequencies. In my experience. a good VNA and a decent cal kit can deliver really good accuracy for impedances in the 5R to 2000R range at low frequencies. Much better than the uncertainty stats given for a default VNA with fairly grim uncertainty factors for the various contributors to overall uncertainty. An uncorrected 51.1R resistor would give an additional error of over 2% for this when measuring typical LCR parts at low frequencies. I think the error will be worse for some types of measurement. Also, even a decent VNA will typically struggle a bit when measuring the Q of inductors on the HF bands but in my experience the results can be quite good with a good VNA and cal kit as long as the Q is in the order of 50-100 and the inductance is in a typical/sensible range. I'd expect an uncorrected 51.1R load to magnify the uncertainty of a Q measurement quite a bit. Up at microwave frequencies the impact is going to be less but down at a few MHz to maybe 100MHz I think a VNA can deliver really good performance as long as the 50R load has a decent reflection coefficient down at these frequencies. --- Quote ---Bud: Now the problem is to know your cal load RL, that you need to measure against a decent calibration kit on a VNA or from the manufacturer data. --- End quote --- Agreed, but based on the physics of the component I'd expect the return loss of a decent 50R cal kit load to be in the ballpark of 70dB at a few MHz. Even a cheapo homebrew load made from selected chip resistors will be close to this at a few MHz if selected to have a load resistance very close to 50.00R at LF. Obviously, this would have to be verified as you say, but I don't think it's rocket science to make a reasonable 50R SMA load. A couple of years ago I posted up a measurement of my homebrew SMA load. It was quite a few years old by then and quite worn but it still managed 40dB return loss by 3GHz and the LF resistance was within maybe 20 milliohms of 50R. This was made using a pair of hand selected 100R chip resistors mounted on a decent quality SMA PCB end launcher. I chose resistors and an end launcher type that gave very good results across this range of LF through 3GHz. The plot below of my homebrew load was taken with an ENA VNA calibrated by a decent 13GHz Ecal module. It isn't quite this good today because it has been used a lot in the last two years and is showing increasing signs of wear. |
| G0HZU:
In terms of a typical application for that old homebrew load I used it as part of my homebrew cal kit to design and verify a precision (SMA connector based) resistive splitter a few years ago. This was needed to help me service an ancient HP8405A vector voltmeter and verify the performance. I used the cal kit to check out various SMD chip resistors (when piggybacked) to make up the resistors in the splitter. I wanted to match the performance of a decent commercial splitter up to the 1GHz range of the HP8405A. I needed ultra low port VSWR and sub 1degree phase balance and very good amplitude tracking on both arms of the splitter. To get this I needed resistances in each arm that were very close to 50.00R across LF to as close as 1GHz as possible. So I needed a decent cal kit for this. I used my old/lowly HP8714B VNA and the homebrew cal kit to do all this because that is all I had back then. When I got the 4 port ENA VNA and the 4 port Ecal module I remeasured it formally as a 3 port device (to 3GHz!) and you can see the results below. It was very impressive indeed for port VSWR, phase tracking and amplitude tracking across LF to 1GHz as you can see in the plots below. It degrades a bit by 3GHz but is still about as good as a typical commercial precision splitter up to 1GHz :) |
| hendorog:
--- Quote from: G0HZU on July 25, 2018, 10:58:57 pm ---In terms of a typical application for that old homebrew load I used it as part of my homebrew cal kit to design and verify a precision (SMA connector based) resistive splitter a few years ago. This was needed to help me service an ancient HP8405A vector voltmeter and verify the performance. I used the cal kit to check out various SMD chip resistors (when piggybacked) to make up the resistors in the splitter. I wanted to match the performance of a decent commercial splitter up to the 1GHz range of the HP8405A. I needed ultra low port VSWR and sub 1degree phase balance and very good amplitude tracking on both arms of the splitter. To get this I needed resistances in each arm that were very close to 50.00R across LF to as close as 1GHz as possible. So I needed a decent cal kit for this. I used my old/lowly HP8714B VNA and the homebrew cal kit to do all this because that is all I had back then. When I got the 4 port ENA VNA and the 4 port Ecal module I remeasured it formally as a 3 port device (to 3GHz!) and you can see the results below. It was very impressive indeed for port VSWR, phase tracking and amplitude tracking across LF to 1GHz as you can see in the plots below. It degrades a bit by 3GHz but is still about as good as a typical commercial precision splitter up to 1GHz :) --- End quote --- That is impressive - I made a splitter as well but as I recall it is not even close to that performance. As a matter of interest, what does your eCal measure itself as? I am curious how close that is performance wise to a physical cal kit. |
| G0HZU:
--- Quote ---That is impressive - I made a splitter as well but as I recall it is not even close to that performance. --- End quote --- Thanks. A fair bit of design effort went into it and it would have been far more sensible to just buy a splitter ;D I used the homebrew cal kit, the HP8714B VNA and a suitable test fixture to measure the best resistor type to help me reverse engineer a decent model of the chosen chip resistor. I then designed and simulated a (0.02" Rogers 4003C) circuit board using Genesys and Sonnet EM with my resistor models to try and get the lowest port VSWR across at least 1GHz. I used a precision PCB mill with decent end mill tools to make the PCB I had a good result with the very first PCB design and I think there was an element of luck in the phase matching. I didn't expect it to be that good and I only needed a few scrapes with a scalpel in the right places on the PCB to get the phase that flat. This was taken with the SMA connectors done up with a torque spanner as the phase shifts with connector tightness in the GHz region as you would expect. I'm not sure it's possible for me to directly measure the load in the Ecal because it is all controlled by the VNA via USB. I think the Ecal is at its very best above a few hundred MHz. By that I mean probably better than a typical 85033E mechanical kit. Below this (especially down at a few MHz) I think the mechanical kits (and my homebrew kit) are marginally better for both 1 port and 2 port measurements. But that is based on my experience of using all three. When I designed my SMA cal kit I went to similar (nerdy) lengths to verify I had the delays and corrections in my user cal kit as close as possible. I developed various passive 2 port test fixtures here that can easily show even slight errors in the cal kit's user file when the VNA test fixture s2p data is post processed on a computer. This work really highlighted to me why the 85033E cal kits and the Ecal modules are so expensive and I'm impressed how consistent all our 85033E cal kits are at work. |
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