quixotron said:
I think you maybe right. I'll double-check tomorrow.
Right. The VNA has two female n-type connectors attached to it. I use the 8532B? 50 ohm n-type cal kit assigned to the VNA. They use n-type male standards, open, short and the 50 ohm line termination. As I understand, the reference plane begins at the point where the standard is connected. Suppose I connect the standards, calibrate and then connect an n-type to sma male adapter, the vna wouldn't account fo the adapter since the reference plane is at the n type connection and not at the sma end of the adapter, right? So, I would need an sma cal kit? I'll ask one of the emc guys about it. Although, I think that sometimes I bugg them with alot of questions.
So, if you have N type standards, it would be reasonable to calibrate at the end of an N-type cable, then use your N to SMA adapter to convert to the pcb. In this case, you may include the extra adapter as part of your "port extension" where you dial-in the extra delay. A bit more error, but it may not be significant.
One thing I forgot to mention in my previous descriptions that is quite important. When you dial-in extra electrical delay, i'm assuming that the transmission line on your PCB is 50 ohms, not some other impedance. This is critical. If it is not 50 ohms, then there will be too much error when you dial-in the extra delay.
When I say "transmission line on the pcb" I don't mean coax. I mean some other transmission line structure, like microstrip, stripline, or coplanar stripline to name a few. You can solder 50 ohm coax cable to the pcb if you want, but that's not what I meant. As a matter of fact, if you are not sure that the printed (microstrip etc.) tx line from SMA up to the antenna feedpoint is 50 ohms or not, you can indeed solder some semi-rigid microcoax (50 ohms of course) as a substitute to get your reference plane accurately up to the antenna feedpoint. Semirigid is a pleasure to work with as it is very easy to get in tiny diameters, is easy to strip and form and also tack-solders down quite easily. The main negative is the narrower your coax, the higher the loss and so potentially the greater measurement error.
quixotron said:
Right. I noticed that when i finished eating lunch, i might have had smeared barbecue sauce over the dut. I had a testrun with my device, I received a different reading, I had my co-worker measure it to see if it was my process. She too had a different measurement. but she said she reeated the measurement so many times. I also tapped the dut with my hand very slightly, tthis changed the resonse dramatically. This is one reason why we use wrenches on some of the othermore expensive vnas. especially the new agilent pnas, $450K USD!!!!!.
I get into a habit of just handtightening my SMA connectors until every so often I get bad errors and intermittents and I regret not using the wrench more often. Hasn't been a problem with N type or APC-7, but it has with SMA or APC3.5.
quixotron said:
Very interesting. So let me get this setup right:
1: You connect some RG cable to the VNA terminals. Probably some ntype to sma adapter. The Rg cable has sma connector types at both ends. I need to calibrate with the RG cable.
2: You attach a pcb to the cable, with a Tx line across it. So there is a piece of coaxial cable mounted onto a pcb, how do you do that, solder it? I can account for the pcb, by connecting it to the RG cable and then inputting an electrical delay, as I observe the locus/cusp converge to a single point near the shorted out resistance line on the smith chart. this should move my reference plane right upto the antenna feedpoint(where i connect my antenna).
This happens if I place a piece of metal, copper tape?? across the feedpoint terminals on the coax/pcb right?
Well, as i said above, I don't necessarily mean you solder coax down to your board, but this section of line needs to be 50 ohms.
As for shorting out the antenna feedpoint, you can use a small shred of copper tape, but I do it a bit more quickly using an old tuning tool that is plastic handle with a small metal blade at the end. The larger the piece of metal is, the more distorted your smith chart display will be. After all, this piece of metal is a sort of antenna with a complex impedance, so the smaller the better.
quixotron said:
I have an n type connection on my vna. Then I calibrate it. I then connect an ntype to male sma adapter. it is small, probbaly 2 inches. Should i account for it in the electrical delay? i probably should, i'll ask the emc guys.
Why would you need a coaxial? I'm a newbie Rf engineer for the record. Only have two years of industry experience.
Sounds like you aren't quite getting it, either that or I didn't quite understand your last few sentances. But I think I've covered it above. You only calibrate once, at the end of the coax as close to the antenna as you can get with your cal standards. Then you extend the electrical length (on my analyzer it is referred to as "port extension")
quixotron said:
Coaxes are leaky structures? Good stuff.
No, the coax is not leaking. What is happening is that RF current from the analyzer is coming down the inside of the coax and flowing out the end of the coax onto the antenna. But the currents that want to flow on the antenna don't exactly match with the currents that want to flow out of the coax (because of the impedances of the antenna shapes), so you get an imbalance between the coax and the load, or in other words, the exact amplitude and phase of the currents don't exactly match up. This causes some of the excess current that doesn't know exactly where to go, to flow down the outside of the coax starting from the open end of the coax.
quixotron said:
Sounds good. You have been of great help. I really appreciate it, Ron. I apologize for the interrogation procedure. I have agilent's guide to vnas but its a bit siplistic, doesnt offer test examples or procedures. i got it off wikipedia. I'll have to look at the HP reference manual some more.
You are welcome, and don't type so fast, you are making enough errors to cause a bit of confusion.