on-board measurement of RF parameters: - RF Cafe Forums

 Post subject: on-board measurement of RF parameters:

Posted: Tue Apr 24, 2007 1:26 pm 

Hi,

I have little experience of measurement, especially on-board input-impedance of an antenna, or of a filter, etc.
Or, .... any suggessions are welcome.
like where we can be trained to perform those test, etc.
thanks a lot.

 Post subject:

Posted: Wed May 16, 2007 4:27 pm 

Well, I can tell you how I do it. I usually work at 900 MHz, so I don't know how well this method will scale with frequency, though it should if you use the right cables.

First of all, you need a vector network analyzer. The 8753D or later model from HP/Agilent is a nice choice. The important thing when using a VNA is to calibrate the ports. More on that later.

On your board, you need a way to disconnect the DUT (device under test) from the rest of the circuitry. Normally, there is a DC blocking cap in line with most parts of an RF circuit, so the easiest thing to do is to remove the DC blocking capacitor. Next, take a short section of RG174 cable with a SMA connector on one end and strip back the other end to expose the shield and center conductor. You will also need a complete SMA RG-174 cable with SMA connectors on both ends for calibration.

Solder the shield of the test cable to a ground point very close to the antenna, filter, or whatever you are testing. Solder the center conductor to the pad of the DC block you just removed so that the cable is now electrically connected to the DUT.

Now, connect the other cable (the one we are using for calibration) to one of the ports of the VNA; Lets use port 1 for this example. Using the instructions for the VNA, perform a 1-port calibration on port 1. THis will involved you connected calibration standard loads to the end of the cable; there will be a 50-ohm, a short, and an open standard. Once complete, the VNA is now calibrated, including the length of cable you are using with the DUT.

You should now be able to connect the cable soldered to the DUT to port 1 of the VNA and perform your measurements.

You can do the same trick with 2 cables if you need to do 2 port analysis for filters.

Please post any questions you have if this isn't clear enough.

- Steve Montgomery
The RF Troubleshooter
www.rftroubleshooter.com

 Post subject:

Posted: Fri May 18, 2007 9:18 am 

Hi!
I have some questions. Can I similarly to do measurement of a dipole antenna and input resistance of a chip? And can I use other 50-Ohm cable (for example, RG-58 )?
Thanks in advance!

 Post subject:

Posted: Fri May 18, 2007 11:02 am 

You should be able to mesure dipole antennas that way.

Testing the input resistance of a chip can be a little trickier. Most RFIC receiver inputs are differential and you need a balun to make them single ended. The balun needs to be arranged such that it also acts as a DC block. Usually, this balun is designed to match the input of the RFIC to a 50-ohm source (like an LNA or SAW filter). So what you are really testing is the match of the balun. Does that make sense?

You can use RG-58, but it is very thick and rigid compared to RF-174 and will be difficult to manage if it is soldered directly to a PCB.

And now a shameless plug. You should check out my website from time to time. I just redid it and it now contains an "Ask The Guru" forum where you can ask questions like this and it will contain a bunch of new articles that I am writing. Look at my sig for the address.

_________________
Steve Montgomery
http://www.rftroubleshooter.com

 Post subject:

Posted: Mon May 21, 2007 6:51 am 

Thank you for the answer. I necessarily will visit your website, when I will have free time. In this topic I have else one question. Why it's possible, to measure the dipole antenna without a balun? If I am not mistaken, a dipole antenna has a balanced input.

 Post subject:

Posted: Mon May 21, 2007 10:38 am 

You are right. Technically a dipole antenna is balanced. And the proper way to feed it is through a balun. However, a lot of people just ground one of the poles and drive it as a single-ended load. You loose a few dB performance, but it is much easier to deal with.

So you want to test it the same way you are going to drive it. If you are using a balun in your circuit, then use one when you test it. If you are driving it as a single ended load, don't use a balun.

When you said dipole in your last post, I had a standard monopole antenna in mind. So many people refer to those as dipoles. Sorry for the confusion.

_________________
Steve Montgomery
http://www.rftroubleshooter.com