Hi All,
I have a balanced 50 ohm 915MHz filter on a PCB that I need to test. Would it work OK if I simply shunted one leg of the balanced filter's input and output with a 50 ohm resistor to ground, while sweeping the other balanced input and output with a VNA? Is this the easiest way to test a balanced circuit for gain and return loss with a typical VNA? (I have no room at all to place 1:1 baluns on my PCB for the (unbalanced) 50 ohm VNA).
Thanks!
-Bill
If you put 50 ohms to ground on one leg and drive the other (listen to the other) with a 50 ohm generator (receiver), the filter will be terminated in 100 ohms. That's not what you want, is it? Why not just put the baluns external to the filter? Use twisted-pair enameled wire to connect from the balanced balun output to the filter. If you make it short enough, it won't matter much if it's not exactly 50 ohms, but you should be able to get pretty close to 50 if you use the right wire size. In any event, if you calibrate the VNA at the ends of the twisted pair, all should be OK. (Calibrate using 50 ohm/49.9 ohm SMT resistors---I've had good luck with 0805 size, but 0603 or 0402 if you can deal with it will work too. Short and open and conneting one port to the other should be obvious. You can check how well your baluns are doing their jobs by reversing the leads at the junction; that should ideally give you just a phase inversion.)
If the baluns are working well, you won't be able to tell how well the filter is balanced just using them to connect to the filter input and output ports. You'll need a different test for that.
Cheers, Tom
Thanks Tom -- much appreciated!
Best Regards,
-Bill
Tom Bruhns wrote:
Have a look on Agilent's website. I believe that the "proper" method is to put a connector on every trace going into or out of the device under test, and then to measure in turn between every possible pair of two ports, whilst all other ports are terminated in very good 50 Ohm loads. Then all of these separate traces are put into some software and it will give you the answers you need. The whole process is so long and complicated that instead of doing that, some people go and buy / hire / "borrow for evaluation, because we're thinking of buying" a 4 port VNA or if they don't really need an accurate answer then they use baluns etc.
Chris
Unless your parameters are truly difficult, just plug it in to your unbalanced (normal) VNA. Works pretty well up to 6GHz, of course terminations are paramount.
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