Balun for network analyzer measurement

[rvargo]

Hi, I'm trying to design a matching circuit for a 13.56MHz loop antenna. The matching circuit is differential . In an app note, they recommend a series inductor + a capacitor to ground on each side of the differential circuit.

I was planning on using a network analyzer to measure the impedance. My network analyzer is single ended, so I was going to use a PCB mounted balun to convert the differential signal to a single-ended signal.

The datasheet of the balun is attached. My questions are:
-Is using a balun the best way to do this?
-On the balun datasheet, should I be using Schematic A or Schematic B? (Schematic B has a grounded center tap)
-Any other advice?

Thanks for any help.

 

[biff44]

buy one from mini circuits

 

[mtwieg]

Hi, I'm trying to design a matching circuit for a 13.56MHz loop antenna. The matching circuit is differential . In an app note, they recommend a series inductor + a capacitor to ground on each side of the differential circuit.

Are you sure it doesn't say the opposite? A series inductor + shunt capacitor will kill the signal...

Are you sure you really need to convert it to unbalanced? The network analyzer probably doesn't care...

The datasheet of the balun is attached. My questions are:
-Is using a balun the best way to do this?

Well by definition what you're describing is a balun. A transformer isn't necessarily the best way to implement a balun though. I would try bias Ts first.

-On the balun datasheet, should I be using Schematic A or Schematic B? (Schematic B has a grounded center tap)

Either would work. Just leave the center tap floating.

 

[rvargo]

Sorry, I mis-typed. It is a series cap and a parallel cap.

Could you explain why you don't think the network analyzer would care if it is balanced? I am by no means an expert in using network analyzers. My thought was that it had to be single-ended.

 

[mtwieg]

Balanced or unbalanced shouldn't affect the impedance measured by the NA. It really only matters when you use it in order to prevent radiation from and to the matching network and cable. I may be wrong, but I don't think that will affect its measured impedance.

 

[rvargo]

So here's a part from mini-circuits. Just so I'm sure I'm hooking it up properly:

--Balanced signal would connect to the primary pins (Pins 6 and 4)
--Unbalanced signal would connect to secondary (Pin 1), and ground would connect to Pin 3

Is that correct?

 

[FvM]

I'm under the impression, that the pulse CX2041NL may be better suited for 10 MHz range than mini-circuits TCM1-1 due to it's wider frequency range. But basically each of them should work, if you apply standard open/short/load calibration at the transformer secondary. A general problem with balanced network analysis is the unknown common mode to differential coupling of the transformer in combination with the common mode impedance of the device under test. Unfortunately both are neither measured nor calibrated and a balun measurement. Therefore, if these effects are expected to be large, the differential one port measurement may be carried out as a singled ended two port measurement, calculating the differential and common mode impedances and mutual coupling from the aquired S parameters.

Measurements of a 13.56 MHz RFID coil and it's impedance matching network is howver a rather simple problem and should work with a balun.