How to measure characteristic impedance of transmission lines on a real RF PCB?

[Alice Nguyen]

Hi all,

I've got a trouble in measuring real characteristic impedance of very small transmission line on a real RF PCB.

The problem is the transmission lines are printed on multi-layer PCB and very small (approximation of serveral mil). And I need to measure characteristic impedance of these transmission lines for accurate impedance matching. In addition, I have got a Network Analyzer. The cable is coax, and SMA connector. So Which cable and connectors should I need in my case?

May anyone help me? Thanks a lot.

 

[FvM]

Suitable test methods are specified in IPC-D-317A and IPC-2141. They are based on TDR-measurements.

 

[Alice Nguyen]

Thanks FvM for your reply

I've downloaded IPC-D-317A and IPC-2141 as well as TDR-measurements. Is IPC is a standardization for TDR measurements?

 

[Alice Nguyen]

I've read about TDR methods.

However, I do not have TDR systems. What I have is VNA (Vector Network Analyzer). So what connectors or probes should I need to measure characteristic impedance of lines on PCB? And how to measure?

 

[FvM]

Connectors can be the same as in TDR setup discussed in IPC-2141 (SMA). You have a transmission line of unknown impedance and unknown electrical length. By terminating it with a variable resistor as in the IPC setup, you can cancel the reflection at the line end and get a single, roughly frequency independent reflection at the line start. If you calibrate the VNA reference plane to this location, you can measure the line impedance directly. Minimal s11 ripple indicates matching at the line end.

Another option is transform the frequency domain s11 data to time domain by a FFT.