Control Impedance of 50 ohm for PCB

[TXRX]

Hi,

I understand from the PCB manufacturer that he measure the impedance 50ohm with TDR (Pulse), he measure the propogation delay of the pulse and calculate the impedance.

Is this impedance will be the same if i test it with VNA at frquency of 5GHz ?

 

[biff44]

Measuring the propagation delay of a TDR pulse will not tell you anything about if the line is 50 ohm impedance or not. So the answer to your question is NO.

If he sends a TDR pulse down the line, and then looks for reflections (at the start of the line, further down the line, etc) and measures the MAGNITUDE of those reflections, then that will tell you if it is a 50 ohm line (the first big reflection) and if there are other problems (like reflections from via holes, bends, etc) further down the line.

Small TDR reflection amplitudes will also show up as a good return loss vs frequency on a VNA.

 

[TXRX]

Hi,

So how the PCB manufacturers measure the 50 ohm impedance without the VNA ? (control impedance )

 

[jiripolivka]

A simple reflectometer is all you need, and a good 50-Ohm termination. For testing, 50-Ohm connectors are used on microstrip line ends.
The function of such reflectometer is included in a VNA, and TDR can be used for longer lines, to indicate impedance deviation from that of the connectors and external 50-Ohm lines

 

[TXRX]

Which is more accurate for measuring impedance at 5GHz: the TDR or VNA ?

 

[jiripolivka]

Which is more accurate for measuring impedance at 5GHz: the TDR or VNA ?

Both methods can indicate impedance deviation from 50 Ohms. Accuracy depends on calibration.

 

[TXRX]

The TDR not measure the impedance at 5GHz, The VNA do it. How can you say that the TDR is accurate ?

 

[jiripolivka]

The TDR not measure the impedance at 5GHz, The VNA do it. How can you say that the TDR is accurate ?

Learn something about TDR. Pulse spectrum can extend to >5 GHz in fact.

 

[TXRX]

Theoreticaly you right - but i am not sure that the PCB manufacturer can reach to 5GHz with their equipment

 

[FvM]

TDR is able to determine trace impedance in the dimension range of PCB with standard test structures. From the PCB manufacturers viewpoint, impedance control is a single parameter optimization matter, there's no option to control e.g. impedance at 1 GHz and 5 GHz selectively.

Measuring impedance frequency selective may be interesting for substrate characterization which is beyond PCB manufacturer's job. It's expected that the designer has chosen a substrate with sufficient RF performance. TDR measurement with high time resolution or equivalent frequency domain measurements can be however meaningful for the designer.

 

[TXRX]

Hi FVM,

Do you think that i can rely on the impedance measurement with TDR from the manufacturer when the board frequency operation is 5GHz?

I use FR-4, I can use substrates with stabilize Er - But the price is big issue.

 

[FvM]

You are asking about FR-4 performance, not measurement reliability. The main drawback of FR-4 is the high microwave loss factor, Permittivity is said to be fairly constant, however. If you feel a need to apply a systematic correction for 5 GHz versus 1 GHz permittivity, you should do it. But I think it's below the fabrication tolerances of impedance controlled PCBs and can be ignored.

Here'a previous Edaboard thread containing some information.
https://www.edaboard.com/threads/139592/

 

[TXRX]

Hi FVM,

The loss factor is high but i can live with it, my problem is the behaviour of Er at frequency which effect on the impedance.

What is the way that the PCB manufacturer can test the 50 ohm impedance at 5GHz ? i check with PCB manufacturers and they have not a VNA.

 

[FvM]

I think I have explained this before. A PCB manufacturer won't measure at 5 GHz and don't use a VNA. Strictly speaking, a VNA doesn't measure transmission line impedance directly. A VNA with time domain analysis option could possibly do a windowed measurement.

Transmission line impedance is understood as a wide band respectively time domain parameter in PCB design and manufacturing. I already mentioned the option to use a systematic correction factor for typical FR-4 dispersion in your design.

 

[Azulykit]

Cost vs. Performance. That is the hardest trade, particularly when using a potentially marginal material. It has been my experience that FR-4 starts to suffer significant loss at these frequencies and above. There is also a significant (higher) price difference between the "microwave" materials and FR-4. Your application will have to dictate your choice.

Good. Fast. Cheap. .......pick two!

I concur that often PCB manufacturers have trouble making measurements in the microwave and also mm bands. The same is true of some of the modeling tools. Good performance above 2 GHz is usually expensive.

 

[TXRX]

The loss is not a issue at 5GHz.

I intend to put a coupun to measure the Er, see attached - i will use "The two microstrip-Line Method", What do you think ?