how to accurate know the length of the transmission line?

[liletian]

Hi Guys
I designed a half wavelength transmission line, I would like to know the accurate length of the transmission line. I planned to connect the transmission line to a 200 Ohms resistor at the end on the PCB. Measure the S parameter through network analysizer and the calculate the length of the transmission line accurately.
I have a few questions here:
I connect the resistor directly to the end of the transmission line, is it correct? Will there be any losses significent which will affect the measurement?

Second, what will be the accuracy of the PCB fabrication, can they maintain a fabrication accuracy of 1mm?

3. What will be the length of half wavelength the transmission line at 2GHz with PCB dieelectrical constant of 4?
Thanks a lot

 

[FvM]

1. The length can be determined more easily from a resonance measurement of a shortened line with loose coupling of the open end, I guess. With the S11 measurement, the calibration accuracy affects the result. I don't excatly understand the purpose of the 200 ohm termination for your measurement.
2. 1 mm to ??
3. Depends on the exact layout

P.S.: This post should be better appended to your similar previous thread, I think.

 

[liletian]

Yes, I agree with you on the 200 Ohms resistor here.

why it should be a shortened line? You mean short the end to measure S parameter? Will a open end better suitable for microstrip line?

1. The length can be determined more easily from a resonance measurement of a shortened line with loose coupling of the open end, I guess. With the S11 measurement, the calibration accuracy affects the result. I don't excatly understand the purpose of the 200 ohm termination for your measurement.

Yes, I agree with you on the 200 Ohms resistor here.

why it should be a shortened line? You mean short the end to measure S parameter? Will a open end better suitable for microstrip line?


2. 1 mm to ??
I mean 1mm round error. For example. 2 cm, it actually value will be 20+- 1mm
3. Depends on the exact layout

Yes, I know. Is there a software I can use to calculate for microstrip line?

thanks a lot
P.S.: This post should be better appended to your similar previous thread, I think.

 

[FvM]

why it should be a shortened line?

The basic idea is to measure a resonator. Both open and short will introduce a certain length deviation compared to the continuous transmission line. So strictly spoken, the best setup depends on the intended use of the transmission line and your measurement capability. Because the propagation speed will be also frequency dependent, the most exact measurement is a S21 difference measurement with PCB transmission lines of different length. This way, all errors introduced by the coax to PCB transitions and also a VNA phase offset can be cancelled out.

 

[liletian]

I do not really get you. The microstrip line( I will use microstrip line) length is fixed. (sorry, it should be microstrip line). I can measure open end to recalculate the length. I agreed the length will not be exactly same as I connected the line to the transistor. But should be close, right?
Also, since the mirostrip length is fixed. i can not measure the S21 of difference length of microstrip line. Any other suggestions?

Thank you for your very helpful discussion.

The basic idea is to measure a resonator. Both open and short will introduce a certain length deviation compared to the continuous transmission line. So strictly spoken, the best setup depends on the intended use of the transmission line and your measurement capability. Because the propagation speed will be also frequency dependent, the most exact measurement is a S21 difference measurement with PCB transmission lines of different length. This way, all errors introduced by the coax to PCB transitions and also a VNA phase offset can be cancelled out.

 

[FvM]

I was describing a method to determine most exactly the properties of a specified transmission line geometry on a particular substrate. It involves making test structures of different length.

I didn't yet understand how the transmission line is terminated in your design, so I can't know which measurement method will introduce the smallest error. If it's a resonator application, you would prefer a similar termination (in terms of open/short) for the measurement.

You have also been asking for the expectable fabrication accuracy. I guess, that the largest error contribution will come from the Er variation of usual substrates.