How to measure 50-ohm transmission line by VNA

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myem

Joined: 12 Dec 2003
Posts: 137

14 Apr 2004 15:31 vna display s11 s22 smith

I want to measure 50-ohm microstrip line by VNA.
Because VNA just can measure s-parameter.
How can I measure it ?
If I get the S-parameter from VNA , how can transfer it into Z0 ??

plasma

Joined: 20 Dec 2001
Posts: 577
Helped: 40

15 Apr 2004 0:15 vna smith chart s11

one of the VNA feature is the "SMITH CHART"
so push the smith button and you will get the impedance
of your microstrip line on your VNA.

StoppTidigare

Joined: 16 Jul 2002
Posts: 156
Helped: 1
Location: Northern earth

15 Apr 2004 0:57 how to measure 50 ohm line impedance

Hi I've never seen a VNA in real life but I've read about the animal Start with an open circuit. and measure S11. At lenght l from the opencircuit the impedance will be Zoc=-j*R0*cot(beta*l).

The impedance at length l from the open circuit is
S11 corresponds to a certain impedance S11_oc=Zoc-Z0/(Zoc+Z0), solve for Zoc.

replace the opencircuit with a short, measure S11. Solve for Zsc
Zsc at length l will vary as j*R0*tan(beta*l)

As you can see from the mathematical relations the characteristic impedance (real) R0 will be found as sqrt(Zoc*Zsc)

Kindest regards,
StoppTidigare

lguancho

Joined: 07 Jan 2004
Posts: 266
Helped: 5
Location: Singapore

16 Apr 2004 7:59 how to make 50 ohm transmission line

Hi, if you just want to see the performance of your 50 ohm transmission line (presumably, you have a jig with suitable connectors) with VNA, just do a full 2port calibration across your frequency band of interest. After calibration, you can see the insertion loss and return loss.

If you have done the design properly, you should get good return loss ( better than -15dB) at port 1 and port 2, that is, the line is well matched to the 50 charactersitic imped of VNA. If you display the VNA in smith complex chart format, you should see your S11, S22 plots centered at middle of the dsplay, at your frequency band.

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flyhigh

Joined: 12 Nov 2003
Posts: 561
Helped: 32
Location: somewhere over the rainbow

22 Apr 2004 9:28 s parameter 50-ohm transmission line

Hi,

besides everything being said so far, just make sure your 50Ohm calibration standard is better, i.e. has lower reflection than the 50 Ohm line you are measuring. It is because the reflection standard provided with some callibration kits is not great at all frequencies, it varies and should be better than -30dB for all of the freq range you are interested in. You cannot measure reflection beyond that standard more accurately (and you might not need it accurately as well so this should not be the limitation in general).

flyhigh

mozee

Joined: 25 Apr 2004
Posts: 11

25 Apr 2004 2:43 50 ohm transmission line

complicated answers to a simple question!!! after calibrating your cables and connectors, connect your sample and measure S11 [or S22 if its symmetrical], press the appropriate button to get the Smith Chart. The center of the Smith Chart is typically 50 Ohms-- see where your curve falls-- good VNA's will allow a marker that tells you the impedance [S,Z,Y] directly.