How to measure the characteristic impedance is 50ohm or not?

[yellowtooth]

Recently, I met a problem that the return loss is not as good as expected.

Maybe the connector on our PCB is not 50ohm.

One port of the connector is N type, the other port is not standard type, something like coaxial cable.

And I cannot connect two connectors together to form a two N type ended connector.

So I can only connect one of them to the vector network analyzer's prot1 to measure S11 or Zin. While the other port is just let open ended.

I wonder how to know if it has 50ohm characteristic impedance.

Please tell me if you know.


thank U.

 

[VSMVDD]

Re: How to measure the characteristic impedance is 50ohm or

plugs and sockets are provided you fitted them right
50 or 75 ohms
1-1 or 1-1.5 ratio of interaction
with the feeder
around 1db insertion loss per connector
chances are its the coax thats the problem

use heliax and proper ntype for your unit

make your own patch cable to the analyzer

this is best to be shure

also use silver solder

make sure of analyzers needs
if its 75R inputs use a balun

to test if the impedance is matched between two units use a vswr meter
if the standing wave ration is 1-1.5 between your unit and the analyzer
then the input maybe is 75 R

and 50 R from your unit as expected
hence the missmatch you observe

so try that

youll need several good quality vswr meters

to get the whole system balanced and usable for testing
http://www.irational.org/sic/radio/tech.html#VSWR

 

[Gakusei]

Re: How to measure the characteristic impedance is 50ohm or

The easiest way to find the characteristic impedance is to calculate it.

First measure the diameter of the outside of the inner conductor and the inside of the outer conductor. Be carefull to do this in the right place, as there are steps in the diameters to allow the connectors to fit together. You can now calculate the impedance using the following equation:

Zo = 138.1 x Log10 (Outer Dia/Inner Dia)

The impedance should be either 50 or 75 ohms. If you get a different answer, you probably measured the diameters in the wrong place.

If you can post a picture of the connector, I can probably tell you what type it is.

 

[VSMVDD]

Re: How to measure the characteristic impedance is 50ohm or

99.9 % of analyzers use 75 R inputs

50 to 75 R = 1-1.5 ratio balun is needed

or the analyzer will missmatch

so... all the maths and i bet from a picture
you cant tell a 50 R bnc from a 75 R bnc becouse
i cant...

its a measure of the diameter of the center spike and the material the dialectric is made of in the plug only {all very strange stuff but true}

so its prob a 75R input and 50 R unit under test senario so use vswr meter to check 1-1 match

 

[nycheung]

Re: How to measure the characteristic impedance is 50ohm or

Could you explain more about your 2 port device? Is it just a 50ohm microstrip-line?

If you guess the mismatch is caused by the connector, you can simply connector it to another known-good 50 device and measure the S11 again.

ny

 

[xxargs]

Re: How to measure the characteristic impedance is 50ohm or

99.9 % of analyzers use 75 R inputs

ehh, you mean 50 Ohm, I newer seem VNA with 75 Ohm impedance...

you must have very rare VNA from TV/FM-radio field if you have 75 Ohm inputs...

50 to 75 R = 1-1.5 ratio balun is needed

or the analyzer will missmatch

so... all the maths and i bet from a picture
you cant tell a 50 R bnc from a 75 R bnc becouse
i cant...

75 Ohm BNC have smaller center pin compare to 50 Ohm, and if you put 50 Ohm male centerpin in 75 Ohm female centerpin one time - connectors is ruined...

but serious - NEVER USE BNC in network analyzer work, connectors is not good in VSWR/retur loss-stability - Use N-connectors or SMA.

its a measure of the diameter of the center spike and the material the dialectric is made of in the plug only {all very strange stuff but true}

so its prob a 75R input and 50 R unit under test senario so use vswr meter to check 1-1 match

so VNA as VSWR-meter is not good enough ?? ;-)

---

you can measure objects (and connector) with different impedanse to make S-parameters on each unit and in simulator coupling togheter with rigth impedances on port and make measures.

For example i measured qrystal filters (origin for 1 Kohm//5pF impedances) to S-parameters (and filter looks very bad in 50 Ohm measure enviroment) and in simulator and rigth port impedances (1 kOhm // 5 pF) sems OK and in simulator find out this filters _need_ 5 pF paralell reactive impedanses on port to working properly.

 

[VSMVDD]

Re: How to measure the characteristic impedance is 50ohm or

dont try to pick an argement man

just solve the guys problems and stay chilled
take a powder and sit on your hands for a while


the problem has nothing to do with his setup analyzers etc

and something you all missed
is you cannot leave open ended connections and gain a match

you must add a dummy load plug to any open ended connections

 

[yellowtooth]

Re: How to measure the characteristic impedance is 50ohm or

Thank U all !

I will try to find some pictrue and upload it.

Added after 6 minutes:

dont try to pick an argement man

just solve the guys problems and stay chilled
take a powder and sit on your hands for a while


the problem has nothing to do with his setup analyzers etc

and something you all missed
is you cannot leave open ended connections and gain a match

you must add a dummy load plug to any open ended connections

In this case, the connector is very special, and I have no other stuff to plug in it.

So, what I can do is just left it open ended.

 

[yellowtooth]

Re: How to measure the characteristic impedance is 50ohm or

now I upload the picture of the connector

 

[yellowtooth]

Re: How to measure the characteristic impedance is 50ohm or

It seems that there is a network analyzer has a funcion called "TIME DOMAIN OPTION"


Maybe it can test S11 of any point in the DUT.


So, if some where, S11 is bad, then we can say in this point, the impedance is not continuous, i.e. not 50ohm.

 

[Gakusei]

Re: How to measure the characteristic impedance is 50ohm or

I can't tell from the pictures what the connector type is, but it looks from the inneruter ratio that it is 50Ω. What does this fixture do and what normaly connects to the outputs? you may find that the equipement that connects to the outputs is a reasonable match, which will make it possible for you to make a measurement of the complete system.

A time domain measurement may well give you a good idea of the performance of the fixture, but if the unit is resonant (a filter for instance) it won't work.

BTW You can get 75Ω VNA's, but the only time I've ever seen one was when I hired one by mistake! DOH!

 

[yellowtooth]

Re: How to measure the characteristic impedance is 50ohm or

Thank U for your reply.

We put a PCB (circuit board) on the fixture , and use a screw to make the board tightly connected to fixture.

The RF signal just go through the screw.

Under the screw, there is a hole which act as a outer conductor, and a bar which act as a inner conductor, just like a coaxial construction.


What I want is to confirm the coaxial construction is 50ohm.

I also know that to left it open ended is not a good way, but what can I do?

Just solder a 50ohm resistance to it?

 

[nycheung]

Re: How to measure the characteristic impedance is 50ohm or

I think you can solder a 50ohm resistor across the +ve and gnd, but you need to make sure it is really 50ohm at your test frequency.

 

[Gakusei]

Re: How to measure the characteristic impedance is 50ohm or

A normal 50Ω resister will be too reactive above a MHz or two. You won't be able to get any meaningfull measurement that way. Could you find a type'N' connector which would fit on your board?

Can you tell me what this board is supposed to do?

Added after 6 minutes:

Can you post a photograph of the connector?

 

[yellowtooth]

Re: How to measure the characteristic impedance is 50ohm or

the figures are just the JPG files uploaded above.

the inner bar is about 5mm (diameter), the outer is about 11.5mm (diameter).

So it is not same as the N type 50ohm load's diameter.

 

[Gakusei]

Re: How to measure the characteristic impedance is 50ohm or

If you put those two numbers into the equation I gave you earlier, you get an answer of 49.95Ω. The inner will look bigger than a type 'N' male connector, because the end of the inner is stepped down to fit into the female connector.

 

[yellowtooth]

Re: How to measure the characteristic impedance is 50ohm or

It seems the only way is to calculate the characteristic impedance by formula.

Fortunately, the length of the bar is about λ/8 for the frequency concerned.

So, when I just leave it open ended, the input impedance (Zin) is just -jZ0.

And I can just measure the Zin to calculate Zo what I want the most.




Any how, thank all of U for the reply.

 

[brumi]

Re: How to measure the characteristic impedance is 50ohm or

Before all you need to repalce all yours connectors and cable to 50 Ohm.
When you do that you start with with a very good matching system.
After that you can test the swr and looses.
Always start with a know system.
Never use unknow connectors or cable.
This are very simple rules but very effective.