Testing RF Waveguides: Using VNA for Freq, Power & Avg

[faheem rana]

Hi there

i am trying to test RF Waveguides and testing them.

Something does not look right to me. i am using Freq range, data points, power and avg option

is there anything special we use on VNA for testing Waveguides.

Regards,
Faheem

 

[f95toli]

I think you need to describe your setup and problem in more detail.

A VNA is not really a complicated device although setting them up can be a bit tricky until you get used to it. Measuring a 50 ohm waveguide should be straightforward.
How are you connecting the waveguide to the VNA (an adapter I presume)? And how is the VNA calibrated?

 

[faheem rana]

Hi there,

Thanks for your reply

We are using these setting

· Start: 14.0GHz
· Stop: 14.5GHz.
· Number of points : 1601
· Average: On
· Averaging: 16 times
· IFBW 70kHz
· Power: -5dBm
· Calibration: Full 2 ports
Power during calibration: -5dBm
Averaging: On

I am ok using VNA and yes it is 50 OHM WG.
We are connecting WG to VNA using adaptors "G4603 UBR140 adaptor, 11.9 to 18GHz"
VNA is calibrated uisng k type connector cal kit as i normally use for all RF test. I think i need to do some setting on VNA but i am struggling with it.
Is there any special settings for testing WG?Regards,
Faheem

 

[f95toli]

No there are no special setting. The only time you need to tell a VNA to do something "special" is if you are measuring devices such as mixers or perhaps using a more complicated calibration scheme (de-embedding etc).. The only thing I would do is to reduce the IF BW to something lower (say a 1kHz or so); just in case the losses are high for some reason. You can always temporarily reduce the number of points a bit to speed up the measurement.
Measuring a 50 ohm waveguide should be no different than measuring a piece of 50 ohm coax.

Have you tried just replacing a waveguide with another 50 ohm line? I.e. just a piece of coax with k-connectors. This will tell you if something has gone wrong when you e.g. calibrated the VNA,

 

[faheem rana]

Hi there,

I have tried using low IFBW but insertion loss is still 1.5 dB out of spec.

I have checked my calibration many times and it is fine.

Need to learn some magic :(

Thanks for your hellp

Regards,

Faheem

 

[davenn]

I am ok using VNA and yes it is 50 OHM WG.
We are connecting WG to VNA using adaptors "G4603 UBR140 adaptor, 11.9 to 18GHz"
VNA is calibrated uisng k type connector cal kit as i normally use for all RF test. I think i need to do some setting on VNA but i am struggling with it.
Is there any special settings for testing WG?

waveguide usually ISNT 50 Ohms, usually much higher
As a rule of thumb, waveguide wave impedance is approximately 500 ohms for standard rectangular waveguide.
I have never seen 50 Ohm WG I doubt that it exists

you told us almost everything except the important part
what is the WG designation type

 

[the_emi_guy]

Hi there,

I have tried using low IFBW but insertion loss is still 1.5 dB out of spec.

I have checked my calibration many times and it is fine.

Need to learn some magic :(

Thanks for your hellp

Regards,

Faheem

Are you mating the waveguide to coaxial adapters directly to the VNA or through cables?

 

[f95toli]

waveguide usually ISNT 50 Ohms, usually much higher
As a rule of thumb, waveguide wave impedance is approximately 500 ohms for standard rectangular waveguide.
I have never seen 50 Ohm WG I doubt that it exists

I'd assumed the OP refers to something like a coplanar waveguide which are generally 50 ohm..
But if we are actually talking about a rectangular waveguide things get much more complicated, especially if the only a standard calibration kit is available (as opposed to a calibration kit for that type of waveguide). In that case you would presumably need to do some type of de-embedding which I mentioned above which -depending on the type of VNA used- could be quite tricky.
I must admit I wouldn't even know where to start in that case. You would at the very least need the Touchstone files for the adapters (unless they are in the VNA software).

 

[davenn]

I'd assumed the OP refers to something like a coplanar waveguide which are generally 50 ohm..

who knows !

well actually, when you look at the adaptors he listed, its referring to 50 Ohm coax to standard rectangular WG ... so definitely not coplanar WG which is more related to striplines

I think he misunderstands the function of the adaptors and how they transform 50 Ohm coax to WG transmission lines

hence why I asked for the WG type info