S parameters

[RFrag]

Hi, I am very new to RF engineering. I wanted to set up a circuit before which I was asked to check the S parameters of the DUT. Now my understanding is that I check the S parameters to understand at what frequency, the DUT will transmit maximum power and at what frequency it won't. With this I can expect the same behaviour of the DUT when connected in the circuit setup. Is that correct?

Also, when I measure the S parameters, the values vary based on the movement of the cables from the VNA to the DUT. How can I minimize that?

 

[KlausST]

Hi,

Yes. The test result is only as good as the test arrangement.

Avoid (long) cables.
Avoid their movement (by fixing them)
Avoid the cables influence at all. By using the right cable (shielded, twisted, correct characteristic impedance...) or by using buffers, differential signaling.

Some hints to find out the "quality" of your test arragement.
* Do a test with the DUT as reference (always use the same DUT)
* Do the test again (immediately) to find out precision
* Do the test on different day times
* Do the test in a different room
* Do the test on a different (identical) arrangement
* let the whole test be done by a different person.
....

The less variation you see the better your test arrangement, the more convincing are the results.

Klaus

 

[G4BCH]

Now my understanding is that I check the S parameters to understand at what frequency, the DUT will transmit maximum power and at what frequency it won't. With this I can expect the same behaviour of the DUT when connected in the circuit setup. Is that correct?

The S-parameters tell you the small signal performance of the device in a 50 ohm system at your chosen operating voltage and current and input signal power. They do not directly tell you how it will perform in your circuit, for that you must account for the input and output circuitry.

 

[D.A.(Tony)Stewart]

- start with a design spec.

- e.g Tx/Rx to 6GHz, insertion loss 0.01 resolution <0.1dB, accuracy <0.2dB or better, IP3, LO Leakage, etc etc

Minimum 4 calibration types , preferred 6 with reference R to match e.g. 50 or 75 ohms or whatever.
Only use semi-rigid coax with suitable connectors. Consider buying or making your own with SMA or better. Not all SMA are equal and use a mini torque wrench tool to protect the flash gold plating.

 

[BigBoss]

If the measurement is disturbed by cable movement, you have to use Professional Grade VNA Cables.You cannot use any 50 Ohm Cable in VNS Measurements, VNA Cables are expensive but Rigid and Consistent Behaving Cables.

GORE® VNA Microwave/RF Test Assemblies

Gore's VNA Microwave Test Assemblies provide exceptional performance for precision test applications with frequencies through 67 GHz and with constant and/or highly repetitive cable movement.

www.gore.com

There are other companies that manufacture high quality VNA Cables.

 

[volker@muehlhaus]

If the cable is sensitive touching (without moving/bending it) that could be an issue with unwanted modes. One example is symmetric antenna with coax feed, with no balun in between.

 

[D.A.(Tony)Stewart]

Semi-rigid copper tubing is relatively cheap compared to VNA grade flex coax. That's what I used for the Iridium front-end chipset ATE setup with 0.01 dB stability in normal use. If you need flex, then the last section can be very short.

I would use same for accelerometers as the capacitance modulation from vibration introduces error.