On 7/19/22 11:44 AM, F1AMM wrote:
It puts a CW signal out at a frequency, and uses a bridge to measure the impedance (or reflection
coefficient) at that frequency by comparing the amplitude and phase of the signal from the oscillator vs whats at the device.
This part of the text (once translated into French) is not clear (understandable). Can you be more descriptive please.
There's no pulses - each measurement is made with a steady state signal.
For S11 measurements, it's like feeding a zero ohm output impedance signal generator through a 50 ohm resistor to the input of the unit under test. That forms a voltage divider.
You measure the signal generator output and the voltage (including phase) at the junction.
If the UUT is 50 ohms, then the voltage at the junction is exactly 1/2 and the phase is the same.
If the voltage isn't 1/2 or the phase difference isn't zero, then the UUT isn't 50 ohms, and you can calculate what it actually is.
That's sort of over simplified - there isn't such a thing as a perfect generator or a perfect 50 ohm resistor, and there's physical sizes involved, etc.
So what you do is measure 3 known impedances: 0, 50, and infinite.
From that you can calculate correction factors to turn raw measurements into calibrated numbers.
Furthermore, because the VNA comes from a world of microwaves, using directional couplers, the whole system is designed and the calibration algorithms are developed in terms of S parameters, which work better for those kinds of things. S parameters are defined in terms of incident and reflected waves, but that's just an alternate representation of the current and voltage at the various points.
If you were doing power engineering, you'd be working in Watts and Vars.
If you're doing low frequency systems people often work in Admittance and Impedance (e.g. Smith charts).
The S parameter heritage means that most of the papers and analytical tools is in terms of incident and reflected (a and b) waves, reflection coefficients, transmission coefficients, etc. So the calibration process tends to focus on developing ways to take your measurements of the standards and apply that mathematically to measurements of an unknown and produce calibrated S parameters, which is the de facto standard.
For instance, you'll see T parameters or ABCD chain matrices as alternate forms. I'm sure someone, somewhere, has developed calibration equations for those representations.
And there are other ways to do the measurement - A so-called 6 port analyzer requires only precise power measurements, with no phase measurement. That's handy when you need measurements for which mixers or phase detectors and such aren't available. (mm wave and terahertz, for instance)
I use the .Sp1 files to recalculate everything, which makes it possible to simulate from the frequency response of a real object (an antenna for example); for example to test an adapter, on paper (actually in Excel)
Yes, and if you want to fool with Python, scikit-rf has a whole set of tools to work with all the different representations, plot them, convert back and forth, and even do a variety of VNA calibrations.
