How to know the PCB layout trace length by VNA Polar diagram ?
http://www.av.it.pt/medidas/data/Man...rt_example.gif
There is a polar diagram in VNA(Vector Network Analyzer)
We are able to know the PCB layout trace length by phase variation.
My questions are :
1. How to operate ? I have found several VNA handbooks, but I have not found the
operation process.
2. What?s the detailed principle ? Why can we know the PCB layout trace length by phase variation?
Where can I find these information ? Such as books, application Note, data sheet, or the key words that I should google ?
Thanks a lot~!
VNA measure amplitude and phase assuming 50 ohm source unless made for cable TV then 75 ohm.
Depending on dielectric constant and wave velocity, length will cause a fixed delay and their may be parasitic L, C
THe trace width to gap ratio is tuned to 50 Ohm by design and S11 determines tolerance errors on dielectric constant assumed as well as etchback factor on trace and finished copper mean gap.
The phase shift significantly increases with frequency in linear fashion as starting around 10% of wavelength, as dφ/dt =f and thus significant points are 1/4λ & 1/2λ where phase is 90 & 180 deg where impedance goes from conjugate if reactive to matched.
Thus assuming the dielectric constant was known, wave velocity and f are known and thus time delay can be estimated from length or visa versa using the 90 or 180 deg wavelengths. Often dielectrics have a 10% minimum tolerance unless TDR S100 tests are specified (est.) from board shop.
If the Characteristic Impedance of this line is 50 Ohm ( I presume that VNA has 50 Ohm Zo) there is something wrong.
A lossy cable ( or trace whatever ) should draw a spiral rather than circles on S21 Vectorial Plot Diagram.
Looks like an almost lossless transmission line + a fixed (6 dB) attenuator, with loosely coupled resonator (which shows up as a small extra circle).
Without knowing the test setup and VNA calibration, everything is rather vague.
A simple way to measure a transmission line's electrical length is to turn the port extension knob until the circles shrink to a small line and read the number.