Learn about vector network analysis Smith chart and filter bandwidth measurement!

博宇讯铭

Learn about vector network analysis Smith chart and filter bandwidth measurement! [Copy link]

Vector network analyzer is a magical measuring instrument, its function is very powerful and worth exploring. Today, Boyu Xunming engineers will briefly introduce the Smith chart in the vector network analyzer and the bandwidth measurement of the filter, hoping to make more people understand and be interested in it.
In order to better test and explain, Boyu Xunming engineers specially used Dingyang SSA1000X vector network analyzer for explanation. Let's take a look at how it is measured. Vector network analyzer
Smith chart
First, open the vector network function under Mode, and open the Smith chart under meassetup.
The Smith chart is a tool for mapping complex reflection coefficients to the impedance of the test equipment. In the Smith chart, the straight impedance plane is reshaped to form a circular grid, from which resistance and reactance (R+jX) can be read.

Each point on the Smith chart represents a complex impedance (r±jx) consisting of a real resistance (r) and an imaginary reactance (x).
The horizontal axis (solid line) represents the real part of the difference between impedance and resistance. The center of the horizontal axis always represents the system impedance. The rightmost value is infinite ohms (open circuit). The leftmost value is zero ohms (short circuit). The
dashed circle intersecting the horizontal axis represents constant reactance.
The dashed arc tangent to the horizontal axis represents constant impedance.
The upper half of the Smith chart is the area where the reactance component is positive and thus produces inductance.
The lower half is the area where the reactance component is negative and thus produces capacitance.
Not only can you see the changes in the Smith chart, you can also measure the bandwidth of the filter. Why did the engineer use the SSA1000X vector network analyzer for testing this time? It is because it has a tracking source function. It can form a single-port vector network analyzer. Today we will use the vector network function of this instrument to measure the bandwidth of the filter:
first prepare a filter (the frequency range and bandwidth of the filter bandwidth can be introduced)
then we turn on the vector network function of SSA1000x, select MODE--vector network analysis, before measuring, we first do a straight-through calibration (normalization), if you have a calibration piece, you can do an enhanced response calibration, so as to measure the standing wave and impedance characteristics of the input port.
Let's see how Boyu Xunming first does a straight-through calibration to measure the filter:
Calibration process: set the start frequency and end frequency, output power 0dbm, use the RF cable to connect the TGSOURCE and rfinput ports;
select the calibration menu---normalization (wait for the calibration to complete); disconnect the cable at RFinput, connect the filter and
select meas-s21.
At this time, if the waveform display is incomplete, you can adjust the resolution of each grid through the amplitude menu scale-, and finally use the Ndb bandwidth marked under MARKFN to measure the filter bandwidth (N is 3)

BW: indicates filter bandwidth
Cent: indicates center frequency
Q: indicates quality factor (center frequency to bandwidth)

吾妻思萌

I learned a very good article. The real and imaginary impedance response under frequency is really profound.