How to Use a Network Analyzer for Cable Testing

安泰测试设备

How to Use a Network Analyzer for Cable Testing [Copy link]

This application tests cables other than the nominal 50Ω, including coaxial, twisted pair, and differential high-speed data lines, including impedance parameters, S parameters (insertion loss, standing wave, Smith diagram, etc.), and can also draw eye diagrams.

According to the performance of the cable, such as frequency range, length, and whether it is differential, set the time domain gating, and you can gate according to the location of the cable connection to obtain the various parameter results of the actual physical cable. The gated gating test results correspond to the tested cable, excluding connectors, fixtures, and other test cables.

Necessity and difficulty:

Since the cable under test is not a 50Ω standard coaxial cable, it may be a high-speed data line, differential line, etc. When testing with a network analyzer, the test port is a standard 50Ω coaxial cable, so an adapter or fixture is required when connecting the cable under test. The S parameters of these connectors and fixtures are unknown, and their influence needs to be removed to obtain the actual parameters of the cable under test.

The commonly used methods at present are de-embedding or fixture removal methods, which require precisely designed fixtures and microstrip calibration parts, and remove the S parameters of the fixture after calibration. The difficulty lies in the production of the fixture and its calibration parts. Usually, the parameters of the calibration parts are theoretical design values, which have a certain gap with the actual values. In addition, the calibration and obtained S parameters of the fixture itself may cause fluctuations in the measured data curve, or even errors.

This method adopts a universal calibration method and locates the cable under test through the time domain gating function. The measured data automatically excludes the influence of connectors and fixtures.

Test principle:

Time domain analysis is a function option of vector network analyzer. In time domain analysis, the measured value is a function of time. For transmission in a homogeneous medium, the time axis is equivalent to the distance axis. In theory, any measured value such as impedance Z, admittance Y or S parameters can be characterized in the time domain using impulse response or step response.

The vector network analyzer obtains the time domain test parameters through the data results of the frequency domain analysis parameters through FFT inverse transformation, filtering and windowing. The horizontal axis is the time axis, and the pulse response or step response is analyzed. For transmission in a uniform medium, the time axis is equivalent to the distance axis. Fast Fourier (FFT) forward and inverse transform is the basis for the vector network analyzer to achieve time domain analysis. When using a vector network analyzer for time domain analysis, it is necessary to define the fuzzy distance based on the electrical length L of the device under test, thereby defining the frequency interval Δf; it is necessary to define the electrical length resolution (time interval resolution) based on the requirements, thereby defining the frequency width SPAN. Maximum unambiguous distance time (length) resolution Note that the single-ended ( S11) test distance and time, the signal round trip, is twice that of the double-ended unidirectional transmission (S21). If the electrical length of the cable under test is less than twice the maximum unambiguous distance, the end of the single-ended (S11) test of the connecting cable must be connected to a matching load, otherwise an open or short circuit at the end will generate an ambiguous signal within the test range.

Use the "time gate" to select a specific part of the impulse response and suppress the rest. The time gate is selected and configured in the time domain analysis state. The time segment selected by the time gate corresponds to a certain position in the test channel. In the time domain analysis mode, select the time gate corresponding to the cable connection monitoring position point. After opening the time gate, switch to the frequency domain for monitoring.

Test method:

The DUT is a cable, which can be any impedance within 10Ω~1kΩ; the tested cable is welded with a 50Ω coaxial connector, such as SMA, N; if the DUT is a differential cable, each cable pair is welded with two pairs of 50Ω coaxial connectors, and the outer shell conductors of each pair of connectors are interconnected and connected to the DUT shielding layer. Calibrate before testing. The test window is divided into two channels:
Ch1 cable insertion loss: S21 test, using a bandpass impulse response, gated at the first peak, and containing at least one pair of time domain side lobes;

Ch2 Cable Impedance: Z11 test, low-pass step response, gated, including 50%~80% of the DUT length.

Test verification:

DUT: 75Ω coaxial, frequency range 3GHz, length 914mm S21 insertion loss initial test results:

S21 time domain gating settings:

S21 insertion loss, gated selection correction test results:

Z11 impedance test results:

This is the end of this article on how to use a network analyzer for cable testing. For more instrument knowledge, please learn about Xi'an Antai Testing.