Pre-assembly measurements in isophase stable cable using the net-split VNA(2)

Measurement of phase match of phase stable cable
If the phase of the reflection parameter (S11) is measured at one port of the cable instead of the phase of the transmission parameter (S21), the requirement of consistency in the installation of the cable joints will be faced. Because the phase of the reflection parameter measured at the port is the result of the superposition of multiple reflection vectors, in which the reflection of the joints has a greater impact, and due to the complexity of the joint installation process (for example, the need to weld) it is difficult to ensure that the reflection characteristics of each joint is the same, in particular, the phase characteristics. Therefore, it is necessary to use the time-domain gate measurement method of the vector network analyzer to eliminate or reduce the reflection effect of the connectors. The so-called time-domain measurement method of network characteristics is to add an ideal shock function or step function signal at the input of the device under test, observe its output waveform and reflection waveform, and determine the characteristics of the device under test by its transient response. In the vector network analyzer time domain measurements are linked to frequency domain measurements by Fourier transform and Fourier inverse transform. That is, the vector network analyzer performs frequency scanning measurements to obtain frequency domain measurements, and the Fourier inverse transform to obtain time domain measurements. As shown in P1, the frequency domain display of the reflection measurement of the cable and P2, the time domain (distance domain) display of the reflection measurement of the same cable (about 30 cm long, with an open terminal) are shown.

It can be seen that the fluctuations in the frequency domain measurement display curve are due to the superposition of the reflection vectors from the cable's joints and open ports, which can also be seen in the phase display in the frequency domain of P3.

If it is necessary to phase match the cable at a certain frequency point, as mentioned earlier, it is difficult to deduce from the reflection characteristics whether the cable transmission phase characteristics are consistent due to the inconsistency of the reflection characteristics of the cable joints. This is a connector impedance matching good cable, by its time domain measurement display curve can be seen in the connector reflection return loss of -40dB or more, but the resulting reflection phase curve undulation of about 5 °.

Vector network analyzer time-domain gate measurement method, is in the time domain measurement of the curve multiplied by a gate function in the time domain, the center of the time position and gate time length can be adjusted. It is generally used to filter out responses from certain different paths. We can use the time-domain gate method to filter out the reflection of the cable joints, and then the processed time-domain measurement curve, through the Fourier transform back to the frequency domain display, to get the filtered joints affected by the cable reflection phase curve used in lieu of the transmission phase measurements. p5 is the time-domain curve filtered out of the impact of the joints. p6 is the frequency domain phase curve filtered out of the impact of the joints.

                                         
It can be seen that the undulation of the reflection phase curve in the frequency domain drops below 1°. It is very convenient for the phase matching measurement of more than two cables. At the same time, you can use the reference plane setting function of the vector network analyzer to compensate the phase display curve in the frequency domain to the zero straight line, so as to get the electrical length of the cable. p7 and p8 show the phase display of another cable in the same state.

Notes on phase matching of phase stable cable
Measuring the phase of the reflection parameter (S11) instead of the transmission parameter (S21) at one port of the cable by the time-domain gate method reduces the requirement for consistency in the installation of the cable joints, but it does not prevent phase measurement errors caused by inconsistencies in the cutting of the cable ports. We can estimate the effect of the error by analyzing the cable port characteristics. Generally we consider the open port of a cable to be an equivalent capacitance, which measures about 0.1pF or less in the GHz band at frequency. At 50 ohms characteristic impedance -jX/Z0 is about 50 or more (where X = 1/wC, and the phase error will be about 1° as seen on the impedance circle diagram).

Therefore, what we need to pay attention to is the cleanliness of the cable cutting port, do not leave metal residues, so as not to form an inductive effect between the inner and outer conductors of the cut end face, resulting in measurement errors.

Of course, in the time domain display curve of the vector network analyzer, you can read out the difference in the position (distance) of the reflection points of the two cable cutting ports as a reference for cable length pre-matching. But such a readout is not as good as the direct phase readout, which can get the phase matching information directly. In addition, since the time-domain measurement of the vector network analyzer is obtained by the Fourier inverse transform of the frequency measurement, its time resolution is the reciprocal of the frequency scanning range, i.e., the wider the frequency scanning, the higher the time resolution obtained. For a 3GHz cable, measured with a 3GHz vector network analyzer, the minimum distance resolution is only 50mm, i.e., the time domain display of two cables with a 50mm difference is the same. A cable with a 50mm difference will transmit with a phase difference of ±180°. Therefore, direct use of time-domain measurements does not solve the phase matching problem.

Using the vector network analyzer's time-domain gate function to perform phase pre-matching measurements during the production of isophase cable sets reduces the number of repetitive installations and improves the efficiency and success rate, especially for low-volume production models

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Related Artical:

 1. Pre-assembly measurements in isophase stable cable using the net-split VNA(1)