Prerequisites for digital cable Testing

Introduction

With the increasing market demand for symmetric digital cables (hereinafter referred to as digital cables) for integrated cabling, many optical cable manufacturers have begun to develop and produce such cables for this manufacturer to meet market demands. At the same time, the manufacturer found that although the digital cable and the early municipal cable are both twisted wires, the requirements for digital cables are much higher than those for standard and practical use. This is because the local cable is only used for low-frequency electrical signal transmission, generally not greater than 1 MHz), while the digital cable transmits high-speed digital signals, up to 1200 MHz, as early as 100 MHz, the No. 5 cable also exceeded. Many strict parameters are defined in the standard to enable digital cables to transmit high-frequency digital signals. At the same time, many parameters are added only when the frequency is high. This puts forward new requirements for production equipment and testing methods. Manufacturers tend to focus on the production of equipment, while ignoring the establishment of a complete testing method. Or simply think that the cable test can be completed as long as an advanced test device is purchased. In this case, the actual cable performance is not consistent with the actual cable performance. It should be pointed out that a complete testing method should include advanced testing equipment and excellent testing personnel. Only when the above two conditions are met can the digital cable test be well completed.

I. Test Equipment

International instruments used for digital cable testing can be divided into two categories based on their testing principles. The first method is the time domain reflection method. The time domain reflection method injects a narrow pulse into one end of the cable and detects the reflection signal at the injection end. Then, the reflected signal is digitalized, And the quantified information is processed by various data, including Fourier transformation, to obtain the cable performance value. Its principle is very similar to that of OTDR. Because the narrow pulse reflects a wide spectrum electrical signal in the frequency domain, when the "Composite Signal" is transmitted within the cable, the uneven point of the cable will cause reflection, the reflected signal is returned to the injection end, and the detector detects electrical signals at the injection end. Therefore, from the injection of an electrical signal to the reception of a signal, the electrical signal degrades the length of two cables. As a result, the signal with a high frequency degrades twice, and the signal becomes very small. This directly affects the dynamic range and accuracy of the test instrument. At the same time, there are also "obfuscation" of signals from multiple reflection signals to the receiver, which may further reduce the test accuracy. However, the finished instrument manufactured based on this principle can be small in size, easy to carry, and low in cost. It is particularly suitable for the inspection of engineering cabling and semi-finished products. However, it is difficult to meet the precision requirements for finished cable detection.

The second method is the standard frequency-domain scanning method. Generally, the vector network analyzer is used as the core component, or a discrete Scanning Signal Source and receiver are used as the core component. balun is used as the balance device to perform Impedance Matching on the test port. This method is a standard reference method. We will discuss how different test devices manufactured based on this method have an impact on the test results.

When people choose this type of digital cable testing equipment, the performance indicators of the testing equipment are often ignored. Some indicators of the test device are directly related to the correctness of the test results.

We will discuss the above point of view with the background of system noise as an example. System Noise Floor) determines the minimum electrical signal level that the System can receive and the dynamic range of the System.

In digital cable testing equipment, system noise mainly comes from the internal Crosstalk of the system. It can also be divided into the noise background of the same end and the near-remote noise background. Next Noise Floor (Noise Background of adjacent ports at the same end) is used to measure the Noise level of adjacent ports at the nearest or remote end. The Noise background of near-remote end is used to measure the Noise level between the adjacent and remote connection ports; the former will affect the performance test results of the near-end crosstalk test in the wire test, and the latter will affect the performance test results of attenuation, remote crosstalk, and the calculated parameters such as ELFEXT in the cable test.

Assume that a test device has the following performance indicators:

Near-remote Noise Background: ≤-60dB 1 ~ 300 MHz

(ATT Noise Floor)

Background noise of adjacent ports at the same end: ≤-60dB 1 ~ 300 MHz

(NEXT Noise Floor)

We calculate the minimum requirement of Noise Background of the test equipment based on the parameters specified by the IEC61156-5 standard, and compare it with the performance of the above-assumed instrument. When the absolute value of the data given in the current table is greater than 60 dB, the data marked with "=" may fail. If the absolute value is close to 60 dB, the data is marked with "*"), and the test result is not highly reliable. It should be pointed out that the test value and the noise background should have a certain margin. Here, we provide a 3 dB margin requirement.

A. Relationship between CAT5e and CAT6 digital cable attenuation tests with A cable length of 100 m and 30 m with Noise Floor.

Table 1

It can be found that such a system is credible when testing the electrical performance of M cables. However, during long cable testing, data is incorrect at a high frequency. Obviously, ATT Floor Noise will affect the test length of the cable. In fact, the maximum length that the system can test can be obtained according to a simple mathematical formula, CAT5e is 57/25) m, CAT6 is 57/33) m. Fortunately, the attenuation trend of cables follows certain rules. Therefore, the attenuation of high-frequency points of cables with a large length can be accurately tested for the attenuation value of lower-frequency points. After fitting through the formula, the test accuracy can still be satisfied.

B. Test the relationship between CAT5 and CAT6 with a cable length of M and NEXT Noise Floor.

Table 2

It can be seen that the NEXT Floor Noise of the test device will affect the test of the near-end Crosstalk of the cable. The crosstalk of a cable is unpredictable because its crosstalk is not like attenuation. Therefore, the test result is incorrect. In addition, it is worth noting that no matter how good your cable crosstalk performance is, the final result will be less than or equal to 60dB. The result shows that the margin between the crosstalk indicator of the cable and the value specified by the standard is small, which is worrying. You cannot know the real crosstalk performance of the cable. The increase in length will make the test results untrusted.

C. Relationship between calculated ELFEXT and Floor Noise

We know that ELFEXT = IOFEXT-α, while IOFEXT is measured, α is the attenuation of the cable), so IOFEXT = ELFEXT + α

Table 3

In this way, error data will be generated in some frequency areas, and an error conclusion will be given. We can see from the table that there is basically no margin in the system when testing CAT6. It is worth mentioning that the increase in length will also make the test results worse.

It should be noted that the table shows the worst possible condition for compliant cables. For well-designed digital cables, the final test results will show that the cable performance is average, or even false positives are deemed to be unqualified. The following is an example of the possible impact of Noise Floor on the test result. In fact, these factors are only one of the many factors.

Therefore, when selecting a digital cable testing device, in addition to concerned about whether the working principle of the device meets the standards, you must also care about the performance indicators of the testing equipment, whether the digital cable testing is satisfied.

Ii. testers

In addition to a set of advanced testing equipment, we should also have qualified testers. A qualified tester should have a brand new test concept, solid theoretical basis and rich operation experience.

The maximum test frequency of the digital cable has reached 1200 MHz and is already in the microwave segment. If the early municipal cable test results mainly assess the total cable set parameters. In some cases, distribution parameters are more important in digital cable testing. As a result, minor changes in the cable status, such as the pitch, may cause failure in the digital cable test. Even the cable position affects the test results.

Iii. Conclusion

The test results of digital cables have two fundamental factors: whether the cable performance can be reflected. First, advanced testing equipment; second, excellent testers, both of which are indispensable. The tester analyzes the test results based on the theory they have mastered, draws correct conclusions, and reports them to the cable designer. In this way, the quality of digital cables can be guaranteed.

1. DSP-4000 Series Digital cable Tester
2. Coaxial Cable and optical fiber cable Detection