NVP is the abbreviation of "Nominal Velocity of Propagation", which refers to the percentage of the transmission speed of signals in a twisted pair cable relative to the speed of light in a vacuum. The calculation formula is as follows:
NVP = (V1/V2) × 100%
- V1: Represents the propagation speed of the electrical signal. (Note: It does not refer to the actual propagation speed of the electrical signal inside the twisted pair, but the speed relative to the cable. Due to the twisting of the twisted pair, the length of the cable is shorter than the length of the core wire; moreover, the density of the twisted pair varies, which leads to different values of V1.)
- V2: Represents the propagation speed of light in a vacuum.
Through the above formula, we can derive the propagation speed of the electrical signal: V1 = V2 × NVP%. As mentioned in the note above, the calculated propagation speed is not the actual propagation speed of the electrical signal. It exists only for the test instrument to calculate the length of the twisted pair cable and is a relative value. Therefore, the NVP of different twisted pair cables needs to be measured and set, rather than being set based on the NVP marked on the selected copper material.
Then, what is the significance of its existence? Here, we have to mention the process by which the test instrument calculates the length of the twisted pair cable. As we all know, the length between two points is equal to the average speed passing through the two points multiplied by the time taken to pass through them. The average speed relied on by the test instrument to calculate the length of the twisted pair cable is exactly this NVP value. Then, how is the time obtained? The time mentioned here is precisely the propagation delay of the twisted pair cable. The propagation delay of the twisted pair cable is calculated by the test host based on the transmitted electrical signal and the received reflected electrical signal. The calculated time value is the actual time taken from the near end to the far end. By multiplying this time by NVP, the length of the cable can be calculated.
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When it comes to this point, we have to talk about the setting of the NVP value in the test instrument. Usually, the NVP value in a FLUKE tester is 69%. If this fixed value is used to measure all types of twisted pair cables, errors in length will inevitably occur. As mentioned earlier, different twisted pair cables require re-setting of the NVP value because their pitch (twisting density) is different. Generally, the higher the grade of the twisted pair cable, the denser its twist pitch. Therefore, when a fixed value is used to measure the length of the twisted pair cable, the measured value of the twisted pair cable with a higher grade will be longer than the actual length. In this case, the electrical performance indicators (such as insertion loss, near-end crosstalk, and return loss) measured by the test instrument cannot be accurately compared with the length. This is because the longer the length of the twisted pair cable, the worse its performance will be.
Typically, before testing a twisted pair cable, we measure a section of twisted pair cable of the same type with a known length. This measurement is equivalent to calibrating the NVP value of the test instrument. When the length measured with the set NVP value is consistent with the actual length of the twisted pair cable, we set this value as the NVP for the test.
Different brands and types of twisted pair cables have different requirements for the NVP value. Therefore, every time the test instrument is used for measurement, the NVP value of the instrument must be calibrated. Only in this way can the measured length be closer to the actual length.
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