Twisted pair and coaxial cable

Overview of Twisted Pair and Coaxial Cables

During the differential transmission of twisted-pair wires, for the external interference signal, the two twisted-pair wires are equivalent to the same conductor, and the amplitude and polarity of the interference potential induced by the interference signal on the two wires are the same. At the input end of the differential amplifier, if the CMR value of the receiving differential amplifier is large enough for this two-wire common mode input, it can be well suppressed.

The coaxial cable gets its name from its construction. Coaxial cable is also one of the most common transmission media in local area networks. The pair of conductors used to transmit information is selected according to the structure that a cylindrical outer conductor is sheathed outside the inner conductor (a thin core), and the two conductors are isolated from each other by insulating materials. The center of the central axis is on the same axis, so it is called a coaxial cable. The reason why the coaxial cable is designed is to prevent the transmission of abnormal signals from external electromagnetic waves.

Coaxial cable transmission video uses impedance matching and unbalanced transmission external end equipotential (shielding) to achieve the ideal effect, while twisted pair transmission video uses parasitic reactance differential cancellation to ensure frequency response, and differential reception is used to suppress external Interference to ensure that the indicators.

Since the characteristic impedance of the twisted pair is not constant, it varies depending on the type, length and wiring environment of the twisted pair. The above impedance transformation adjustment is only an approximate impedance when a typical twisted pair is commonly used. There are huge differences (environmental media cannot be unified), the wiring is inevitably bent, and the exact length of the wire is not easy to grasp, so that its characteristic impedance cannot be accurately adjusted. This reduction of common mode rejection ratio and reflection distortion caused by the unsatisfactory ideal differential balanced transmission conditions and impedance mismatch are aspects that need to be paid attention to when transmitting video over twisted pair cables.

Twisted pair and coaxial cable

Application comparison and precautions between twisted pair and coaxial cable

1) High frequency response to video twisted pair is better than coaxial cable

Since the parasitic reactance of the coaxial cable is relatively large (although its value is constant), it is only suitable for transmitting video signals with a resolution of 300 to 350 lines without high-frequency compensation or equalization. The parasitic reactance between the twisted pairs is small (the ideal situation tends to be zero), and the loss of high-frequency components of the transmitted video is small, especially suitable for the transmission of high-definition video with more than 400 TV lines. It can even transmit video signals modulated into radio frequency.

2) Transmission distance

Coaxial cable without repeater can not travel as far as twisted pair. According to the technical indicators of a typical dedicated chip for video transmission over twisted-pair cables: the effective transmission distance between a pair of transceivers is 1,000 feet (304.8 m). Adding relays to transmit twisted pairs is not as good as coaxial cables. Adding multi-stage relay amplification between twisted-pair transceivers will have some problems, and the effect is not easy to achieve ideal. The main problem is probably that the balance conditions are not easy to meet, as well as compensation and impedance matching. aspect reasons. The coaxial cable does not have these problems. Regardless of the actual wiring environment and the wire diameter, the signal wire and the ground wire (shielding layer) can always be in a constant state. Determine the amount of equalization compensation required by the cable. When the distance is long, you can add several levels of repeater amplification.

3) Interference and anti-interference

As long as the shield of the coaxial cable is grounded and in good contact, it will usually not be disturbed and will not interfere with others. The degree of interference of the twisted pair basically depends on the CMR value of the differential amplifier at the receiving end. Commonly used dedicated chips CMR value is generally between 70 ~ 90 dB (10 MHz). The author uses an AT&T Category 4 unshielded cable (including 4 pairs of twisted pairs) to simultaneously transmit 2 channels of video, 2 channels of audio, 1 channel of FM radio, computer data signals and 1 channel of voice telephone. The work shows that: common line transmission Whether it is digital or analog, modulated or unmodulated, video or audio, as long as the balanced transmission method is adopted, there will generally not be mutual interference and crosstalk problems. If some of the signals use unbalanced transmission, for example, the computer data stream uses RS-232C or RS-423 format or the audio broadcast uses single-ended transmission, interference and signal crosstalk may occur.

In the application of twisted pair transmission of video, in addition to the above, the following two issues should be paid attention to:

(1) Not all twisted pairs can be used to transmit video. Low-frequency twisted-pair cables such as telephone lines and power double-stranded cables are not suitable for video transmission due to the limited available frequency band. The balanced interference suppression conditions of the multi-strand parallel lines used for parallel data transmission are better, but they cannot cancel the parasitic reactances like twisted-pair lines, and the attenuation of high-frequency components is more serious. For regular projects, standard communication lines with a transmission rate of more than 10 Mbps should be selected for EIT / TIA category 3 or above, and all the entrances and exits of transmitted video, voice and data should use balanced plugs and sockets such as RJ45 or RJ11.

(2) The balanced transmission mode of twisted pair is most suitable for transmitting composite full TV signals, transmitting component video (YUV; YCaCr; YIQ and RGB, etc.) and computer VGA or RGB signals, and the conditions of balance and phase are not easy to grasp.

To sum up, it can be considered that in general occasions using twisted pair to transmit video, as long as the balance conditions are met, the transmission quality of the signal can be guaranteed. In the occasions where digital data signals and video need to be transmitted at the same time, the advantages of using twisted pair cables are more obvious than coaxial cables, and they are more in line with the principle of integrated wiring. .

The introduction of twisted pair and coaxial cable is over. I hope that through the above statement, everyone has a new understanding.