This article introduces you to transmission line theory

In modern communication technology, whenever in the transmission of high-speed signals, especially in the process of radio frequency and microwave signal transmission, it is inevitable to encounter signal attenuation caused by return loss and radiation loss. How can this be improved?

Analysis of the advantages and disadvantages of various transmission lines

In order to bind the electromagnetic wave in a certain cross section and prevent the signal from radiating to the outer space, people usually use a guiding system called "metal waveguide".

                                   

                                           The guiding system of metal waveguides

Metal waveguide is actually a hollow metal tube, it is characterized by high frequency signal attenuation, high transmission efficiency. However, the disadvantage of waveguide is that it can only conduct high-frequency signals, and can not be transmitted for medium and low frequency signals. This is the opposite of a traditional two-wire system.

So is there a transmission line that can cover all the frequencies? In order to solve this problem, scientists put forward a new solution -- microstrip transmission and coaxial transmission technology, this paper mainly discusses the coaxial transmission technology.

                       

                                          Coaxial line guide system

Coaxial line transmission is composed of separated conductor columns, which can transmit transverse electromagnetic wave. From the perspective of the two separated conductor columns and the transmission of transverse electromagnetic wave, coaxial line transmission technology has relatively less signal attenuation and wide frequency band coverage. Theoretically, it can cover the whole frequency band, which is a better choice for signal transmission.

Coaxial line transmission has another advantage, high frequency electromagnetic wave signals are transmitted through the space electric field, the master device can simultaneously transmit Power to the slave device through the coaxial line, which is called PoC (power-over-coax), PoC and PoE work in a similar principle.

                                                                      

                                           Schematic diagram of coaxial transmission line power supply

Channel equalization technique

Cross talk between channel codes

In digital communication system, due to the multipath effect, channel band limit and other factors, the symbol tail will be formed at the receiving end. The tail part is superimposed with the adjacent symbol, which forms the intersymbol interference, resulting in the sampling signal distortion and the wrong decision.

                                                                  

Signal loss caused by inter-code crosstalk and code element recovery by channel equalization technology

In order to improve the transmission performance of communication systems in fading channels, engineers often take an anti-fading measure to eliminate or weaken the ISI problem caused by multipath delay in broadband communication, which is called "channel equalization".

Transmission line network model

Channel equalization is to eliminate inter-code interference, its core idea is to compensate the characteristics of the channel or the whole transmission system, and make the impedance of the transmission line and the impedance of the load equal by compensating the network.

                                                     

                                                                        Transmission line equivalent model

In the high-speed transmission system, the transmission line will produce many parasitic parameters, so the equivalent segmented transmission model can be established by using the blocking container

                                                     

According to the signal transmission model, the output signal will be distorted after the high-speed signal passes through the long transmission line. If the signal is not processed, the signal will not be accurately identified, resulting in intercode interference crosstalk

                                       

                                       Transmission line signal distortion causes at least intercrosstalk

The specific method of impedance matching is to add a transfer function on the basis of the transmission line, so that the entire transmission system can realize impedance matching, which is called "all-pass filtering" or "equalizing circuit".

                              

Balanced transmission implementation method

In order to eliminate the intercode interference, so that the circuit to achieve the balance effect, generally use two methods, one is pre-weighted, the other is de-weighted.

The pre-weighting technique is to increase the amplitude of the rising and falling edge of the signal, and keep the amplitude unchanged elsewhere. The de-weighting is to keep the amplitude of the rising and falling edges of the signal unchanged, while the signal weakens elsewhere.

According to the above piecewise transmission line transmission model, Feed Forward Equalizer (FFE) is generally done at the sending end and Decision Feedback Equalizer (DFE) at the receiving end.

                                    

                                                         Modeling of equalized transmission technology

Feedforward equalization FFE is the most commonly used equalization technique in SerDes systems. Usually, the SerDes sender uses FFE technology to pre-balance the signal. FFE uses different weights for delayed signals (w-1, w0,... , wn) add. Control the weight of the size of the adjustment balance strength

                                                                                                                           

                                                         The principle of pre-weighted feedforward equalization

Decision feedback equalization DFE is also implemented through digital high-frequency filters. Unlike FFE, DFE is a nonlinear equalization technique: the signal after the decision is a digital signal. Thus, the DFE can only amplify the high-frequency signal, but not the high-frequency noise

                                           

                                                         The decision feedback aggravates the principle

In DFE circuits, a Continuous Time Linear Equalizer CTLE (Continuous Time Linear Equalizer) is usually added to the front stage. CTLE functions as a combination of high-pass filter and low-pass filter to realize full-pass filtering of signals

                                                                      

                               Coaxial equalization technology theory can cover the whole frequency band

Microchip based on CoaXpress package solution

A Coaxpress-based video transmission scheme introduced by Microchip is coaxial transmission based on equalization method.

EQCO125X40 Integrated equalizer, CDR, and cable drive. It can send/receive signals on a single cable or PCB tracking pair, downstream transmission in 1.25 Gbps/12.5 Gbps 8b/10b encoding, and upstream transmission in 20.833 Mbps/41.666 Mbps 8b/10b encoding. In the application of ADAS graph transmission, the video signal is transmitted without loss through the coaxial line to the platform of high computing power for further processing. This algorithm has been accepted by more and more customers.

                                       

                              Microchip's image transmission solution based on CoaXpress technology

The chip is mounted on Microchip PolarFire video platform. Customers can easily complete product development and shorten the development process by using the free IP installation package provided by Microchip. Its authorized agent Excelpoint can provide corresponding technical support and guidance. Help customers develop products more efficiently.