Differential signal
Differential transmission is a signal transmission technology, which is different from the traditional practice of one signal wire and one ground wire. Differential transmission transmits signals on both wires. The two signals have the same amplitude and opposite phase. The signals transmitted on these two wires are differential signals. Differential signal, also called differential mode signal, is relative to common mode signal.
Let's use a method to make a metaphor for the differential signal. The differential signal is like two people on a seesaw. When one person is lifted up, the other is lifted off-but their average position remains the same. Continuing the analogy of the seesaw, a positive value can indicate that the person on the left is taller than the person on the right, while a negative value indicates that the person on the right is taller than the person on the left. 0 means that both people are at the same level. Applied to electricity, these two seesaws are represented by a pair of wires labeled V+ and V-.
Features
Strictly speaking, all voltage signals are differential, because one voltage can only be relative to another voltage. In some systems, "system ground" is used as the voltage reference point. When'ground' is used as a voltage measurement reference, this signal planning is called single-ended. We use this term because the signal is represented by the voltage on a single conductor. On the other hand, a differential signal acts on two conductors. The signal value is the voltage difference between the two conductors. Although not very necessary, the average value of these two voltages will always remain the same.
It is conceivable that an equal voltage added to the two conductors at the same time, the so-called common mode signal, has no effect on a differential amplifier system, that is, although the input effective signal amplitude of a differential amplifier is only It takes a few millivolts, but it can be indifferent to a common-mode signal up to a few volts. This index is called the common-mode rejection ratio (CMRR) of the differential amplifier. The general operational amplifier can reach more than 90db, and the high-precision operational amplifier can even reach 120db. Because interference signals generally exist in the form of common-mode signals, the application of differential signals greatly improves the signal-to-noise ratio of the amplifier system.
advantage
1. Strong anti-interference ability. Interference noise is generally equal in value and loaded onto two signal lines at the same time, and the difference is 0, that is, noise does not affect the logical meaning of the signal.
2. It can effectively suppress electromagnetic interference (EMI). Since the two wires are close together and the signal amplitudes are equal, the coupling electromagnetic fields between the two wires and the ground wire have the same amplitudes, and their signal polarities are opposite, and their electromagnetic fields will cancel each other out. Therefore, the electromagnetic interference to the outside world is also small.
3. Accurate timing positioning. The receiving end of the differential signal is the point at which the difference between the signal amplitudes on the two wires undergoes positive and negative transitions, which is used as the point where the logic 0/1 transitions are judged. However, ordinary single-ended signals use the threshold voltage as the transition point of signal logic 0/1, which is greatly affected by the ratio of the threshold voltage to the signal amplitude voltage, and is not suitable for low-amplitude signals.
Disadvantage
If the area of ​​the circuit board is very tight, the single-ended signal can have only one signal wire, and the ground wire must go to the ground plane, while the differential signal must go two wires of equal length, equal width, close proximity, and at the same level. This situation often occurs when the chip's pin spacing is so small that it can only pass through one trace.
Clock data recovery (CDR: clockdatarecovery)
Clock recovery, as a core function necessary for high-speed serial communication, has been used more and more widely. There are clock recovery modules in Ethernet, PCI-Express, and Aurora. In contrast, the traditional parallel transmission method of simultaneous transmission of clock and data cannot achieve a bandwidth of more than 1Gb/s.
Simply put, the so-called clock recovery is: according to the reference clock, the clock signal is extracted from the data signal. Correspondingly, only serial data is transmitted on the channel, and there is no clock signal on the channel. The data receiving end receives the serial data and performs clock recovery.
In SERDES, the basis of clock data recovery
Usually the CDR protocol runs at a higher data rate and longer transmission distance, which brings great design challenges.
In SERDES (Serializer-Deserializer) applications, as the name implies, the CDR receiver must recover the embedded clock from the data. More precisely, the clock is obtained from the exchange of data signals.
The CDR transmitter first sends data serially, and then converts the data into an 8b/10b encoding scheme. The encoding process obtains 8-bit data and converts it into 10-bit symbols. The 8b/10b encoding method can transmit an equal number of 0s and 1s on the data line, thereby reducing inter-symbol interference and providing enough data edges so that the receiver can lock the phase on the received data stream. The transmitter multiplies the system clock to the transmission bit rate and sends 8b/10b data on the TX differential pair at this rate.
The task of the CDR receiver is to lock the phase on the RX differential bit stream first, and then the receiver performs data bit alignment according to the recovered clock, and then uses the receiver's reference clock for word alignment. Finally, the data is decoded by 8b/10b for use by the system.
In a CDR system, the sending and receiving systems usually have completely independent system clocks. These two clocks are very critical in a specific range of variation, which is about hundreds of PPM.
CDR circuit and jitter
The main design challenge of the CDR interface is jitter, that is, the deviation of the actual data transmission position relative to the expected position. Total jitter (TJ) consists of deterministic jitter and random jitter. Most jitter is deterministic, and its components include inter-symbol interference, crosstalk, duty cycle distortion, and period jitter (such as interference from switching power supplies). Generally, random jitter is a by-product of semiconductor heating problems, and it is difficult to predict.
The transmission reference clock, transmission PLL, serializer, and high-speed output buffer all affect the transmission jitter. For a given bit period or data eye, the transmission jitter is usually described by the percentage of unit interval or UI (unit interval). For example, the transmission jitter of .2UI means that the jitter is composed of 20% of the bit period. For transmission jitter, the lower the UI value, the better, because they represent less jitter.
Likewise, the CDR receiver will specify the maximum amount of jitter that can be tolerated at a given bit rate. The typical bit error rate (BET) standard is 1e-12. Receive jitter is still specified by UI. A larger UI indicates that the receiver can tolerate more jitter. The typical receiver specification is .8UI, which means that 80% of the bit period can be noise, and the receiver will still be able to receive data reliably. Jitter is usually quantified with a statistical bell-shaped distribution, which has an ideal edge position at its fixed point.
Channel equalization (Channelequalization)
Channel equalization (Channelequalization) refers to an anti-fading measure taken to improve the transmission performance of a communication system in a fading channel. It is mainly to eliminate or reduce the inter-symbol interference (ISI) problem caused by multipath delay in broadband communication.
The mechanism is to compensate for the characteristics of the channel or the entire transmission system. In view of the constant or variable parameter characteristics of the channel, the data rate is different, and there are multiple structural methods for equalization. Roughly divided into two categories: linear and nonlinear equilibrium. The main difference between a linear equalizer and a nonlinear equalizer is the method in which the output of the adaptive equalizer is used for feedback control. It is more difficult to equalize with communication channels. Generally, equalization is performed at the baseband after demodulation at the receiving end. Therefore, the baseband equalization technology is widely used.
In practice, an adaptive filter is generally added to achieve channel equalization. The filter is used to compensate the distorted pulse. The demodulation output sample obtained by the judge is the sample after the equalizer is corrected or the inter-symbol interference is removed. The adaptive equalizer directly adjusts the gain according to a certain algorithm directly from the actual digital signal transmitted, so that it can adapt to the random change of the channel, so that the equalizer always maintains the best state, and thus has better distortion compensation performance.
UCOAX Produce AWG 36-46 Micro Coaxial Cable, these cables are widely used for Drone/AR/VR/Notebook/Smartphone/Medical Probe and others.
Ucoax provides a wide range of standardized and customized connectivity solutions in high-frequency & high-speed applications. These solutions ensure the reliable transmission of signals, data and power in highly demanding applications.
Our selection of cable assemblies range from simple jumpers to power and high-speed data cables to complex harnesses. They are used in a wide variety of applications and industries to interconnect components, sub-systems, and equipments.
Our investment into making sure we are the best cable tooling and assembly company you can choose is second to none in our category, and our capabilities are vast. As we serve many different industries, we maintain inventory on applicators and connectors from Molex, JST, TE, Deutsch, Hirsose, and many others. Our library of crimp tools is maintained and calibrated regularly by our engineering staff, ensuring we always have the right tool for any assemblies for wire cables.
Multi-Core Coaxial Cable,Multicore Cable,Multi Core Coaxial Cable,UL Wire Colours
UCOAX , https://www.jsucoax.com