Useful Information | Differential signal is also called differential mode signal, do you really understand it?

Latest update time：2020-07-01

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Differential signal

Differential transmission is a signal transmission technology. Different from the traditional method of one signal line and one ground line, differential transmission transmits signals on both lines. The amplitudes of the two signals are equal and the phases are opposite. The signal transmitted on these two lines is a differential signal. Differential signals are also called differential mode signals, which are relative to common mode signals.

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 a voltage reference point. When 'ground' is used as a reference for voltage measurements, the signal arrangement is called single-ended. We use this term because the signal is represented by the voltage on a single conductor.

A differential signal, on the other hand, is applied to two conductors. The signal value is the difference in voltage between the two conductors. Although not strictly necessary, the average value of the two voltages will often be the same.

It can be imagined 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 to say, although the effective input signal amplitude of a differential amplifier only needs a few millivolts, it can be indifferent to a common-mode signal of up to several volts.

This indicator is called the common mode rejection ratio (CMRR) of the differential amplifier. General operational amplifiers can reach more than 90db, and high-precision operational amplifiers 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:

Strong anti-interference ability: Interference noise is generally loaded onto two signal lines at the same time and with the difference being 0, that is, the noise has no effect on the logical meaning of the signal.

It can effectively suppress electromagnetic interference (EMI): because the two wires are close to each other and the signal amplitude is equal, the amplitude of the coupled electromagnetic field between the two wires and the ground wire is also equal, and their signal polarities are opposite, so their electromagnetic fields will cancel each other out. Therefore, the electromagnetic interference to the outside world is also small.
Accurate timing positioning: The receiving end of the differential signal is the point where the difference in the signal amplitudes on the two lines changes positively or negatively, which is used as the point for judging the logic 0/1 transition. Ordinary single-ended signals use the threshold voltage as the signal logic 0/1 transition point, which is greatly affected by the ratio of the threshold voltage to the signal amplitude voltage and is not suitable for low-amplitude signals.

shortcoming:

If the area of the circuit board is very limited, the single-ended signal can have only one signal line, and the ground line runs on the ground plane, while the differential signal must run two lines of equal length, equal width, close proximity, and on the same plane. This situation often occurs when the chip pin spacing is very small, so that only one line can pass through.

Clock Data Recovery

Clock recovery is a core function that must be possessed by high-speed serial communication and is increasingly widely used. Ethernet, PCI-Express, and Aurora all have clock recovery modules. In contrast, the traditional parallel transmission method of transmitting clock and data simultaneously cannot achieve a bandwidth of more than 1Gb/s.

Simply put, clock recovery is to extract the clock signal from the data signal based on the reference clock. 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.

The basis of clock data recovery in SERDES

Typically, CDR protocols operate at higher data rates and longer transmission distances, thus bringing significant design challenges.

In SERDES (Serializer-Deserializer) applications, as the name implies, the CDR receiver must recover the embedded clock from the data. More precisely, it obtains the clock from the exchange of data signals.

The CDR transmitter first sends the data serially and then converts the data into an 8b/10b encoding scheme. The encoding process takes the 8-bit data and converts it into a 10-bit symbol.

8b/10b encoding transmits an equal number of 0s and 1s on the data line, reducing intersymbol interference and providing enough data edges for the receiver to phase lock on the received data stream. The transmitter multiplies the system clock to the transmit bit rate and sends 8b/10b data at that rate on the TX differential pair.

The task of the CDR receiver is to first lock the phase on the RX differential bit stream, then the receiver performs data bit alignment according to the recovered clock, and then performs word alignment with the receiver's reference clock. Finally, the data is 8b/10b decoded for system use.

In CDR systems, the transmit and receive systems usually have completely independent system clocks. These two clocks are critical within a certain variation range, which is about hundreds of PPM.

Signal Equalization

Channel equalization is an anti-fading measure taken to improve the transmission performance of a communication system in a fading channel. It is mainly used to eliminate or reduce the inter-symbol interference (ISI) problem caused by multipath delay in broadband communication.

Its mechanism is to compensate for the characteristics of the channel or the entire transmission system. There are various structural equalization methods for the constant or variable parameter characteristics of the channel and different data rates.

Generally, it can be divided into two categories: linear and nonlinear equalization. The main difference between linear equalizer and nonlinear equalizer is the method in which the output of adaptive equalizer is used for feedback control. It is more difficult to equalize the baseband channel, and it is usually equalized in the baseband after the receiving end demodulates, so baseband equalization technology is widely used.

In practice, an adaptive filter is generally added to achieve channel equalization. The filter is used to compensate for the distorted pulse, and the demodulated output samples obtained by the decision maker are samples that have been corrected by the equalizer or after the inter-symbol interference has been removed.

The adaptive equalizer continuously adjusts the gain based on a certain algorithm directly from the actual digital signal being transmitted, so it can adapt to the random changes in the channel, allowing the equalizer to always maintain the best state, thereby achieving better distortion compensation performance.

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