Digital Signal Integrity Tutorial (page 5)

  

 

Page 1 Page 2 Page 3 Page 4 At this page (5)
⚛ Noise Output Levels ⚛ Problems Caused by Ground Bounce ⚛ Termination of transmission lines ⚛ Driving Interconnecting Channels ⚛ Interaction of Signals (Crosstalk)
⚛ Peak Totem-Pole Current (Crossover Current) ⚛ Ground Bounce Noise Troubleshooting ⚛ Termination Method on the Load Side ⚛ Distributed Channel Line Capacity ⚛ Differential Signal Transmission
⚛ Charging Current/Discharge Capacity Output ⚛ Signal Loopback ⚛ Termination at Source Signal Side ⚛ Termination of Channel Lines ⚛ Evolution of Differential Transmission Technologies
⚛ Ground Bounce ⚛ Transmission Lines ⚛ Summary of Use of the Different Termination Methods ⚛ Logical Interconnection and Bus Drivers Families  

 

 

Interaction of Signals (Crosstalk)

When two conductors are close together and parallel to each other they exhibit mutually capacitive and inductive coupling: current flow in one conductor causes reverse flow to the second, while any change in voltage at some point in the first conductor is capacitively imprinted on the second.

The mutual interaction of neighboring signals is called crosstalk. In the case of digital signals, where each signal is independent of each other, the mutual interaction is undesirable, introduces noise to the system and degrades the signal quality.

Inductive and capacitive coupling of digital signals occurs during signal level change and not as long as a signal is in a steady state. The coupling is greater as the frequency increases and the up/down times of the signals are reduced.

Mutually coupled conductors are always near or between lines or levels of the reference voltage (grounding). When the distance between the two conductors increases, the mutual interaction is weakened, as the coupling of each conductor with the ground is predominant. The crosstalk phenomenon is enhanced as the distance between signal lines decreases.

Figure 5-16 schematically illustrates the mechanism of interaction of two adjacent, parallel-spaced conductors.

Interaction of signals (crosstalk)

 

Figure 5-16

 

Along the AB conductor, a signal pulse moves to the point B. At the location where the pulse is at a random time, electromagnetic fields present a parasitic current flow and corresponding voltages in the conductor CD, both to point D and to the C.

Due to the capacitive coupling between the two conductors, a parasitic SC signal is transmitted in the same polarity to points C and D. A corresponding but reverse polarized parasitic signal SL is also propagated to points C and D due to the inductive coupling between Pipelines.

The parasitic signal to point D is called forward crosstalk and because its two components (capacitive and inductive) have reverse polarity, it is minimized by the inductive component being prevalent. If the AB and CD conductors are in homogeneous material (e.g., striplines in a printed circuit), then the parasitic signal forwards equals zero.

In the case of the parasitic signal to point D, the situation is different. The inductive and capacitive signal components have the same polarity and are summed, creating the so-called reverse crosstalk. Parasitic backward interaction is also the main cause of noise in the overall phenomenon.

The magnitude of the two parasitic signals depends on the rise/fall times of the main signal, the distance between the conductors, the length of time the two conductors are parallel, and finally the termination devices at the edges of the conductors. If the CD does not end at its ends, the parasitic signals will be reflected and magnified by the effect of the interaction. The reverse crosstalk has a maximum limit, after which it no longer depends on the length of the parallel conductors.

To address the phenomenon of signal interaction, the following solutions are proposed:

  • A) Increasing the distance between the conductors, which, however, requires more space in the printed circuit.
  •  
  • B) Reduce the distance of each line from the line or ground plane. This solution, however, has as a side effect the reduction of the characteristic resistance of the conductors and the increase of the required driving power respectively.
  •  
  • C) shielding or ground stripping lines, with ground lines or by designing the conductors at separate levels of the printed circuit and interfering ground or supply levels between them.
  •  
  • D) Stopping the two-end conductors to minimize reflections of parasitic signals.
  •  
  • E) Providing lower rising and falling rates of the digital signal using control circuits with output level change rate control.
  •  
  • F) In printed circuits, critical signals (eg clock lines) must use striplines to minimize the forward crosstalk.

 

 

Differential Signal Transmission

The previous paragraphs examined the transmission of the digital signal through a signal conductor and the corresponding ground line. Grounding serves as a reference voltage for both the driving circuit, which generates the signal, and the driven circuit, which receives the signal. Logic signal levels are compared with the reference voltage, and only have meaning in relation to it. The signal is transmitted along the line and returns via the ground line. This mode of transmission is called single-ended mode.

For conductors longer than one meter, the signal/noise ratio is such that it creates problems in the secure transmission of the signal. Differential signal transmission techniques are therefore used.

Differential mode uses a pair of conductors between the driver and the driven circuit. The one conductor transfers the signal to its positive form while the second conducts an equal signal but with opposite polarity from the first one.

The differential receiver operates as a differential amplifier and reproduces the logic signal by the difference of the two input signals, as illustrated in Figure 5-5-17 below.

Basic principles of differential signal transmission

 

Figure 5-17

 

The logical state represented by the differential signal is determined by the difference (VOD) of the "rectangular" VOA signal and the "complementary" VOB. The threshold voltages VTL and VTH of figure 5-17 are precisely determined by the VOD difference.

At first glance, differential signal transmission has the serious drawback of using two conductors instead of one. At the same time, however, it has the fundamental advantage of great immunity to noise.

This immunity is primarily due to the ability of the receiving circuitry to reject any noise voltage that is simultaneously applied to both lines. Any voltage offset common to both inputs (common-mode noise) does not affect their difference (VOD) and consequently neither the received logical state. The ability of the receiving circuit to reject common noise is expressed by the common-mode rejection ratio (CMRR).

Due to the rejection of the common noise signal, the differential transmission circuits are immune to electromagnetic interference, to the interaction of neighboring signals, and noise to ground bounce.

Differential transmission can also help in cases where signal levels are too low. Such levels are used to reduce power consumption and increase transmission speed. In these cases, the difference of the two signals actually doubles the signal levels (+V - (-V) = 2V), making it easier to handle.

As far as the return of the signal is concerned, no earthing conductor is required for it to occur. In theory, everything that is transmitted to one duct returns from the other. Without ground return current, the design of the conductors or the reference voltage levels is not critical.

Because one of the two differential signals uses the other, it is easier to accurately control the logic status change time than in the case of transmission through a conductor. More accurate time control allows for the transmission of differential signals at a higher speed than transmission through a single conductor.

The above advantages of differential transmission require exactly the same characteristics of the two transmission channels. The basic design rules in this case are:

 

  • A) The two ducts must have the same length. Otherwise, the signals cease to be similar and complementary when they reach the receiving circuit. As long as this happens, a return current appears through the ground and noise is probably generated in the system.
  •  
  • B) The two conductors should be as close as possible to each other (in the case of a printed circuit) or a twisted pair (in the case of cables). When this happens, any external parasitic noise also affects the signal of the two conductors, so it is completely rejected during reception as a common-mode signal. In addition, the mutual coupling of the two nearby conductors largely eliminates electromagnetic interference emitting to other circuits.
  •  
  • C) The distance between the two conductors must be constant. Between the two ducts, coupling occurs when they are close to one another. This coupling results in the reduction of the impedance of the two conductors to a value different from their characteristic ZO. The new impedance is called differential impedance and depends on the distance between the conductors. Keeping this distance stable, a uniform impedance is achieved over the entire length of the conductors, which, as mentioned in previous paragraphs for the transmission lines, is particularly important to avoid reflections.

 

 

Evolution of Differential Transmission Technologies

Differential transmission methods of the digital signal were initially used to interconnect, by means of a pair of wires, systems that abstracted from each other. The high immunity of differential signals to noise has been the main cause of their use for longer than one meter lengths (Figure 5-18).

Differential transmission methods are described by standards that determine the desired characteristics of the differential signal and the driving and receiving circuits.

One of the oldest differential transmission models via a pair of twisted-pair ZO-rated cables of 100Ω is the RS-422. The wide range of signals (± 2V to ± 5V) and the low transmission rate (<30Mbps) of the standard make it ideal for transmission in a high-noise environment.

Digital signal transmission technologies

Figure 5-18

 

One of the newer differential transmission models, called LDVS (Low Voltage Differential Signaling), has seen significant improvements in the field of transmission speed and power consumption. Using the LVDS standard, speeds up to 40 times higher (400Mbps) are achieved from older differential transmission methods, reducing the power consumption at the same time up to 10 times. The performance of the LVDS is due to the minimization of the voltage signals of the differential signals. In addition, the LVDS standard can be used for transmission of conductor pairs, coaxial cable and printed circuit boards.

The following table 5-6 summarizes the development of RS-422 and LVDS differential transmission technology:

 

Standard

RS-422

LVDS

Amplitude of Differential Signal

± 2-5 V

± 250 - 450 mV

Threshold Voltage

± 200 mV

± 100 mV

Transmission Rate

<30 Mbps

> 400 Mbps

Static Power Consumption  (Driving Circuit with 4 Outputs)

60 mA

8 mA

Static Power Consumption (Receiving Circuit with 4 Inputs)

23 mA

15 mA

Signal Propagation Delay (Driving Circuit)

11 ns

1.7 ns

Signal Propagation Delay (Receiving Circuit)

30 ns

2.7 ns

 

Table 6.5