【正文】 Differential Signaling Introduction Reading Chapter 6 12/4/2023 2 Agenda ? Differential Signaling Definition ? Voltage Parameters Common mode parameters Differential mode parameters ? Current mode logic (CML) buffer Relate to parameters Modeling simulation ? Timing parameters Clock recovery Embedded clock ? AC coupling Common mode response Issues with simulation ? 8B10B encoding DC balanced codes ? Duty Cycle distortion Cycle 12/4/2023 3 Single Ended Signaling ? All electrical signal circuits require a loop or return path. ? Single ended signal subject several means of distortions and noise. Ground or reference may move due to switching currents (SSO noise). We touched on this in the ground conundrum class. A single ended receiver only cares about a voltage that is referenced to its own ground. Electromagic interference can impose voltage on a single ended signal. Signal passing from one board to another are subject to the local ground disturbance. ? We can counteract many of these effect by adding more ground. ? As frequencies increase beyond 1GHz, 80% of the signal will be lost. 12/4/2023 4 Review of threshold sensitivity ? The wave is referenced to either Vcc or Vss. Consequently the effective DC value of the wave will be tied to one of these rails. ? The wave is attenuated around the effective DC ponent of the waveform, but the reference does not change accordingly. Hence the clock trigger point between various clock load points is very sensitive to distortion and attenuation. Tx Vss Vref Vss Rx2 Vref Long line Vss Rx1 Vref Short line 12/4/2023 5 Differential Signaling ? Any signal can be considered a loop is pleted by two wires. ? One of the “wires” in single ended signaling is the “ground plane” ? Differential signaling uses two conductors The transmitter translates the single input signal into a pair of outputs that are driven 180176。 out of phase. The receiver, a differential amplifier, recovers the signal as the difference in the voltages on the two lines. ? Advantages of differential signaling can be summed up as follows Differential Signaling is not sensitive to SSO noise. A differential receiver is tolerant of its ground moving around. If each “wire” of pair is on close proximity of one and other. electromagic interference imposes the same voltage on both signals. The difference cancels out the effect. Since the AC currents in the “wires” are equal but opposite and proximal, radiated EMI is reduced. Signals passing from one board to another are not subject to the local ground disturbances. As frequencies increase beyond 1GHz, up to 80% of the signal may be lost, but difference still crosses 0 volts. There are still loss issues for differential signaling but only e into play in high loss system. Most single ended systems assume approximately 15% channel loss. 12/4/2023 6 Differential Signaling Cons ?The cost is doubling the signal wires, but this may not be so bad as pared to adding grounds to improve single ended signaling. ?Routing constraint: Pair signals need to be routed together. ?Differential signal have certain symmetry requirements that may pose routing challenges. 12/4/2023 7 Differential Signal Parameters ? Voltage on line 1 = a ? Voltage on line 2 = b ? Differential voltage d = ab ? Common mode voltage c= (a+b)/2 ? Odd mode signal, o = (ab)/2 ? Even mode signal, e = (a+b)/2 ? Signal on line 1 a = e+o ? Signal on line 2 b = eo ? Useful relations。 o = b/2。 e = c Line 1 Line 2 Reference 12/4/2023 8 Propagation Terms to Consider ?Differential mode propagation ?Common mode propagation ?Single ended mode (uncoupled) propagation This is when the other line is not driven but terminated to absorbed reflections. ?Transmission line matrixes will reflect these modes. 12/4/2023 9 Differential Microstrip Example Z0 Z s e??S s e 1 . 9 9 2nsft?S s e L 1 1 C 1 1???Z s e 3 9 . 7 4 4 ??Z s eL 1 1C 1 1??Sc 1 . 9 9 6nsft?Sc L 1 1 L 2 1?( ) C 1 1 C 2 1?( )???Z c o m m 4 7 . 4 2 2 ??Z c o m mL 1 1 L 2 1?C 1 1 C 2 1???Sd 1 . 9 2 4nsft?Sd L 1 1 L 2 1?( ) C 1 1 C 2 1?( )???Z d i f f 6 6 . 1 8 5 ??Z d i f f 2L 1 1 L 2 1?C 1 1 C 2 1???i n d u c ta n c e an d c ap ac i ta n c e m at r i x e sC 1 1C 1 2?C 2 1?C 2 2??????L 1 1L 1 2L 2 1L 2 2??????T r an s m i s s i o n l i n eC 2 2 C 1 1??C 1 2 C 2 1??C 2 1 6 . 6 9 0 0 0 0 1013?f a r a din??C 1 1 4 . 1 7 7 0 0 0 1012?f a r a din??L 2 2 L 1 1??L 1 2 L 2 1??L 2 1 1 . 2 9 1 0 0 0 109?h e n r yin??L 1 1 6 . 5 9 8 0 0 0 109?h e n r yin??SE: single ended = uncoupled 12/4/2023 10 Differential Impedance ?Coupling between lines in a pair always decreases differential impedance ?Differential impedance is always less that 2 times the uncoupled impedance ?Differential impedance of uncoupled lines is 2 times the uncoupled impedance. 12/4/2023 11 Propagation Velocities ?For TEM structures, (striplines) Differential mode, Common Mode, and single ended velocities are the same ?For Non TEM and QuasiTEM structures (microstrip) Differential mode, Common Mode, and single ended velocities and impedances are not the same. Common mode can be converted to differential mode at a receiver and result in a differential signal disturbance. 12/4/2023 12 Example of Common Mode ? Line 1 and line 2 have the same DC offset. This is DC mon mode. It can be defined as an average DC for time duration of many UI cycles value as well. ? Line1 and line 2 have the same AC offset ? This is AC mon mode ? AC mon mode also result from time differences (skew) between si