【正文】 12/4/2020 Differential Signaling 下載 Common Mode Signal ? The DC mon mode signal is 1 ? The AC mon mode signal is .2 v peak to peak Some may specifications may call this v peak from the DC average ? We will add this mon mode to the signals “a” and “b” Pl o t c o m m o n m o d e v o l t a g e0 1 2 3 4 50. 80. 911. 1aibi?2tins17 12/4/2020 Differential Signaling 下載 Add 150 ps skew to signal b Pl o t i n d i v i d u a l l i n e v o l t a g e s a n d o f f s e t v o l t a g e0 0. 83 1. 67 2. 5 3. 33 4. 17 510. 330. 3311. 672. 333aibioff setitins? Waveforms do not look so good. ? We even have what appears to be nonmonotonic behavior. 18 12/4/2020 Differential Signaling 下載 Differential signal looks OK 0 0. 83 1. 67 2. 5 3. 33 4. 17 521012a b?tnsPl o t D i f f e re n t i a l v o l t a g em a x a b?( ) m i n a b?( )? 3 . 5 6 2?? However we lost differential signal amplitude. ? It used to be 4 peak to peak and now is . 19 12/4/2020 Differential Signaling 下載 Common mode measurements are different 0 1 2 3 4 50. 511. 52aibi?2tinsPl o t c o m m o n m o d e v o l t a g emeana b?2??????1?maxa b?2??????m i na b?2??????? 0 . 9 4 4?max meana b?2??????maxa b?2??????? meana b?2??????m i na b?2??????????????0 . 5 0 4?? Average is still 1. Peak to peak is but peak is ? AC mon mode signals can be converted to differential 20 12/4/2020 Differential Signaling 下載 PWB structures that introduce Skew An escape from a BGA or connector pins introduces skew This is an example of skew pensation 21 12/4/2020 Differential Signaling 下載 Bends introduce skew Back to back bends pensate for skew from frequencies below 2 GHz. 22 12/4/2020 Differential Signaling 下載 More Terms: Balanced and Unbalanced ? Good Agilent Technologies article on balance and unbalanced signaling mo/downloads/ ? Unbalanced signaling in reference to ground ? Balanced signaling is referenced only to the other port terminal. If each channel is identical, then this suggests a virtual AC ground between the two terminals. It is often useful to allow this AC ground to be a DC voltage to biasing devices. 23 12/4/2020 Differential Signaling 下載 Ether 10/100BASET example T N 1T P 150 ? 50 ? 50 ? 50 ? Transformer Filter Commonmode choke Unbalanced Balanced 24 12/4/2020 Differential Signaling 下載 Low Voltage Differential Signaling: LVDS ? 200MHz – 500 MHz Range ? Published by IEEE in 1995 ? Lacks robustness for GHz Signaling ? Well suite distributing system clocks ? Good noise margin ? Common mode impedance has wide range provide buffer design flexibility ? Differential impedance is optimize around 100 ? ? Differential receiver switching thresholds are tighter than for single ended logic. ? Most device require external termination and bias resistors ? Does not have capacitance or package spec. This severely limits GHz operation 25 12/4/2020 Differential Signaling 下載 Current Mode Logic ? Emerging technology ? No real spec yet but can infer operation from spec’s like PCI Express? , Infiniband?, USB, SATA, etc. ? Tx and Rx lines are separate ? The Tx driver steers current between the differential terminals ? AC coupling between Tx and Rx with a series capacitor provides mon mode design flexibility ? Termination is in buffers. This may require pensation or a band gap reference to insure a tight resistance range. 26 12/4/2020 Differential Signaling 下載 Example of Simple CML Differential Behavioral Circuit Vcc Vss I_source r_termn, C_term r_termp, C_term 0 10 20 3001D a t a P u l s e s0 10 20 3001D a t a W a v e sw a v e t( ) 1 ep u l s e t( ) 3? p e rns????W a v e p t( )W a v e p t( ) 1? r _ t e r m n? b a la n c e p? W a v e n t td?( ) 1?W a v e n t td?( ) r _ t e r m n? b a la n c e n?Positive Terminal Negative Terminal This exponent determines wave shape This switch time offset Balance between for FET switch 27 12/4/2020 Differential Signaling 下載 Example of Sensitivities: I, balance, C Vcc I_source More prominent for faster edges 28 12/4/2020 Differential Signaling 下載 Example of Sensitivities: Slew, Skew, R Vcc I_source +/skew R/F slew 29 12/4/2020 Differential Signaling 下載 Serial Differential ?GHz transmission will have many UI’s of data in transit on the interconnect at any points in time. ?Hence it bees useful to think of this as serial data transmission. ?Often multiple single channels are ganged in parallel to achieve even higher data throughput. 30 12/4/2020 Differential Signaling 下載 AC coupling issues ?Series capacitors can build up charge difference between differential terminals for the following reasons. Unequal numbers off zero and ones Duty cycle (UI) distortion. ?The solution is to use a data code that is “DC” balanced. 8B10B (8 bit 10 bit) with disparity is one such code ?Tight UI control is a basic requirement for keeping the signal eye open 31 12/4/2020 Differential Signaling 下載 Eye Diagram ? The eye diagram is a convenient way to represent what a receiver will see as well as specifying characteristics of a transmitter. ? The eye diagram maps all UI intervals on top of one and other. ? The opening in eye diagram is measure of signal quality. ? This is the simplest type of eye diagram. The are other form which we will discuss later Eye Diagram 32 12/4/2020 Differ