Class2_13_14_GHz_Differential_Signaling

Class2_13_14_GHz_Differential_Signaling - GHz Diffe ntial S...

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12/4/2002   GHz Differential Signaling High Speed Design
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12/4/2002 Introduction 2 ISI (Inter-Symbol Interference) Frequency dependant loss causes data dependant jitter which is also called inter symbol interference (ISI). In general the frequency dependant loss increase with the length of the channel. The high frequencies associated with a fast edge are attenuated greater than those of lower frequencies. The observable effect on a wave received at the end of a channel looks as if the signal takes time to charge up. If we wait long enough the wave reaches the transmitted voltage. If we don’t wait long enough and a new data transition occurs, the previous bit look attenuated. Hence a stream of bits will start or finish the charge cycle at different voltage point which will look to the observer as varying amplitudes for various bits in the data pattern.
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12/4/2002 Introduction 3 Effect of increasing channel length Notice the effect on the lone narrow bit verses the wider pulse that is representative of multiple bits. The lone pulse looks more and more like a runt as the channel length increases Tx channel Rx channel Rx channel Rx channel Rx
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12/4/2002 Introduction 4 Simulation of a lossy channel ISI Example is 1 meter of FR4 at 1GHz Notice the loss creates the edge to edge jitter and the max voltage is not reached on the runt pulse This is ISI. Tx Rx Rx
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12/4/2002 Introduction 5 How can we fix the runt pulse? Solution: Boost the amplitude of the first bit. The means we drive to a higher voltage at the high frequency component and a lesser voltage at lower frequency. Transition bit
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12/4/2002 Introduction 6 Equalization The previous slide illustrates the concept of equalization. Normally the max current is supplied on the transition bit and reduces on subsequent bits. Thus if we reference to the transition bit to a transmitter this equalization is commonly called “de”-emphasis . If we talk about the a the non- transition bit in reference to a receiver or passive network we might call this “ pre”-emphasis . Although the two may be considered the same, the former is used more commonly.
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12/4/2002 Introduction 7 Equalization Philosophy – First step Given the channel has a complex loss verses frequency transfer function, H ch ( ϖ ) The FFT of an input signal multiplied by the transfer function in the frequency domain is the response of the channel to that input in the frequency domain. tx(t) Tx( ) If we take the IFFT of the previous cascade response we get the time domain signal of the output of the channel. We talked about this last semester. rx(t)=IFFT(Tx( )*H ( ))
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12/4/2002 Introduction 8 Equalization Philosophy – The punch line Given the response of the output: Tx( ϖ )*H ch ( ) Look what happens if we multiply this product by 1/ H ( ). The result is Tx( ).
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Class2_13_14_GHz_Differential_Signaling - GHz Diffe ntial S...

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