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High - Speed Serial Interface Circuits and Systems Woo - Young Choi Dept. of Electrical and Electronic Engineering Yonsei University Lecture 9 : Tx Finite Impulse Response (FIR) Equalizer
High-Speed Serial Interface
Circuits and Systems
Woo-Young Choi
Dept. of Electrical and Electronic Engineering
Yonsei University
Lecture 9: Tx Finite Impulse Response (FIR)
Equalizer
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Tx FIR
Tx
DriverChannel RxCTLE
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Tx FIR
Discrete Time Domain Implementation?CTLE
Bilinear transformation of s-domain filter into z-domain
Tapped and Delay
s-domain analysis (CT) vs z-domain analysis (DT)
Possible to approximately implement with Finite-Impulse Response (FIR)
2 1~
1
zs
T z
1
1
2 1
1
z
T z
(T: Unit Interval) sTz e
12~
12
sT
sT
Simple to implement
No stability problem (No feedback)
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Tx FIR
FIR
Tx FIR much easier to implement
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Tx FIR
FIR coefficient determination
CTLE:
Frequency-domain analysis X =
Time-domain analysis for FIR
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Tx FIR
*
Tx
DriverChannel Rx
*=Random
Bit Sequence
Inter-Symbol Interference
=
Pulse Response
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Tx FIR
FIR coefficient determination
Example (From Prof. Palermo of TAMU class notes)
Tx Channel Rx
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Tx FIR
Use 3-tap FIR for simplicity
h[0]
h[-1]
h[1]
1
0
1
w
w
w
0.13
~ 0.60
0.27
1 0 1[0] [ 1] [ 2] 0w h w h w h
1 0 1[1] [0] [ 1] 1w h w h w h
1 0 1[2] [1] [0] 0w h w h w h
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Tx FIR
Tx Channel Rx
1
0
1
w
w
w
0.13
0.60
0.27
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Tx FIR
Tx Channel Rx
FIR High Pass Filter
1
0
1
0.13
0.60
0.27
w
w
w
1 2[ ] 0.13 0.60 0.27W z z z
sTz e2j fTe
Corresponding frequency response?
1z ( 0)H f 0.2 14dB
1z 1
( ) 12
H fT
0H dB
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Tx FIR
Tap coefficients?
Minimum Mean-Square Error (MMSE) algorithm
for pre-, post-cursor reduction with the given tap number
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Tx FIR
How many taps?
(From Prof. Palermo of TAMU class notes)
3-tap FIR gives significant performance enhancement
Tx FIR CTLE Decision-Feedback Equalizer (DFE)
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Tx FIR
2-Tap FIR (main and 1st post-cursor) also called Pre-/De-Emphasis
Normal
Waveform
Nominal
swing
De-emphasis
Waveform
Nominal swing
Pre-emphasis
WaveformNominal
swing
Pre-emphasis: Enhance high-frequency components
De-emphasis: Reduce low-frequency components
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Tx FIR
• Tx FIR can be easily implemented with CML drivers
– D1=D0
Pre-emphasis
WaveformNominal
swing
– D1≠D0
(Lecture 7)
Vd,pp = ISS RL
Vd,pp = (C0 - C1) x 50
Vd,pp = (C0 + C1) x 50 More taps can be implemented by adding parallel CML drivers with proper polarity
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Tx FIR
Homework: (Due on 11/16)
An FIR filter has the z-domain system function given asH(z)= a + b z-1 + c z-2.
(a)Draw the block diagram for an NRZ transmitter equalizerimplementing above system function. Your block diagram shouldcontain only delay elements with delay T, the unit interval, and thedrivers having the required gain.
(b) Determine how much high-frequency peaking above equalizerprovides at the Nyquist frequency (=1/2T) compared to the DC gain.Use the linear scale.
(c) Show the circuit schematic of an differential current-mode driverimplementing above equalizer.
