Compact lowpass ladder filters using tapped coils 2009 International Symposium on Circuits and Systems, Taipei Nagendra Krishnapura Varun Gupta 1 Neetin Agrawal 2 Department of Electrical Engineering Indian Institute of Technology, Madras Chennai, 600036, India 1 currently at the Indian Institute of Management, Ahmedabad, India 2 currently at Texas Instruments, Bangalore, India 25 May 2009
23
Embed
2009 International Symposium on Circuits and Systems, Taipeinagendra/papers/isc09-tappedlcfil-sl.pdf · 2009 International Symposium on Circuits and Systems, Taipei Nagendra Krishnapura
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Compact lowpass ladder filters using tapped coils
2009 International Symposium on Circuits and Systems, Taipei
Nagendra Krishnapura
Varun Gupta1
Neetin Agrawal2
Department of Electrical EngineeringIndian Institute of Technology, Madras
Chennai, 600036, India
1currently at the Indian Institute of Management, Ahmedabad, India
2currently at Texas Instruments, Bangalore, India
25 May 2009
Pulse shaping filters in serial links
• Spiral inductors occupy large chip area
LC ladder filters for pulse shaping
+
Vs
-
+
Vo
-
L4L2 L6
+
Vs
-
+
Vo
-
L4L2 L6
L2 + L4 + L6
• Use a single spiral with multiple taps to save area
Outline
• Single inductor with multiple taps versus multiple inductors
• Effect of coupling between inductors in a ladder filter
• Cancelling the effect of coupling
• Seventh order Bessel filter using a single spiral
• Simulation results
• Conclusions
7.5GHz Bessel filters for 10Gb/s data
50Ω 50Ω
+
Vo
-
+
Vs
-
1.1788nH 0.5382nH
0.9584pF 0.3412pF 0.0740pF
50Ω
+
Vs
-
+
Vo
-
50Ω
0.9617pF
1.1726nH
0.3688pF
0.3458nH
0.0469pF
0.7449nH
0.2228pF
L2 L4
C1 C3 C5
C5
L4
C3
L2
C1
L6
C7
Single spiral versus multiple spirals
3T
110µm sq.
1.173nH
3T
85µm sq.
0.74nH
3T
60µm sq.
0.35nH
4T
135µm sq.
2.2632nH
total area = 56375µm2 total area = 38025µm2
1.1726nH 0.3458nH0.7449nH 2.2632nH
* The area of the single spiral is incorrectly given as 27225µm2 in the paper
Coupling between adjacent inductors
R
+
Vs
-
+
Vo
-
RL2 L4
C1 C3 C5
M24
R
+
Vs
-
+
Vo
-
RL2+M24 L4+M24
C1 C3 C5
-M24
Vo(s)
Vs(s)=
1 − s2M24C3
D5(s)
• Zeros at ±√
1/M24C3
• Undershoot
• Reduced attenuation
Coupling between alternate inductors
R
+
Vs
-
+
Vo
-
RL2 L4
C1 C3 C5
M26
C7
L6
Vo(s)
Vs(s)=
1 − s2(C3 + C5)M26 − s4C3C5L4M26
D7(s)
• A pair of zeros on the real axis
• A pair of zeros on the imaginary axis
• Undershoot, notch, reduced high frequency attenuation
Step response with coupling
0 50 100 150 200−0.2
0
0.2
0.4
0.6
0.8
1
1.2
[Vo
lts]
time / ps
Fifth order Bessel filter
Ideal Bessel
with k=0.3
Magnitude response with coupling
0.1 1 10 100 −60
−50
−40
−30
−20
−10
0
GHz
dB
Fifth order Bessel filter
Effect of coupling on the step response
0 0.1 0.2 0.3 0.4 0.5 0.6 0.70
1
2
3
4
5
6
7
coupling coefficient
% u
nd
ers
ho
ot
5th order
7th order
Effect of coupling on the magnitude response
0 0.1 0.2 0.3 0.4 0.5 0.60
2
4
6
8
10
12
coupling coefficient
de
gra
da
tio
n in
att
en
ua
tio
n a
t 2
0G
Hz [
dB
]
5th order
7th order
Cancelling the effect of coupling between adjacent coils
M24
L2 L4 L2+M24 L4+M24
-M24
M24
L2 L4 L2+M24 L4+M24
-M24Lc3=M24
Lc3=M24
Cancelling the effect of coupling between adjacent coils