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Outline
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Direct conversion architecture
Time-varying DC offsets
Solutions on offset
Harmonic mixing principle
FLEX pager receiver
Individual receiver blocks
Conclusion
CMOS Direct Conversion
I
90º
Q
Pros
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Simple architecture
No image problem
No 50ohm interfaces
High integration level
Low cost, low power
Cons
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DC offsets
Flicker noise
LO leakage
Even-order distortion
Time-Varying DC Offsets
LO Leakage
Zero IF + Offset
• The offset originates from self-mixing.
• It can be as large as mV range at the mixer output.
• It varies with the environment and moving speed of
the mobile.
• The offset signal bandwidth also changes with time.
• The maximum bandwidth can be as large as kHz range.
DC offset
Frequency
Flicker noise
Frequency
Broad Band
High-pass corner
Frequency
Signal
Frequency
Offset-Free
Narrow Band
DC Offsets
Time-Varying DC Offsets
Solutions on Offset
• Autozeroing or correlated double
sampling
• AC coupling or high pass filtering
• Digital cancellation
• Double LO frequency method [ISSCC99]
• Adaptive dual-loop algorithm
combined with the mixer [RAWCON00]
• Pulse-width-modulation based
bipolar harmonic mixer [CICC97]
• Square–law based CMOS harmonic
mixer [Our work: RAWCON00]
This work
Conventional
Harmonic Mixing Principle
2flo
Square-law Based Mixer
RF
Voltage
IF
No
Coupling
Current
M3
M4
2
Voltage
LO
M1 M2
• Ideally self-mixing free.
• Traditional voltage controlled switches are replaced by
current controlled time-varying transconductances.
• Current injection is used to reduce flicker noise.
• No noise contribution from LO stage and current source.
-10
-1
-30
-40
-40
-50
Higher corner, Larger BER.
10 -2
-15
-10 -5
-10
-5
0
0
5
5
10
10
15
kHz
• Narrow band modulation
• Significant energy near DC
• High pass filtering is not viable
• DC offset problem
• Flicker noise is significant
50
200
1000
High pass corner (Hz)
10 0
E
D
10 -1 Offset / Signal
A: 0.2
B: 0.4
-2
C: 0.6
10
D: 0.8
E: 1.0
BER
[dB]
dB
-20
-20
C
B
A
C
DC offset effect
4
8
12
Eb/N0 (dB)
B
A
16
DC Offset Effect
00
-60
-60
A: Zero Offset
B: 1e-7 Offset
C: 2e-7 Offset
BER @ 12dB Eb/N0
FLEX 6400, 4FSK
High pass effect
Difficulties in FLEX Pager
4-FSK Pager Receiver
• Harmonic mixers are used to solve time-varying DC offset.
• Peak detectors are used to cancel static DC offset.
• High front-end gain and current injection to reduce flicker noise.
LNA
• Non-quasi-static
phenomenon makes
it unnecessary to do
on-chip matching.
• Off-chip matching by
a single inductor and
a balun.
• |S11|<-20dB @
930MHz
• Both on-chip and offchip inductive loads
were tried.
Double Balanced Mixer
Improve the linearity; Provide constant impedance to LNA;
Current injection provides more than 20dB flicker noise reduction.
Ring Oscillator
It provides 45º phase shift.
AGC
• Gain: -14.5dB~18.6dB.
• The linear resistor R0 is used to improve the linearity.
• The signal level is sensed by the peak detector.
Static DC Offset Cancellation
Zero-IF
4-FSK
Signal
Peak
Detector
Fmin>200Hz
LPF
0
Gain [dB]
-20
-40
-60
-80
-100
0
20
10
Frequency [kHz]
• 5th order elliptic gyrator-C filter
• Pass-band gain –6.2dB, ripple ≤ 0.5dB (≤ 9kHz)
• Stop-band attenuation ≥ 63dB (≥17.8kHz)
30
4-FSK Demodulator
Modified zero-intermediate frequency
zero-crossing demodulator (ZIFZCD)
Clock Recovery Circuit
VCO is a source-coupled multi-vibrator.
Measured Demodulated Signal
The measured
asynchronous/synchronized
direction signal.
The measured
asynchronous/synchronized
speed signal.
The function of demodulator was verified.
Die Photo
RF Front-End
Mixer
Mixer
AGC
LPF
Base Band Circuitry
RF Front-End
Conclusion
• Feasibility of direct conversion has
been demonstrated.
• Proposed harmonic mixing technique
solves self-mixing induced DC offset
problem successfully.
• With the help of static DC offset
cancellation, the total DC offset is
less than 1mV at the receiver output.
• The modified ZIFZCD 4-FSK
demodulator functions correctly.
• A 4-FSK FLEX pager receiver in a
single chip has been implemented.