CMOS RFIC Design for Direct Conversion Receivers Zhaofeng ZHANG

Outline of Presentation Background Introduction Design Issues and Solutions A Direct Conversion Pager Receiver Conclusion

Research Goal Low Cost –Process: CMOS Device is good enough Improved passive components –Integration level Minimize external components Minimize IC area and pin numbers Low Power –High integration = low power –Low power individual block design –System architecture is important

Heterodyne Receivers High IF: more than 2 down-conversions –Best sensitivity –Need off-chip image-rejection SAW filters and channel-selection filters –Highest cost, high power, low integration Low IF –Relaxed image-rejection requirement compared to high-IF –No DC offset problem –Quadrature LO is required –Flicker noise may be a problem –High integration level, low cost

Homodyne Receivers Simple architecture No image problem No 50ohm interfaces High integration level Lowest cost, low power  DC offsets  Flicker noise  LO leakage  Even-order distortion Pros Cons 90º I Q LNA

Origin of Problem  DC offsets  Flicker noise  LO leakage  Even-order distortion  Linearity requirement  Noise requirement  IQ mismatch The mixer: the most critical component! All problems are limited by the mixer design! Our research focus!

DC Offsets & LO Leakage + 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 and changes with time. The maximum bandwidth can be as large as kHz range. LO leakage forms an interference to other receivers. LO Leakage Zero IF

Power Narrow BandBroad Band Frequency Power Frequency Power Frequency Power Frequency Signal DC Offsets Offset-Free DC offset Spectrum Illustration Flicker noise High-pass corner

Existing Solutions on DC Offset AC coupling or high pass filtering Autozeroing or double sampling Offset cancellation in digital domain Double LO frequency method [ISSCC99] Adaptive dual-loop algorithm combined with the mixer [RAWCON00] Pulse-width-modulation based bipolar harmonic mixer [CICC97] However, these methods are either not so effective or too complicated, or not suitable for CMOS process.

Proposed Harmonic Mixing Conventional Our Work f lo =f rf RF Signal f rf BB Signal 0 2f lo =f rf RF Signal f rf BB Signal 0 LO LeakageDC Offset LO Leakage f lo =f rf /2 f lo

Square-law Based Mixer LO leakage free. Ideally self-mixing free. Current controlled switching. No noise contribution from LO stage. LO 2 RFIF Current Voltage Coupling No Vlo+ Vlo- Vrf+ Vrf- 3V3V

Flicker Noise Reduction Flicker noise is proportional to the current. Current injection is used to reduce flicker noise. No noise contribution from current source too. Vrf+ I0I0 Vlo+ Vlo- Vrf- 3V3V

Offset Cancellation LO Input Power (dBm) Gain (dB) > 35dB TSMC0.35 

Noise Performance Injected Current I 0 (  A) Noise 10kHz (dB)

How to improve more? However, flicker noise is still too large due to CMOS devices, minimum noise figure achieved is larger than 10kHz for CMOS harmonic mixer. It requires a high gain and low noise LNA to overcome flicker noise while the front-end linearity suffers. For a narrow-band communication system such as FLEX pager, the noise requirement at low frequency is very tough. It is well known that bipolar device is a good candidate to eliminate flicker noise. But, can we do it in a CMOS process and how good is the device? YES!

Lateral Bipolar Transistor in a Bulk CMOS Process W.T. Holman95 Gate Emitter Collector Base Ground P+N+ Emitter Vertical Collector Lateral Base Gate

Physical Model of LBJT D. Mac98 P-Sub Gate EmitterCollector Base P-Sub M1 Q1 Q2Q3 Pure LBJT: M1, Q3 off, Q1, Q2 on.

Gummel Plot of LBJT TSMC0.35   >40 at mAs max f T  4GHz

LBJT Harmonic Mixer RL OUT-OUT+ VRF+ VRF- IiIi VLO- VLO+M1M2 VDD Q1 Q2

Noise Performance Large LO improves noise.

Even Order Distortion RF Signal f rf BB Signal 0 InterferenceIM2 (f 2 -f 1 ) It is mainly introduced by layout asymmetry and device mismatch. Since direct-conversion, the intermodulation components IM2 will fall into the demodulated signal spectrum. Therefore, good IIP2 is required for homodyne receivers. It is found that varying the loading resister or voltage bias can compensate the device mismatch and improve IIP2 significantly. a 1 x+a 2 x 2 +a 3 x 3 +… f1f1 f2f2

IIP2 Improvement IIP2=18dBm IIP2>40dBm Same DC biasCompensation

LBJT Mixer Performance Technology TSMC 3M2P 0.35  m VDD3V3V Signal Gain+15dB DC offset suppression>30dB Noise 10kHz<18dB 1dB compression point>-20dBm Input-referred IP3>-9dBm Input-referred IP2>+40dBm Power consumption<2.2mW

Summary on Mixer Flicker noise free, corner frequency is below 10kHz. DC offset free, more than 30dB DC offset suppression is achieved. No LO leakage problem. Sufficient IIP2 after bias compensation. High gain and low power consumption. Complete CMOS process. Suitable for CMOS direct conversion applications.

Difficulties in FLEX Pager FLEX 6400, 4FSK dB kHz Narrow band modulation Significant energy near DC High pass filtering is not viable DC offset problem Flicker noise is significant BER Eb/N0 (dB) DC Offset Effect High pass effect High pass corner (Hz) 12dB Eb/N Big Challenges

4-FSK Pager Receiver Fully differential architecture to reject substrate noise. 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. 45  AGC LNA DEMOD VCO RF: Zhaofeng BB: Zhiheng

LNA Non-quasi-static phenomenon makes it unnecessary to do on-chip matching. Off-chip matching by a single inductor and a balun. 930MHz Both on-chip and off-chip 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 Half RF frequency, Provide 45  phase.

Static DC Offset Cancellation Peak Detector Fmin  200Hz Zero-IF 4-FSK Signal

Performance Summary Pager receiver with off-chip ind Maximum Gain:62dB Noise 14.5dB Overall DC offset at LPF output: <1mV (Signal: 400mV) Power dissipation: 58mW Technology: TSMC0.35  m 4M2P Die area: 4.6 mm 2 Front-End Off-chip ind On-chip ind RF/BB gain: 51.13dB 40.33dB 11.5dB 24.0dB 5.8dB 15.0dB IIP3: -26dBm -20.7dBm IIP2: -10dBm -5.6dBm Operating frequency: 930.1MHz LO frequency: 465MHz IQ gain mismatch: < 0.3dB IQ phase mismatch: < 5  RF/BB over LO/BB: > 54dB Self-mixing free Input matching: < -20dB Power dissipation: 52.76mW Baseband (Zhiheng) AGC gain:-14.5dB~18.6dB LPF: Pass-band gain-6.2dB, ripple .5dB (  9kHz) Stop-band attenuation  63dB (  17.8kHz) Offset cancellation: <2mV (under ±100mV input offset) Input Referred Noise: 600 n 10kHz Clock Recovery: Capture range > 550Hz Power dissipation: 5.4mW (including all testing buffers)

Die Photo DEMOD LPFAGCMixer OSC LNA OSC Mixer Base Band Circuitry [Zhiheng] RF Front-End 45  AGC LNADEMOD VCO

Summary on Pager Receiver 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 successfully.

Conclusion Circuit design for direct-conversion has been discussed. –DC offset: more than 30dB improvement –LO leakage: no longer a problem –Flicker noise: corner frequency is less than kHz due to lateral bipolar device. –IIP2: larger than +40dBm after bias compensation. System on chip has been successfully demonstrated using CMOS direct conversion architecture.