Sudheer Vemulapalli

All-digital PLL and transmitter for mobile phones

We present the first all-digital PLL and polar transmitter for mobile phones. They are part of a single-chip GSM/EDGE transceiver SoC fabricated in a 90 nm digital CMOS process. The circuits are architectured from the ground up to be compatible with digital deep-submicron CMOS processes and be readily integrateable with a digital baseband and application processor. To achieve this, we exploit the new paradigm of a deep-submicron CMOS process environment by leveraging on the fast switching times of MOS transistors, the fine lithography and the precise device matching, while avoiding problems related to the limited voltage headroom. The transmitter architecture is fully digital and utilizes the wideband direct frequency modulation capability of the all-digital PLL. The amplitude modulation is realized digitally by regulating the number of active NMOS transistor switches in accordance with the instantaneous amplitude. The conventional RF frequency synthesizer architecture, based on a voltage-controlled oscillator and phase/frequency detector and charge-pump combination, has been replaced with a digitally controlled oscillator and a time-to-digital converter. The transmitter performs GMSK modulation with less than 0.5/spl deg/ rms phase error, -165 dBc/Hz phase noise at 20 MHz offset, and 10 /spl mu/s settling time. The 8-PSK EDGE spectral mask is met with 1.2% EVM. The transmitter occupies 1.5 mm/sup 2/ and consumes 42 mA at 1.2 V supply while producing 6 dBm RF output power.

1.3 V 20 ps time-to-digital converter for frequency synthesis in 90-nm CMOS

We propose and demonstrate a 20-ps time-to-digital converter (TDC) realized in 90-nm digital CMOS. It is used as a phase/frequency detector and charge pump replacement in an all-digital phase-locked loop for a fully-compliant Global System for Mobile Communications (GSM) transceiver. The TDC core is based on a pseudodifferential digital architecture that makes it insensitive to nMOS and pMOS transistor mismatches. The time conversion resolution is equal to an inverter propagation delay, which is the finest logic-level regenerative timing in CMOS. The TDC is self calibrating with the estimation accuracy better than 1%. It additionally serves as a CMOS process strength estimator for analog circuits in this large system-on-chip. Measured integral nonlinearity is 0.7 least significant bits. The TDC consumes 5.3 mA raw and 1.3 mA with power management from a 1.3-V supply.

All-digital PLL and GSM/EDGE transmitter in 90nm CMOS

A 1.2V 42mA all-digital PLL and polar transmitter for a single-chip GSM/EDGE transceiver is implemented in 90nm CMOS. It transmits GMSK with 0.5/spl deg/ rms phase error and achieves -165dBc/Hz phase noise at 20MHz offset, with 10 /spl mu/s settling time. A digitally controlled 6dBm class-E PA modulates the amplitude and meets the EDGE spectral mask with 3.5% EVM.

The First Fully Integrated Quad-Band GSM/GPRS Receiver in a 90-nm Digital CMOS Process

We present the receiver in the first single-chip GSM/GPRS transceiver that incorporates full integration of quad-band receiver, transmitter, memory, power management, dedicated ARM processor and RF built-in self test in a 90-nm digital CMOS process. The architecture uses Nyquist rate direct RF sampling in the receiver and an all-digital phase-locked loop (PLL) for generating the local oscillator (LO). The receive chain uses discrete-time analog signal processing to down-convert, down-sample, filter and analog-to-digital convert the received signal. A feedback loop is provided at the mixer output and can be used to cancel DC-offsets as well to study linearization of the receive chain. The receiver meets a sensitivity of -110 dBm at 60mA in a 1.4-V digital CMOS process in the presence of more than one million digital gates

A 24mm 2 Quad-Band Single-Chip GSM Radio with Transmitter Calibration in 90nm Digital CMOS

The RF transceiver is built on the Digital RF Processor (DRP) technology. The ADPLL-based transmitter uses a polar architecture with all-digital PM-FM and AM paths. The receiver uses a discrete-time architecture in which the RF signal is directly sampled and processed using analog and DSP techniques. A 26 MHz digitally controlled crystal oscillator (DCXO) generates frequency reference (FREF) and has a means of high-frequency dithering to minimize the effects of coupling from digitally controlled PA driver (DPA) to DCXO by de-sensitizing its slicing buffer.

A 0.8mm 2 all-digital SAW-less polar transmitter in 65nm EDGE SoC

EDGE is currently the most widely used standard for data communications in mobile phones. Its proliferation has led to a need for low-cost 2.5G mobile solutions. The implementation of RF circuits in nanoscale digital CMOS with no or minimal process enhancements is one of the key obstacles limiting the complete SoC integration of cellular radio functionality with digital baseband. The key challenges for such RF integration include non-linearity of devices and circuits, device mismatches, process parameter spread, and the increasing potential for self-interference that could be induced by one function in the SoC onto another.

A discrete time quad-band GSM/GPRS receiver in a 90nm digital CMOS process

We present the receiver in the first single-chip GSM transceiver that incorporates full integration of quad-band receiver, transmitter, memory, power management, dedicated ARM processor and RF built-in self test in a 90 nm digital CMOS process. The architecture uses direct RF sampling in the receiver and an all-digital PLL in the transmitter. The receive chain uses discrete-time analog signal processing to down convert, down- sample, filter and analog-to-digital convert the received signal. An auxiliary feedback is provided at the mixer output that can linearize the entire receive chain. The receiver meets a sensitivity of -110 dBm at 60 mA in a 1.4V digital CMOS process

Time-to-digital converter for RF frequency synthesis in 90 nm CMOS

We propose and demonstrate a 20 ps time-to-digital converter (TDC) in 90 nm digital CMOS. It is used as a phase/frequency detector and charge pump replacement in an all-digital PLL for a fully-compliant GSM transceiver. The TDC core is based on a pseudo-differential digital architecture that makes it insensitive to NMOS and PMOS mismatches. The time conversion resolution is equal to an inverter propagation delay, which is the finest logic-level regenerative timing in CMOS. The TDC is self calibrating with estimation accuracy better than 1%. Measured INL is 0.7 LSB. The TDC consumes 1.3 mA from a 1.3 V supply.

Spur-free all-digital PLL in 65nm for mobile phones

After the first-ever all-digital PLL (ADPLL) [1] for Bluetooth radios has proven benefits of CMOS scaling and integration, demonstrators for more challenging wireless standards have emerged [2–6]. In the ADPLL, however, the digitally-controlled oscillator (DCO) and time-to-digital converter (TDC) quantize the time and frequency tuning functions, respectively, which can lead to spurious tones and phase noise increase. As such, finite TDC resolution can distort data modulation and spectral mask at near integer-N channels, while finite DCO step size can add far-out spurs and phase noise. Also, a major underreported issue is an injection pulling of the DCO due to harmonics of the digital activity at closely-spaced frequencies, which can also create spurs. This work addresses all these problems and demonstrates RF performance matching that of the best-in-class traditional approaches.

Elimination of spurious noise due to time-to-digital converter

We propose an improved architecture of a multi-GHz all-digital phase-locked loop (ADPLL) that is free from spurious tones caused by the finite resolution of the phase detection process. These tones appear at the RF output when the synthesized frequency is very close to the integer-N multiple of the reference frequency. The phase detection in the ADPLL is performed by a time-to-digital converter (TDC), whose typical resolution of 10–30 ps is sufficient for the GSM-quality RF operation. While the TDC quantization noise does not normally produce significant phase noise degradation, the near-integer-N condition makes the loop ill-behaved such that the total quantization energy falls close to dc and will not get filtered by the loop filter. The proposed solution of randomizing the TDC quantization noise is verified through comprehensive and detailed simulations.