Spur-free Dynamic Range Research Articles

A 12-Bit Dynamic Tracking Algorithm-Based SAR ADC With Real-Time QRS Detection

A 12-bit successive approximation register (SAR) ADC based on dynamic tracking algorithm and a real-time QRS-detection algorithm are proposed. The dynamic tracking algorithm features two tracking windows which are adjacent to prediction interval. This algorithm is able to track down the input signal's variation range and automatically adjust the subrange interval and update prediction code. QRS-complex detection algorithm integrates synchronous time sequential ADC and realtime QRS-detector. The chip is fabricated in a standard 0.13 μm CMOS process with a 0.6 V supply. Measurement results show that proposed ADC exhibits 10.72 effective number of bit (ENOB) and 79.63 dB spur-free dynamic range (SFDR) at 10k Hz sample rate given 41.5 Hz sinusoid input. The DNL and INL are bounded at -0.6/0.62 LSB and -0.67/1.43 LSBs. The ADC achieves FoM of 48 fJ/conversion-step at the best case. Also, the prototype is experimented with ECG signal input and extracts the heart beat signal successfully.

A High‐linearity Input‐Buffer with high output common‐mode stability for 10bit 3.2GSs ADC

A high-linearity input-buffer with high output common-mode stability for 10-bit 3.2 GS/s ADC is proposed in this Letter. The buffer is mainly composed of a source follower for strong driving ability. To enhance its linearity, a feed forward signal path from input to output is proposed. A replica buffer-based common-mode feedback is designed to achieve high output common-mode stabilisation of input-buffer. The prototype is implemented in 40 nm CMOS process. The output common-mode variation is reduced from 200 to 1 mV. Input-buffer consumes 96 mW in 3.2 GHz sampling rate and achieves 69.3 dB spur-free dynamic range at 1581 MHz input frequency.

A Linear Microwave Electro-Optic Front End With SiGe Distributed Amplifiers and Segmented Silicon Photonic Mach–Zehnder Modulator

The dynamic range of a radio-over-fiber (RoF) link is fundamentally limited by the linearity of electro-optic modulators. Silicon photonic (SiP) processes offer significant potential for integration, but the plasma dispersion effect impacts the realizable spur-free dynamic range (SFDR). This article analyzes the underlying source of nonlinearity in SiP Mach–Zehnder modulator (MZM) and demonstrates fundamental limitations on intermodulation distortion. A SiGe low-noise distributed driver is co-designed for the SiP MZM to linearize and extend the bandwidth over which the SFDR remains high. The RoF transmitter achieves 1–20 GHz bandwidth and an SFDR of 109 dB $\cdot $ Hz2/3 at 11 GHz. To the best of our knowledge, this is the highest SFDR demonstrated for a SiP technology without incorporating predistortion.

Predictive SAR ADC with two-step loading technology for energy reduction

This paper presents a novel two-step loading technology to cut down the analog-to-digital converter (ADC) loading phase energy consumption when prediction algorithms is employed in quantization. By loading initial guess code’s logical 1 and logical 0 in the sequence of time, proposed technology can qualitatively reduce the charge variation between capacitors’ top- and bottom-plate. Consequently, the energy produced by loading phase is partly saved. Also, to verify the proposed method, a 12-bit Vcm-based predictive successive approximation register (SAR) ADC predicting higher 7 bits is designed in 0.13 μm CMOS process with a 0.6 V supply. Given a full-scale ECG signal, the ADC achieves 24.76% loading phase energy reduction in average. Given a 10 Hz full-scale sinusoid signal and a slope signal, the ADC saves 24.5% and 24.45% loading phase energy in average, respectively. Further, given a 41.5 Hz full-scale sinusoid signal, the proposed ADC exhibits 11.76 effective number of bit (ENOB) and 86.5 dB spur-free dynamic range (SFDR) at 10k Hz sample rate.

RF Waveform Synthesizers With Quantum-Based Voltage Accuracy for Communications Metrology

We review our development of Josephson-junction-based programmable reference sources to synthesize spectrally pure waveforms with quantum-based voltage accuracy for characterizing and improving future communication devices and systems. The goal is to provide sources that together span the entire radio frequency band, equivalent to seven orders of magnitude in frequency (20 kHz to 300 GHz). To synthesize waveforms in the microwave frequency band, we are extending the existing Josephson arbitrary waveform synthesizer (JAWS) technology used for ac voltage metrology at audio frequencies (<;20 kHz). Using new microwave circuits and experimental techniques, we demonstrate JAWS-based synthesis of single-tone waveforms at 1 GHz, with -49 dBm output power and 79 dBc spur-free dynamic range (SFDR). To synthesize waveforms in the millimeter-wave frequency band, we are exploiting single flux quantum (SFQ) superconducting digital technology to design circuits that amplify SFQ pulses using a network of pulse splitters. With our initial SFQ-based circuit, we demonstrate synthesis of a 4-GHz sine wave with -74 dBm output power and 70 dBc SFDR.

A linear magnetic flux-to-voltage transfer function of a differential DC SQUID

A superconducting quantum interference device with differential output or ‘DSQUID’ was previously proposed for operation in the presence of large common-mode signals. The DSQUID is the differential connection of two identical SQUIDs. Here we show that besides suppression of electromagnetic interference this device provides effective linearization of the DC SQUID voltage response. In the frame of the resistive shunted junction model with zero capacitance, we demonstrate that spur-free dynamic range of the DSQUID magnetic flux-to-voltage transfer function is higher than 100 dB while the total harmonic distortion of a signal is less than 10−3% with a peak-to-peak amplitude of a signal being a quarter of half flux quantum, 2Φa = Φ0/8. Analysis of the DSQUID voltage response stability to a variation of circuit parameters shows that DSQUID implementation allows for highly linear magnetic flux-to-voltage transformation at the cost of a high identity of Josephson junctions and high-precision current supply.

A High Spur-Free Dynamic Range Silicon DC Kerr Ring Modulator for RF Applications

Silicon photonics offer a low-cost platform for large-scale RF system integration. Spur-free dynamic range (SFDR) for analog and RF photonic components is limited by electrical and optical characteristics of the p- and n- junction used to produce plasma dispersion. We propose a silicon ring modulator and demonstrate design conditions that balance the phase change between the DC Kerr and plasma dispersion effects to produce a broadband, linear electro-optical conversion. A linear phase shifter is tested within a ring modulator and an SFDR of 98 dB $\cdot$ Hz $^{2/3}$ is measured. With predistortion, the SFDR increases to 108 dB $\cdot$ Hz $^{2/3}$ . In addition, the ring supports discrete multitone modulation with 16-QAM subcarriers at a 47 Gbps data rate over a 16 GHz bandwidth.

High-Speed Cross-Correlation for Spectrum Sensing and Direction Finding of Time-Varying Signals

Cross-correlation is a promising method for highly sensitive spectrum sensing. By computing the cross-power spectrum from a pair of sensors and performing time averaging, significant reduction of noise can be achieved, even between incoherent sensors. However, this improvement requires a large number of time-averages and is thus not useful for sensing of time-varying electromagnetic environments. To alleviate this long sample-time requirement, we propose using more than two sensors to capture data and cross-correlate between all possible sensor-pair combinations. This exchanges a long sample-time requirement for more sensors to achieve a given spur-free dynamic range, thus taking advantage of a large number of easily obtained low-cost software-defined radios. We quantify the rate of change of spur-free dynamic range (SFDR) due to attenuating a sensor as well as the rate of change of SFDR for collections of sensors. Then, we show that the proposed approach is effective in detecting time varying signals. Finally, we detail a modification to the method which allows for time-varying, angle-dependent sensing, and results in a new approach for direction finding based upon cooperative cross-correlation.

A new adaptive blind background calibration of gain and timing mismatch for a two-channel time-interleaved ADC

This study investigates a new adaptive blind calibration structure for the calibration of gain and timing mismatch error, which is applied for a two-channel Time-Interleaved Analog-to-Digital Converter (TI-ADC). This technique calibrates the mismatch error in the output of the converter without any interrupt at the typical performance of the TI-ADC. The output of the time-interleaved ADC is approximated by modeling based on Taylor's series. The effect of gain and timing mismatch in the output is represented by the first order of Taylor's series. By using a Least-Mean Square (LMS) and correlation-based algorithm, the approximate coefficients of the Taylor's series are identified. Input signal and its chopped image or input signal and its chopped and delayed image will be correlated in the proposed algorithm. This algorithm can identify the coefficients blindly. It is simpler than the previous similar techniques. Also, this technique does not have the drawbacks of a technique which uses the Taylor's series approximation and filtered-X LMS algorithm. Simulation results confirm these claims and show that by entering six sinusoidal inputs in the whole bandwidth of the proposed technique, 48.2 dB improvement in Spur Free Dynamic Range (SFDR) is achieved.

A Dynamic Tracking Algorithm Based SAR ADC in Bio-Related Applications

This paper presents a novel dynamic tracking algorithm to boost the energy quality of successive approximation register analog-to-digital converter (ADC) in bio-related applications. By introducing two tracking windows that are adjacent to prediction interval, the proposed algorithm can track down the input signal’s variation range, automatically adjust the subrange interval, and then update prediction code. The 12-bit ADC is designed in a 0.13- $\mu \text{m}$ CMOS process with a 0.6-V supply. Given a 41.5-Hz sinusoid signal, the proposed ADC exhibits 11.77 effective number of bit and 85.4-dB spur-free dynamic range at 10-kHz sample rate with a power consumption of 960 nW. Given an electrocardiogram (ECG) signal with baseline drift and vibrations, the ADC achieves 94.42% prediction accuracy. Also, dynamic tracking method provides an approach of ECG feature extraction in time domain.

Linearity optimizations of analog ring resonator modulators through bias voltage adjustments

The linearity of ring resonator modulator (RRM) in microwave photonic links is studied in terms of instantaneous bandwidth, fabrication tolerances, and operational bandwidth. A proposed bias voltage adjustment method is shown to maximize spur-free dynamic range (SFDR) at instantaneous bandwidths required by microwave photonic link (MPL) applications while also mitigating RRM fabrication tolerances effects. The proposed bias voltage adjustment method shows RRM SFDR improvement of ∼5.8 dB versus common Mach–Zehnder modulators at 500 MHz instantaneous bandwidth. Analyzing operational bandwidth effects on SFDR shows RRMs can be promising electro-optic modulators for MPL applications which require high operational frequencies while in a limited bandwidth such as radio-over-fiber 60 GHz wireless network access.

Forward bias operation of silicon photonic Mach Zehnder modulators for RF applications.

In this paper, we demonstrate that forward bias (+0.9V) of a high-speed silicon (Si) optical Mach-Zehnder modulator (MZM) increases the radio-frequency (RF) link gain by 30 dB when compared to reverse bias operation (-8V). RF applications require tunable, narrowband electro-optic conversion with high gain to mitigate noise of the optical receiver and realize high RF spur-free dynamic range. Compared to reverse bias, the forward bias gain rolls off more rapidly but offers higher RF link gain improvement of more than 13.2 dB at 20 GHz. Furthermore, forward bias is shown to result in comparable spurious-free dynamic range (SFDR: 104.5 dB.Hz2/3). We demonstrate through an analytical dc transfer curve the existence of simultaneous high gain and OIP3 and verify the theoretical results with measurement under forward bias at a bias point of around +0.9 V.

A 1.8 V 3 GS/s 7-bit time-interleaved Quasi C-2C SAR ADC using voltage-comparator time-information

This paper presents a 7-bit 15×interleaved SAR ADC that operates up to 3GS/s, using 180nm CMOS technology. The ADC utilizes the transient information of a dynamic SAR voltage-comparator to resolve 2bits per clock cycle, using a time-comparator block. Thus, only 5 clock cycles are needed to resolve 7bits. This results in speed improvement of about 60%, compared to conventional ADC. Also, an improved Quasi C-2C DAC structure with reduced internal node swing and reduced switching activity are utilized, which decreases the power consumption of DAC up to 65%. We employ the above techniques in designing a 7-bit SAR ADC, in which 3bits are resolved with time-comparator blocks and 4bits are resolved with a voltage-comparator. To calibrate the proposed time-comparator block, a calibration process is proposed. ADS simulation of the ADC illustrates an ENOB (Effective Number of Bits)>6.5-bit and SFDR (Spur Free Dynamic Range)=−52.8 dBc for a single SAR converter with sampling at 200MS/s. For the time-interleaved SAR ADC with 15 single SAR converters, ENOB is 6.15-bit and SFDR=−45 dBc with sampling at 3GS/s up to Nyquist frequency. This ADC consumes 150mW at 1.8V supply and achieves a Figure-of-Merit (FoM) of 700 fJ/conv-step.

Broadband high-resolution microwave frequency measurement based on low-speed photonic analog-to-digital converters.

An approach to microwave frequency measurement with a high resolution and a broad bandwidth is proposed based on three parallel low-speed photonic analog-to-digital converters (ADCs) architecture. Through simultaneously bandpass sampling the input microwave signal with the three photonic ADCs, the input frequency can be calculated from the Fourier frequencies of the photonic ADCs using the proposed frequency recovery algorithm. Theoretical analysis and simulation results indicate that the proposed method is applicable for both single-tone and multi-tone microwave signals. By employing three ~1 GS/s@8 bits photonic ADCs, 0-100 GHz frequency measurement with an error of ± 0.5 MHz and a spur-free dynamic range of 94 dB-Hz2/3 over the full band has been numerically demonstrated. Additionally, a proof-of-concept experiment is carried out to demonstrate the effectiveness of the proposed method, where a frequency measurement range of 0-20 GHz with a measurement error of ± 8 kHz is realized by utilizing three photonic ADCs with sampling rates of 27.690 MS/s, 27.710 MS/s, and 27.730 MS/s. Larger frequency measurement range can be achieved by using an optical modulator with a larger bandwidth.

Highly linear dual ring resonator modulator for wide bandwidth microwave photonic links.

A highly linear dual ring resonator modulator (DRRM) design is demonstrated to provide high spur-free dynamic range (SFDR) in a wide operational bandwidth. Harmonic and intermodulation distortions are theoretically analyzed in a single ring resonator modulator (RRM) with Lorentzian-shape transfer function and a strategy is proposed to enhance modulator linearity for wide bandwidth applications by utilizing DRRM. Third order intermodulation distortion is suppressed in a frequency independent process with proper splitting ratio of optical and RF power and proper dc biasing of the ring resonators. Operational bandwidth limits of the DRRM are compared to the RRM showing the capability of the DRRM in providing higher SFDR in an unlimited operational bandwidth. DRRM bandwidth limitations are a result of the modulation index from each RRM and their resonance characteristics that limit the gain and noise figure of the microwave photonic link. The impact of the modulator on microwave photonic link figure of merits is analyzed and compared to RRM and Mach-Zehnder Interference (MZI) modulators. Considering ± 5 GHz operational bandwidth around the resonance frequency imposed by the modulation index requirement the DRRM is capable of a ~15 dB SFDR improvement (1 Hz instantaneous bandwidth) versus RRM and MZI.

High power and high SFDR frequency conversion using sum frequency generation in KTP waveguides.

We characterize the intermodulation distortion of high power and efficient frequency conversion of modulated optical signals based on sum frequency generation (SFG) in a periodically poled potassium titanyl phosphate (KTP) waveguide. Unwanted frequency two-tone spurs are generated near the converted signal via a three-step cascaded three-wave mixing process. Computer simulations describing the process are presented along with the experimental measurements. High-conversion efficiencies and large spur-free dynamic range of the converted optical signal are demonstrated.

Injection-locked semiconductor laser-based frequency comb for modulation applications in RF analog photonics.

A linearized intensity modulator for periodic and pulsed light is proposed and demonstrated. The free carrier plasma effect has been used to modulate the refractive index of the phase section of a three-section mode-locked laser. If injection locked, the modulation induces an arcsine phase response on the three-section mode-locked laser. By introducing this mode-locked laser into a Mach-Zehnder interferometer biased at quadrature, one can realize a true linear intensity modulation. This novel laser suppresses any unwanted amplitude modulation and increases the performance of the linearized intensity modulator. Experimental results have provided a record low static Iπ of 0.39 mA and a spur-free dynamic range of 75 dB.Hz2/3.

Broadband linearization in photonic time-stretch analog-to-digital converters employing an asymmetrical dual-parallel Mach-Zehnder modulator and a balanced detector.

A broadband linearization scheme for time-stretch analog-to-digital converters (TS-ADCs) is proposed based on an asymmetrical dual-parallel Mach-Zehnder modulator and a balanced detector. The theoretical and simulation results indicate that, compared with the differential and arcsine operation method generally employed in TS-ADCs, the proposed scheme has a superior performance on enhancing the spur-free dynamic range and suppressing the even-order distortions and the third-order spurs even under a large modulation depth. Additionally, the proposed scheme realizes online linearization. Therefore, it has the potential to enhance the dynamic range of a broadband TS-ADC in real time.

Radio-over-fiber transmission using semiconductor laser under weak external light injection

Optical injection locking is a promising method to overcome the limitations exhibited by directly modulated distributed feedback lasers in analog radio-over-fiber applications. Here the effects of optical injection locking are investigated experimentally for relatively low injection ratio, which is favored for practical applications. The measured responses of optical injection locking with low injection ratio still show distinctive modulation characteristics, controllable by the frequency detuning and the injection ratios. At the higher positive detuning the resonance peak can extend to 22 GHz (limited only by the analyzer instrument), which is 4.2 times greater than in the free running case. Reduction in the noise level becomes better as the system moves towards negative detuning. The reduction reaches to 5.97 dB at the resonance peak for −20 GHz detuning and a −2.9 dB injection ratio. Moreover, reduction also can be realized in the low frequency regime 0–10 GHz. We also achieved a 9.1 dB spur free dynamic range improvement. The error vector magnitude requirement specification of 3.1 % for 64-QAM OFDMA can be satisfied by an injection locked laser with 5–10 dB enhancements, comparing to a free running laser even after transmission over different transmission distances.

Broadband Compensation of Dispersion in APL Using OPC Based on DFB Semiconductor Lasers

A broadband chromatic dispersion (CD) compensation scheme using optical phase conjugation (OPC) based on a DFB semiconductor laser is proposed and experimentally demonstrated in analog photonic links (APLs). The DFB laser acts as a nonlinear medium to induce four-wave mixing and a pump laser simultaneously. Compared with the conventional OPC based on a semiconductor optical amplifier or a dispersion-shifted fiber, an extra pump laser is no longer required. Experimental results show that the power fading caused by the CD in a 50.4-km APL can be compensated with a 3-dB bandwidth up to 33 GHz. Meanwhile, the spur-free dynamic range is enhanced with 12.6 dB · Hz 2/3 ; the transmission performance is also improved.