Digital Quadrature Demodulation Research Articles

Aliasing Effect on Flux Ramp Demodulation: Nonlinearity in the Microwave Squid Multiplexer

A novel system formed by a Microwave Superconducting Quantum Interference Device (SQUID) Multiplexer (muMUX) and a room temperature electronics employs frequency division multiplexing (FDM) technique to read out multiple cryogenic detectors. Since the detector signal is embedded in the phase of the SQUID signal, a Digital Quadrature Demodulator (DQD) is widely implemented to recover it. However, the DQD also generates a signal that aliases into the first Nyquist zone affecting the demodulated detector signal. In this work, we demonstrate how this spurious signal is generated and a mathematical model of it is derived and validated. In addition, we discuss different proposals to improve the attenuation of this undesired signal. Lastly, we implement one of the proposals in our readout system. Our measurements show an enhancement in the spurious signal attenuation of more than 35 dB. As a result, this work contributes to attenuate the spurious below the system noise.

A low computation detection method of signal nonlinear distortion based on digital quadrature detection principle

This paper presents a digital low computation detection method for signal nonlinear distortion. After presetting ADC sampling rate and completing data sampling, the low sampling rate data extracted from the sampling data complete the autocorrelation analysis to correct and calculate the accurate fundamental frequency. According to the relationship between the fundamental frequency and the sampling rate, the complete periodic segment data of the signal cutting out from the sampling data carry out the digital quadrature detection operation to obtain the value of each harmonic component and nonlinear distortion. Experiments show that the calculation of this method is less than that of FFT spectrum analysis method when measuring the THD lower than 15 order. When calculating the 15-order THD at 1024 points, the calculation time is reduced by 44.26%. Moreover, the digital quadrature detection method has high measurement accuracy with less than 0.6% error of detecting 5-order THD.

Synthetic dispersion interferometry for relative atmospheric pressure sensing.

We present a modified version of the two-arm, two-color, single second harmonic generation heterodyne dispersion interferometer, as introduced by Irby et. al. [Rev. Sci. Instrum.70, 699 (1999) 10.1063/1.1149489]. The amount of optical elements is reduced and digital in-phase and quadrature demodulation is used to retrieve the phase shift from a single photodetector signal. The intrinsic system noise and drift for this device are analyzed by measuring the Allan deviation. We investigate the use of this device for relative atmospheric pressure measurement. Relative pressure measurements are performed in a pressure chamber and referenced against a piezoresistive pressure transceiver to demonstrate the concept. It was found that the deviation was less than 150 Pa and an error estimation has been derived.

Continuous wave desktop coherent superheterodyne X-band EPR spectrometer

This article describes a desktop X-band superheterodyne spectrometer designed to measure EPR spectra in the vicinity of g = 2. The spectrometer operates in the vicinity of the 9.2 GHz frequency in the power range of about 40 mW–1 nW. The device uses Time Locking concept with digital quadrature detection of intermediate frequency (100 MHz) and the synthesis of all signals used in the device from a single driving oscillator. To register CW spectra, a TE102 rectangular cavity with the capability of matching and frequency tuning is used. The microwave part of the spectrometer is implemented in the form of two non-tunable modules based on microstrip technology and commercially available microwave MMICs. Two modes of operation are provided – both with PLL of microwave sources with a highly stable driving oscillator as a reference and an AFC system without frequency modulation. All signal processing after digitization of the intermediate frequency is performed digitally using FPGA resources. It is possible to record EPR spectra both with magnetic field modulation at various frequencies and without modulation. The bandwidth of the receiving path can be selected from the widest 46 MHz to about 1.5 kHz, which allows the spectrometer to be used both for recording weak signals and for recording fast-flowing processes. The spectrometer uses a magnetic system on permanent magnets Sm-Co with magnetic field uniformity of about 3 * 10−5. Changing the magnetic field in permanent magnets is done using scanning coils which provides a field change of >800 G (p-p). The control of the magnetic field induction in the magnet gap is performed by a specially designed precision Hall effect magnetometer, which provides a resolution of 1 mG with an absolute error of no more than 0.2 G. A personal computer for spectrometer control is connected via Ethernet. The spectrometer is stable and easy to use. The high performance of the spectrometer is confirmed by the recorded spectra.

A Novel Digital IQ Demodulation for Interferometric Radiometers

In the frame of an SMOS follow-on operational mission, a new instrument design is being developed based on the lessons learned from MIRAS, the SMOS payload. To reduce hardware complexity and mass, digital In-phase Quadrature (IQ) demodulation is considered. In this schema, Q components are obtained by delaying one clock of the digitized IF signals instead of using phase quadrature analog mixers. The purpose of this article is to formulate this concept for application to interferometric radiometry, establish the required data processing methods, and provide experimental results.

An FPGA-based Direct Sampling and Digital Processing System for Wideband and Narrowband Radar Signal

This paper designs an integrated wideband and narrowband radar signal direct sampling and digital processing system based on FPGA. The system comprises a VPX case, a main board, a direct sampling board, a fiber board. It makes direct sampling of 1.8GHz-3.0GHz echo waves, with 3.2GHz sampling rate. The wideband and narrowband echo waves share the same ADC channel. After ADC sampling, both of wideband and narrowband data are exported to a mix-free digital quadrature demodulation module in FPGA. The narrowband data will be further processed by digital frequency down conversion. The wideband data will be further processed by digital de-chirp operation, with parallel DDS structure and multi-level decimation filters. Data rate of both the wideband and narrowband data after processing is decreased obviously. The designed sampling system cut down the analog radar signal processing operation, and decreases signal distortion. Besides, it integrated the reception, sampling and processing of wideband and narrowband signal, simplified the structure of radar system.

Secondary Nuclear Spin Echo Signals in Thin Yttrium Iron Ferrite Garnet Films

The results are presented of experimental and theoretical study of the phenomenon of secondary nuclear spin echo in magnetically ordered materials in which the formation of additional echo signals is due to dynamic hyperfine coupling. Numerical simulation of the effect of the amplitude (ω1) and the durations of the first (t1) and the second (t2) exciting pulses on the echo signals is performed. It is found that the maximum amplitude of the secondary echo is formed under the conditions ω1t1 = 0.5π and ω1t2 ≈ 0.6π. It is shown that secondary echo signals can be observed upon inhomogeneous excitation of the spectral line ω1 ≤ Δω, where Δω is the inhomogeneous spectral line width. At a temperature of T = 4.2 K, additional double-pulse spin 3τ-echo signals from iron nuclei are experimentally observed in an epitaxial yttrium ferrite garnet film enriched with 57Fe magnetic isotope to 96%. The experimentally observed phase relationships between the primary and secondary echo signals, as well as the dependence of the echo signal amplitude on the amplitude and duration of the exciting pulses, are in good agreement with the results of numerical simulation of the dynamics of nuclear magnetization with regard to the dynamic hyperfine coupling. It is shown that the secondary echo exhibits the effect of spectral line narrowing, and the amplitude of the secondary echo is proportional to the nuclear magnetic resonance (NMR) enhancement factor in magnets, η. In the case of 57Fe NMR in an yttrium iron garnet (YIG) film, the amplitude of the 3τ-echo is two to three orders of magnitude smaller than the amplitude of the primary 2τ-echo, which corresponds to η ≈ 440. The detection of weak secondary echo signals proves to be possible due to the use of a phase-coherent NMR spectrometer with digital quadrature detection at the carrier frequency and signal accumulation.

High‐speed data acquisition of the reader of the SAW RFID system

The measurement of the echo time delays and phases of the surface acoustic wave (SAW) tag by the corresponding reader should be more accurate with the increase of the coding capacity, and the design of the high-speed data acquisition module (DAM) is the key factor for the improvement of the reader performance. Aiming at the large capacity SAW tag, which combines pulse time delay and phase coding, the radio-frequency (RF) band-pass sampling and digital quadrature demodulation scheme is utilised for the reader design. The design of the high-speed DAM includes the analysis, calculation and determination of the sampling frequency, the sampling clock design with cascaded phase-locked loop structure and the driver circuit design etc. Finally, the SAW RFID system is established, and the time delays and phases of the tag reflectors are tested. The measurement results show the effectiveness of the high-speed DAM of the reader.

Sub-Nyquist SAR via Quadrature Compressive Sampling with Independent Measurements

This paper presents an efficient sampling system for the acquisition of synthetic aperture radar (SAR) data at sub-Nyquist rate. The system adopts a quadrature compressive sampling architecture, which uses modulation, filtering, sampling and digital quadrature demodulation to produce sub-Nyquist or compressive measurements. In the sequential transmit-receive procedure of SAR, the analog echoes are modulated by random binary chipping sequences to inject randomness into the measurement projection, and the chipping sequences are independent from one observation to another. As a result, the system generates a sequence of independent structured measurement matrices, and then the resulting sensing matrix has better restricted isometry property, as proved by theoretical analysis. As a standard recovery problem in compressive sensing, image formation from the sub-Nyquist measurements has significantly improved performance, which in turn promotes low sampling/data rate. Moreover, the resulting sensing matrix has structures suitable for fast matrix-vector products, based on which we provide a first-order fast image formation algorithm. The performance of the proposed sampling system is assessed by synthetic and real data sets. Simulation results suggest that the proposed system is a valid candidate for sub-Nyquist SAR.

Digital Recursive Demodulator Based on Kalman Filter

The number of samples used for demodulation determines the demodulation speed of a digital demodulator. The traditional digital quadrature demodulator requires samples covering integer signal periods for demodulation. To improve the demodulation speed and flexibility, a digital recursive demodulator based on the Kalman-filter algorithm is proposed, which can implement demodulation with samples of less than one signal period. Then, the resources consumption and time cost of the Kalman-filter-based demodulator, the widely used digital quadrature demodulator and two other digital recursive demodulators are analyzed and compared. Numerical calculations were carried out to study the measurement uncertainty and the influence of the number of samples on the signal-to-noise ratio of the demodulation result. Finally, the proposed demodulator is applied for capacitance measurement in the industrial application. Experimental results are presented to demonstrate the performance and validity of the proposed demodulator.

Noise Performance of SDR-Based Off-Diagonal GMI Sensors

In this paper, we propose a general model to evaluate the equivalent magnetic noise for an off-diagonal giant magneto-impedance (GMI) sensor utilizing a software-defined radio (SDR) as a digital receiver. Based on this SDR approach, an analog-to-digital conversion (ADC) of the voltage induced by the sensing element is directly performed. The signal processing is digitally achieved in real time. It includes a digital quadrature demodulation, a decimation, and a filtering. The approach allows to give the key rules for quantifying the white noise level of this digital implementation. The voltage noise spectral density of the ADC and of each SDR stage is given. The white noise level expected by the modeling was in good agreement with the measurements. With a sensing element composed of a 400 turn coil wound around a co-rich GMI wire, equivalent magnetic noise levels of 220 and 4.2 pT/ $\sqrt {Hz}$ were obtained at 1 Hz and in the white noise region, respectively. The potential improvement of these performances, allowed by this digital conditioning, as well as limitations is also discussed.

A multi-frequency electrical impedance tomography system for real-time 2D and 3D imaging.

This paper presents the design and evaluation of a configurable, fast multi-frequency Electrical Impedance Tomography (mfEIT) system for real-time 2D and 3D imaging, particularly for biomedical imaging. The system integrates 32 electrode interfaces and the current frequency ranges from 10 kHz to 1 MHz. The system incorporates the following novel features. First, a fully adjustable multi-frequency current source with current monitoring function is designed. Second, a flexible switching scheme is developed for arbitrary sensing configuration and a semi-parallel data acquisition architecture is implemented for high-frame-rate data acquisition. Furthermore, multi-frequency digital quadrature demodulation is accomplished in a high-capacity Field Programmable Gate Array. At last, a 3D imaging software, visual tomography, is developed for real-time 2D and 3D image reconstruction, data analysis, and visualization. The mfEIT system is systematically tested and evaluated from the aspects of signal to noise ratio (SNR), frame rate, and 2D and 3D multi-frequency phantom imaging. The highest SNR is 82.82 dB on a 16-electrode sensor. The frame rate is up to 546 fps at serial mode and 1014 fps at semi-parallel mode. The evaluation results indicate that the presented mfEIT system is a powerful tool for real-time 2D and 3D imaging.

On RF-Pulse-Phase Dependence of Nuclear Quadrupole Resonance Signal Under Short-Repetition-Time Pulse Sequences

In the most conventional pulsed nuclear quadrupole resonance (NQR) systems, radio frequency (RF) pulses for irradiation are produced by chopping a continuous RF wave with a DC pulse. Therefore, RF pulses have phase coherency but NQR signals after each RF pulse have different initial phase. In this case, a phase-sensitive detection (PSD) using the same continuous RF wave is necessary to accumulate NQR signal. Meanwhile, recent improvement of device technology enables a phase controlled RF pulse and a direct sampling of NQR signal in the laboratory frame. We developed a new pulse NQR system using phase controlled RF pulse, in which RF pulses with the same initial phase are produced and NQR signal after each pulse is directly acquired in the laboratory frame, and then the accumulated signal is processed by digital quadrature detection and fast Fourier transform. This paper provides and compares the feature of NQR signals from both types of pulse NQR system. Main findings are that NQR signal obtained by a train of phase coherent pulses and PSD shows the periodical variation of the intensity depending on the offset frequency of the irradiation RF pulse from the resonance frequency but the signal obtained by the phase controlled RF pulse train does not. This feature would be important for the application of NQR to detection system for such as explosives.

A high-field magnetic resonance imaging spectrometer using an oven-controlled crystal oscillator as the local oscillator of its radio frequency transceiver

A home-made high-field magnetic resonance imaging (MRI) spectrometer with multiple receiving channels is described. The radio frequency (RF) transceiver of the spectrometer consists of digital intermediate frequency (IF) circuits and corresponding mixing circuits. A direct digital synthesis device is employed to generate the IF pulse; the IF signal from a down-conversion circuit is sampled and followed by digital quadrature detection. Both the IF generation and the IF sampling use a 50 MHz clock. An oven-controlled crystal oscillator, which has outstanding spectral purity and a compact circuit, is used as the local oscillator of the RF transceiver. A digital signal processor works as the pulse programmer of the spectrometer, as a result, 32 control lines can be generated simultaneously while an event is triggered. Field programmable gate array devices are utilized as the auxiliary controllers of the IF generation, IF receiving, and gradient control. High performance, including 1 μs time resolution of the soft pulse, 1 MHz receiving bandwidth, and 1 μs time resolution of the gradient waveform, is achieved. High-quality images on a 1.5 T MRI system using the spectrometer are obtained.

Investigation on the maximum signal handling capability of fiber optic interferometric sensor based on the digital heterodyne demodulation scheme

The maximum signal handling capability (MSHC) is an important parameter for the fiber optic interferometric sensor (FOIS) system. It is restricted by the heterodyne frequency (HF) and the digital quadrature demodulation algorithm (DQDA) in the digital heterodyne demodulation scheme. In this paper we systematically investigate the MSHC of the FOIS based on the digital heterodyne demodulation scheme through comparison of the HF-determined MSHC and DQDA-determined MSHC. When discussing the DQDA-determined MSHC, two widely used methods are included, which are arctangent and differential-cross multiply (DCM). The comparison results reveal that the arctangent-determined MSHC is 12dB larger than the DCM-determined MSHC. The HF-determined MSHC is between the two types of DQDA-determined MSHC. If the DQDA is arctangent, the MSHC of the system is the HF-determined MSHC. If the DQDA is DCM, the MSHC of the system is the DCM-determined MSHC. Experiments are carried out and the results are consistent with the theoretical analysis. The conclusion in this paper can help us to estimate the MSHC of the FOIS system based on the digital heterodyne demodulation scheme more accurately and provide a useful guideline for the design of the FOIS system with large demodulation dynamic scale.

A DAQ-device-based continuous wave near-infrared spectroscopy system for measuring human functional brain activity.

In the last two decades, functional near-infrared spectroscopy (fNIRS) is getting more and more popular as a neuroimaging technique. The fNIRS instrument can be used to measure local hemodynamic response, which indirectly reflects the functional neural activities in human brain. In this study, an easily implemented way to establish DAQ-device-based fNIRS system was proposed. Basic instrumentation components (light sources driving, signal conditioning, sensors, and optical fiber) of the fNIRS system were described. The digital in-phase and quadrature demodulation method was applied in LabVIEW software to distinguish light sources from different emitters. The effectiveness of the custom-made system was verified by simultaneous measurement with a commercial instrument ETG-4000 during Valsalva maneuver experiment. The light intensity data acquired from two systems were highly correlated for lower wavelength (Pearson's correlation coefficient r = 0.92, P < 0.01) and higher wavelength (r = 0.84, P < 0.01). Further, another mental arithmetic experiment was implemented to detect neural activation in the prefrontal cortex. For 9 participants, significant cerebral activation was detected in 6 subjects (P < 0.05) for oxyhemoglobin and in 8 subjects (P < 0.01) for deoxyhemoglobin.

Software Defined Radio (SDR) and Direct Digital Synthesizer (DDS) for NMR/MRI Instruments at Low-Field

A proof-of-concept of the use of a fully digital radiofrequency (RF) electronics for the design of dedicated Nuclear Magnetic Resonance (NMR) systems at low-field (0.1 T) is presented. This digital electronics is based on the use of three key elements: a Direct Digital Synthesizer (DDS) for pulse generation, a Software Defined Radio (SDR) for a digital receiving of NMR signals and a Digital Signal Processor (DSP) for system control and for the generation of the gradient signals (pulse programmer). The SDR includes a direct analog-to-digital conversion and a Digital Down Conversion (digital quadrature demodulation, decimation filtering, processing gain…). The various aspects of the concept and of the realization are addressed with some details. These include both hardware design and software considerations. One of the underlying ideas is to enable such NMR systems to “enjoy” from existing advanced technology that have been realized in other research areas, especially in telecommunication domain. Another goal is to make these systems easy to build and replicate so as to help research groups in realizing dedicated NMR desktops for a large palette of new applications. We also would like to give readers an idea of the current trends in this field. The performances of the developed electronics are discussed throughout the paper. First FID (Free Induction Decay) signals are also presented. Some development perspectives of our work in the area of low-field NMR/MRI will be finally addressed.

A digital magnetic resonance imaging spectrometer using digital signal processor and field programmable gate array

A digital spectrometer for low-field magnetic resonance imaging is described. A digital signal processor (DSP) is utilized as the pulse programmer on which a pulse sequence is executed as a subroutine. Field programmable gate array (FPGA) devices that are logically mapped into the external addressing space of the DSP work as auxiliary controllers of gradient control, radio frequency (rf) generation, and rf receiving separately. The pulse programmer triggers an event by setting the 32-bit control register of the corresponding FPGA, and then the FPGA automatically carries out the event function according to preset configurations in cooperation with other devices; accordingly, event control of the spectrometer is flexible and efficient. Digital techniques are in widespread use: gradient control is implemented in real-time by a FPGA; rf source is constructed using direct digital synthesis technique, and rf receiver is constructed using digital quadrature detection technique. Well-designed performance is achieved, including 1 μs time resolution of the gradient waveform, 1 μs time resolution of the soft pulse, and 2 MHz signal receiving bandwidth. Both rf synthesis and rf digitalization operate at the same 60 MHz clock, therefore, the frequency range of transmitting and receiving is from DC to ~27 MHz. A majority of pulse sequences have been developed, and the imaging performance of the spectrometer has been validated through a large number of experiments. Furthermore, the spectrometer is also suitable for relaxation measurement in nuclear magnetic resonance field.

An FPGA-based open platform for ultrasound biomicroscopy

Ultrasound biomicroscopy (UBM) has been extensively applied to preclinical studies in small animal models. Individual animal study is unique and requires different utilization of the UBM system to accommodate different transducer characteristics, data acquisition strategies, signal processing, and image reconstruction methods. There is a demand for a flexible and open UBM platform to allow users to customize the system for various studies and have full access to experimental data. This paper presents the development of an open UBM platform (center frequency 20 to 80 MHz) for various preclinical studies. The platform design was based on a field-programmable gate array (FPGA) embedded in a printed circuit board to achieve B-mode imaging and directional pulsed-wave Doppler. Instead of hardware circuitry, most functions of the platform, such as filtering, envelope detection, and scan conversion, were achieved by FPGA programs; thus, the system architecture could be easily modified for specific applications. In addition, a novel digital quadrature demodulation algorithm was implemented for fast and accurate Doppler profiling. Finally, test results showed that the platform could offer a minimum detectable signal of 25 μV, allowing a 51 dB dynamic range at 47 dB gain, and real-time imaging at more than 500 frames/s. Phantom and in vivo imaging experiments were conducted and the results demonstrated good system performance.

Extensible electrical capacitance tomography system for gas–liquid two-phase flow

Electrical capacitance tomography (ECT) has being widely accepted as a laboratory and industry tool to analyse gas/liquid two-phase flow. For system optimisation and function extension and for getting more information of two-phase flow, a novel extensible ECT system based on field programmable gate array (FPGA) and digital signal processor (DSP) is designed. The new scheme can easily realise not only common single-frequency excitation and demodulation but also new multi-frequency excitation and demodulation. Firstly, a review of classical ECT systems is described, and function extension of our new ECT system is introduced compared to the old ECT system in our laboratory. Then, the design scheme of the new system is proposed in detail. And main parts, such as FPGA, DSP and detection circuit are discussed one by one. Meanwhile, the generation of multi-frequency signals, discrete Fourier transform (DFT) demodulation and digital quadrature demodulation of multi-frequency are described deeply. At last, static and dynamic experiments under condition of multi-frequency excitation and demodulation are carried out to verify the performance of this novel ECT. Experimental results show that this system is of high precision and stability in multi-frequency model. And there is still a future of further extension for this ECT system.