Broadband RF Signal Research Articles

Search for a Dark-Matter-Induced Cosmic Axion Background with ADMX.

We report the first result of a direct search for a cosmic axion background (CaB)-a relativistic background of axions that is not dark matter-performed with the axion haloscope, the Axion Dark Matter eXperiment (ADMX). Conventional haloscope analyses search for a signal with a narrow bandwidth, as predicted for dark matter, whereas the CaB will be broad. We introduce a novel analysis strategy, which searches for a CaB induced daily modulation in the power measured by the haloscope. Using this, we repurpose data collected to search for dark matter to set a limit on the axion photon coupling of a CaB originating from dark matter cascade decay via a mediator in the 800-995MHz frequency range. We find that the present sensitivity is limited by fluctuations in the cavity readout as the instrument scans across dark matter masses. Nevertheless, we suggest that these challenges can be surmounted using superconducting qubits as single photon counters, and allow ADMX to operate as a telescope searching for axions emerging from the decay of dark matter. The daily modulation analysis technique we introduce can be deployed for various broadband rf signals, such as other forms of a CaB or even high-frequency gravitational waves.

Compact real-time RF spectrum analyzer with 16 GHz instantaneous bandwidth based on photonic frequency-shifting loops

Spectral analysis of broadband RF signals in real time is of primary importance for numerous applications. So far, the instantaneous bandwidth of real-time spectrum analyzers based on conventional digital techniques is limited to a few GHz. This limitation is set by the clock jitter of the analog-to-digital converters, and by the processing capabilities in real time of digital electronics. On the contrary, analog architectures based on microwave photonics are not constrained by such limitations, and offer potentially a very high instantaneous bandwidth. However, they generally suffer from inherent limitations, such as large footprint and high complexity. Here, we propose a much simpler architecture of RF spectrum analyzer based on frequency-shifting loops. It utilizes only compact commercial telecom components, a single CW laser, and slow electronic resources (10 MSa/s). The probability of intercept is 100%, the instantaneous bandwidth reaches 16 GHz, and the spectral and temporal resolutions are respectively equal to 160 MHz and 50 µs. Our system is expected to open new avenues in embedded applications of microwave photonics.

Bi-directional frequency shifting loops for real-time processing of broadband RF signals

Analog photonic techniques can perform better than conventional digital electronics, which have significant limitations when it comes to processing fast RF signals on the fly. We show that a simple photonic architecture, based on a bi-directional frequency-shifting loop, makes it possible to calculate in real time the cross-correlation function of two broadband signals for about 200 values of their delay simultaneously. Additionally, our architecture also enables to perform spectral analysis of signals with 16 GHz instantaneous bandwidth, 100 % probability of interception, and detection electronics below 10 MSa/s.

Microwave photonics‐based channelized receiving and reconstruction for broadband RF signal

A microwave photonics-based channelized receiver for broadband vector signals is proposed and experimentally demonstrated. A 4-line optical frequency comb (OFC) generated by a dual-parallel Mach-Zehnder modulator (DPMZM) is used as local oscillator (LO) signal in the in-phase/quadrature (I/Q) receiver for coherently down-converting the broadband signal into different channelized bands. By properly setting the frequency difference between the OFC comb line and the broadband signal, either homodyne or heterodyne detection of I/Q receiver is realized. To compensate the amplitude difference and phase difference of adjacent channel signals, an overlap ratio of adjacent slices is introduced for channel equalization and signal reconstruction. The simulated result shows the error vector magnitude of 4-channel channelization for a reconstructed 8 GBaud quadrature phase shift keying (QPSK) signal is very close to the original one. In the experiment, 2-channel channelization for a 2 GBaud QPSK is realized and the whole signal is reconstructed successfully. The experimental result also proves the quality improvement of reconstructed signal with an increase of channel overlap ratio.

Remote broadband RF signal down-conversion with stable phase and high efficiency using a sideband optical phase-locked loop.

We propose a phase-stable high-efficiency down-conversion approach for broadband radio frequency signals transmitted from a remote site. A high power coherent optical local oscillator signal is used at the local site to increase the conversion gain and the spurious-free dynamic range (SFDR) at the same time. A sideband optical phase-locked loop ensures the suppression of the phase noise induced by the fiber transmission and the relative frequency drift of the remote and local lasers, which are essential for the signal transmission and the down-conversion. We first experimentally demonstrate the down-conversion of a single frequency signal at 16.45 GHz to a 250 MHz intermediate frequency (IF) signal with 3 dB gain and 103 dB/Hz2/3 SFDR after 10 km fiber transmission. Then we show the broadband down-conversion capability by down-converting a 1 GHz wide linear frequency modulated pulse signal centered at 11 GHz to 1 GHz with 3 dB gain. Along with a positive gain, the SFDR of the IF signal down-converted from 5 GHz to 40 GHz has reached 97.6 dB/Hz2/3 on average. This approach is suitable for weak broadband remote signal down-conversion with a simple-structured remote end.

Photonic high-fidelity storage and Doppler frequency shift of broadband RF pulse signals.

A microwave photonic system which can simultaneously realize the functions of rapidly tunable Doppler frequency shift (DFS) and high fidelity storage of broadband RF signals is proposed and verified. Single-sideband carrier-suppression modulation combined with dual-AOM frequency shifting ensures large-range and fast-responding DFS. And time-gated semiconductor amplifier (SOA) based fiber delay loop can realize high-fidelity RF pulse storage with high extinction ratio switching and amplification characteristics of time-gated SOA. A spurious rejection ratio greater than 40 dB, tuning range of DFS greater than ± 3 MHz, response speed of DFS less than 30 ns, and high fidelity storage of 4 GHz-12 GHz RF signals with greater than 381 circulations (corresponding 80 us delay time) are realized by the proposed structure. The maximum signal-to-noise ratio (SNR) is 13.6 dB within 381 circulations. Based on the experimental data, the simulation results show that the delay time also could be extended to 10 times more.

Compact High-Efficiency Broadband Rectifier With Multi-Stage-Transmission-Line Matching

This brief proposes a compact high-efficiency broadband microwave rectifier for energy harvesting. The proposed rectifier is designed to operate over a broad bandwidth spanning from 2 to 3 GHz and an input power dynamic range from 0 dBm to 10 dBm. In the proposed structure, a compact broadband low-loss matching network and a dc-pass filter with a broad stopband were designed, leading to a high conversion efficiency in a wide frequency range with compactness. For the matching network, low loss and compact size were achieved by strategically choosing and designing three transmission line segments. Moreover, the proposed dc-pass filter is compact and can effectively block broadband RF signal and its in-band high order harmonics. Theoretical analysis and simulations of the proposed structure were carried out. A prototype was fabricated, which demonstrates a broadband bandwidth of 41.5% (2.0–3.05 GHz) where the RF-dc power conversion efficiency is higher than 70% at an input power of 10 dBm. The measured efficiency remains over 45% within 1.9–3.05 GHz when the input power decreases to 0 dBm. The maximum measured efficiency is 75.8% at an input power of 14 dBm.

A switchable self-interference cancellation system for dual-band IBFD system using a monolithic integrated DML array

A dual-band optical self-interference cancellation (OSIC) system is proposed and experimentally demonstrated for in-band full-duplex (IBFD) system using a monolithic integrated multi-channel directly modulated laser (DML) array, tunable optical delay line (TODL) and optical variation attenuator (OVA), and photodetector. The key component of the OSIC system is the integrated DML array which is measured in terms of the spectral characteristic, P–I curve, and small-signal frequency response. The use of the integrated DML array can greatly save the cost as well as reduce the complexity and power consumption of the system. The proposed system is theoretically and experimentally verified. The theoretical analysis shows that the cancellation depth is associated with the attenuation ratio, the time delay and the microwave frequency of the RF signal. Experimentally, the cancellation depth is more than 43 dB in the frequency of 2.4 GHz and 5 GHz. For a broadband RF signal, cancellation depth of 25 dB is achieved with the bandwidth of ∼260 MHz at a center frequency of 2.4 GHz and 5 GHz. In addition, the switchability between double frequencies of the applied microwave signal makes the OSIC system more flexible and fast without adjusting the TODL and OVA, repeatedly

Angle-of-Arrival Measurement System Using Double RF Modulation Technique

A new technique for determining the angle of arrival (AoA) of an RF signal is presented. It is based on a cascaded modulator structure, where continuous wave light from an optical source is modulated by an incoming RF signal twice. The AoA of an RF signal can be determined by the system output RF signal power. The proposed structure is capable for measuring the AoA of both narrow and broadband RF signals. This overcomes the limitation in all previously reported photonics-based AoA measurement systems, which are applicable to either a single-frequency RF signal or a broadband RF signal. It also does not involve electrical components and high extinction ratio modulators, which are required in reported structures. Experimental results are presented that show an AoA measurement range of 0° to over 65° with errors of less than 1.9° for a microwave signal at 2.65 and 12.62 GHz. Results also show that the system is capable to measure the AoA of a pseudorandom binary sequence signal at a microwave frequency.

Leaky waveguide deflector for 40 GS/s and 6 bits all-optical analog-to-digital converters

Analog-to-digital converters (ADC) are an important part of realizing direct digital receivers in future communication systems, where broad bandwidth, high effective number of bits, and low DC power consumption are an important requirement of achieving it at microwave frequencies. Due to a number of physical limitations of electrical ADC, design of an all-optical analog-to-digital converter (AOADC) is pursued and presented here based on highly stable mode-locked laser based clock signal pulses for optical sampling, and a combination of leaky waveguide optical deflector using electro-optical (EO) polymers and stationary optical windows followed with high-speed photodetectors as optical quantizer. The reported principle of the AOADC is to convert a broadband RF signal into a spatially sampled light using optical deflection angle variation before quantizing it using either binary or Gray coded optical windows. The design and modeling of an AOADC working up to 20 GHz RF frequencies (with Nyquist sampling rate of over 40 GS/s) and for providing a resolution of better than 6 bits with under 4W of power consumption. Performance is currently limited by both optical and microwave attenuation in the available EO polymers.

KSTAR 토카막의 Alfvén파 RF 스펙트럼 측정을 위한 광대역 보우타이 안테나 설계

During KSTAR plasma experiments, torsional <TEX>$Alfv\acute{e}n$</TEX> waves in the frequency of few GHz or below were detected. To understand this plasma waves during the crash of MHD instabilities, an RF spectrometer has been developed for detection of the radiated RF signals in the KSTAR Tokamak. It has the capability of broadband RF spectral measurement (50 ~ 400 MHz). To detect the broadband RF signals which are radiated from the KSTAR systems, a broadband antenna is the key feature of the RF spectrometer. In this paper, a broadband bowtie antenna for detection of <TEX>$Alfv\acute{e}n$</TEX>-waves in the KSTAR Tokamak is presented. Planar-type bowtie antenna is designed and fabricated on an FR4 substrate with thickness of 1.6 mm. The antenna consists of bowtie shaped balanced radiators and broadband planar balun. The antenna is designed to have an input impedance of 50 Ohm, and a taper-shaped balun is adopted for field and impedance matching between 50 Ohm transmission line to 110 Ohm feeding network of balanced radiators. The implemented antenna provides around -3 to 3 dBi of gain for the whole frequency band. The VSWR of the bowtie antenna is less than 12:1 over the frequency bandwidth of 50 to 2000 MHz.

Precision Broadband RF Signal Recovery in Subsampled Analog Optical Links

We present a novel technique for ultrawideband RF disambiguation where we are able to determine the precise Nyquist band from which an optically sampled, and therefore downconverted signal originated. This is accomplished by applying a discrete perturbation to the sampling rate, and measuring the magnitude and direction of the shift in signal location within the fundamental Nyquist band. Once the Nyquist band has been determined, the exact signal frequency can also be recovered. We demonstrate extremely accurate multioctave signal recovery over 1 MHz-40 GHz.

Mach-Zehnder: A Review of Bias Control Techniques for Mach-Zehnder Modulators in Photonic Analog Links

The Mach-Zehnder modulator (MZM) has been widely used for broadband photonic analog links and high-speed digital optical fiber communication systems because it possesses large modulation bandwidth, low driving voltage, and low chirp. The MZM is a very important optical modulator for photonic applications. In an MZM, the input light is split into two paths, each of which is modulated by an electrical signal. Then the two arms are combined to generate an intensity-modulated light or a phase-modulated light at the output of the MZM. An MZM can be made of lithium niobate (LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ), gallium arsenide (GaAs), or indium phosphide (InP), materials that exhibit some anisotropy in their dielectric properties. Theoretically, the relation of output optical field and driving voltage is a cosine function, i.e., nonlinear transfer function. For RF photonics, the MZM has typically two applications: optical harmonic generation for optical frequency multiplication and optical subcarrier modulation for optical signal modulation. For optical frequency multiplication, high transfer-function nonlinearity is preferred. In contrast, high transfer-function linearity is preferred for optical subcarrier modulation. It is well known that a cosine transfer function can present high or low nonlinearity dependent on operation voltage. Specifically, bias voltages determine the degree of nonlinearity or linearity of the MZM transfer function. For optical frequency multiplication such as millimeter-wave generation, the MZM should be biased at some specific bias points, such as minimum transmission, maximum transmission, and quadrature bias points, to enhance nonlinearity [1]?[2]. For optical subcarrier modulation, biasing an MZM in its linear region such as quadrature bias points allows transmitting broadband RF signals with multioctave bandwidth and improves spurious free dynamic range (SFDR). Therefore, care must be taken to maintain and control the MZM bias point for a specific application.

Compact and low-latency instantaneous frequency measurement using 38 cm bismuth-oxide fiber and fiber Bragg gratings

We demonstrated a compact and low-latency photonic approach for broadband RF signal frequency measurement. The photonic approach does not require high-speed electronics for gigahertz RF signal measurement. Frequency-to-intensity mapping is achieved based on a two-tap finite-impulse-response filter architecture, where the time delay between signals is provided by fiber Bragg grating array. With four-wave mixing (FWM) as an optical broadband mixer, sinusoid frequency response is transferred to a DC output, where FWM is achieved in a 38 cm bismuth-oxide fiber. Thus, the resultant signal does not consist of any high-frequency component and no high-speed oscilloscope is needed for the measurement.

Multi-chroic Dual-Polarization Bolometric Focal Plane for Studies of the Cosmic Microwave Background

We are developing multi-chroic antenna-coupled Transition Edge Sensor (TES) focal planes for Cosmic Microwave Background (CMB) polarimetry. In each pixel, a dual polarized sinuous antenna collects light over a two-octave frequency band. Each antenna couples to the telescope with a contacting silicon lens. The antenna couples the broadband RF signal to microstrip transmission lines, and then filter banks split the broadband signal into several frequency bands. A TES bolometer detects the power in each band and polarization. We will describe the design of this device and demonstrate its performance with optical data measured using prototype pixels. Our measurements show low ellipticity beams, low cross-polarization, and properly partitioned bands in banks of 2, 3, and 7 filters. Finally, we will describe how we will upgrade the Polarbear CMB experiment using the focal planes of these detectors to increase the experiment’s mapping speed and its ability to discriminate between the CMB and polarized foregrounds.

A Novel Broadband Photonic RF Phase Shifter

A novel broadband photonic RF phase shifter is proposed that employs single-sideband modulation of the broadband RF signal onto an optical carrier. The phase of the carrier is shifted relative to the modulated signal using an optical allpass filter. Ring-resonator-based optical allpass filters are tuned across the carrier frequency to yield an RF phase shift of over 360 degrees. Results from a proof-of-concept experimental demonstration are presented for modulation frequencies in the range of 2-4 GHz.

Proposal of a nonlinear compensation scheme using an optical chirp controller in SCM/OFM RoF link

An SCM optical FM method can obtain an FM gain, so it is effective on employing an RoF link which transfers a broad-band or multicarrier RF signal. However, the RF signal quality is degraded by intermodulation distortions due to the nonlinearity of LDs and optical frequency discriminator. The nonlinear compensation technique in the RoF link has been investigated in order to obtain a higher RF signal quality. This paper proposes an insertion-type distortion nonlinear compensation. In the proposed scheme, a nonlinear device is inserted between transmitter and receiver and we propose an optical chirp controller as the nonlinear device. The proposed scheme suppresses the higher order intermodulation distortion which degrades RF signal quality in the adaptive transmittance control method. In this paper we theoretically analyze the SFDR of the optical FM method using the optical chirp controller and show that improvement in the SFDR at 9 and 5 dB compared to methods without nonlinear compensation and the adaptive transmittance control method, respectively, can be obtained. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 89(12): 1–11, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20322

Microphotonic wideband tapped optical delay line FIR filter noise analysis

MicroPhotonic broadband RF signal processors utilize true-time-delay methods to perform processing functions that cannot be achieved by conventional electronic methods. Using a tapped optical-delay line, a low-cost adaptive wideband FIR filter can be realized. We analyse the sources of noise for such a filter using commercially available VCSEL sources, PIN diodes, and transimpedance amplifiers. The power at each tap can be equal by using a suitably graded coating, or if the reflectivity is constant, there is an optimum reflectivity that maximizes the signal-to-noise ratio for a given number of taps, as we show in this paper. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 45: 476–480, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20857

High-resolution radio frequency spectral analysis with photon echo chirp transform in an Er:YSO crystal

We demonstrate implementation of the chirp transform algorithm with photon echoes in an Er/sup 3+/:YSO crystal at 1536 nm for application to spectral analysis of RF signals. The principle and features of the technique are presented using a simple picture in the time-frequency plane. Experimental results with low-bandwidth RF signals demonstrate 40-kHz spectral resolution and a time-bandwidth product of 500. Extension to broad-band RF signals is discussed.

Sensor fault detection in etch based on broadband rf signal observation

To minimize machine downtime in semiconductor plasma etching processes, a sensor fault detection system was developed, employing a broadband rf sensor. The work presented here focuses on detecting errors in the transformer coupled power (TCP) measurement and pressure sensor, though the method has potential for several other sensors. To improve the detection sensitivity, we used real-time machine input estimation, and compared it against actual recipe settings instead of employing the raw rf signals directly. For a main etch process on a Lam TCP 9400SE, ±10% relative deviations in TCP measurement, or ±25% in pressure, can be detected with high accuracy: Detection probability PD≈1 and false alarm rate PF≈0 for both.