RF Signal Frequency Research Articles

Wideband finely tunable, ultralow-phase noise microwave generation in a Brillouin cavity.

A novel, to the best of our knowledge, approach to generate frequency-tunable microwave sources with low-phase-noise based on a Brillouin laser frequency comb is proposed and experimentally demonstrated. The Brillouin laser frequency comb is generated by combining stimulated Brillouin scattering, frequency shifting optical injection locking, modulation sideband optical injection locking (MSOIL), and four-wave mixing effects. By beating the generated comb lines, the microwave is generated with an extremely low-level phase noise of -120 dBc/Hz at a 10-kHz offset. The frequency of the microwave signal can be finely tuned in steps of a Brillouin cavity mode spacing (i.e., 2 MHz) and coarsely adjusted to integer times the applied RF signal frequency in the MSOIL unit. Remarkably, the phase noise of the microwave source can be kept at almost the same low level during the whole tuning process over the frequency range of 30-75 GHz. The proposed tunable low-phase-noise microwave generation approach has great potential applications in communications, radars, and metrology.

All-optical instantaneous RF signal frequency measurement system based on a linear ACF with a steep slope

A new microwave photonic structure for measuring the frequency of an RF signal, to the best of our knowledge, is presented. The frequency of an unknown RF signal can be determined by simply measuring the system output optical powers. The proposed frequency measurement system can be designed so that the ratio of the two system output optical powers as a function of the RF signal frequency or the amplitude comparison function (ACF) has a steep linear slope over a wide frequency range. This enables the RF signal frequency to be measured in high resolution and high accuracy. The proposed frequency measurement system has a simple and compact structure, and is free of high-speed photodetectors as well as RF components and instruments. It also has a fast response time compared to many reported photonics-based frequency measurement systems. A proof-of-concept experiment is carried out. Experimental results show a linear ACF with a slope of more than 4.4 dB/GHz over a frequency measurement range of 5–26 GHz and a frequency measurement accuracy of better than ±0.1GHz.

All-optical high-order subharmonic mixer with a high signal-to-noise ratio and stable performance.

A photonics-based high-order subharmonic mixer, which enables a low-frequency LO source to be used for high-frequency RF signal frequency downconversion, is presented. It is based on an optically injected semiconductor laser, which is oscillated in the period-one state, sandwiched between two optical phase modulators. It has the advantages of a simple and compact structure, wide bandwidth, absence of electrical components, reconfigurable subharmonic mixing operation, stable output IF signal performance, high signal-to-noise ratio, infinite LO-to-RF port isolation, and high LO-to-IF port isolation. Furthermore, it is suitable for use in remote antenna applications. We set up a proof-of-concept experiment that demonstrates a reconfigurable second-, fourth-, sixth-, and eighth-order subharmonic mixing operation for different input RF signal frequencies and powers. The experimental results also demonstrate that the proposed structure exhibits a stable output IF signal performance, which overcomes the IF signal phase stability problem in the reported high-order subharmonic mixers.

Effects of Optical Sampling Pulse Power, RF Power, and Electronic Back-End Bandwidth on the Performance of Photonic Analog-to-Digital Converter.

The effects of optical sampling pulse power, RF power, and electronic back-end bandwidth on the performance of time- and wavelength-interleaved photonic analog-to-digital converter (PADC) with eight-channel 41.6 GHz pulses have been experimentally investigated in detail. The effective number of bits (ENOB) and peak-to-peak voltage (Vpp) of converted 10.6 GHz electrical signals were used to characterize the effects. For the 1550.116 nm channel with 5.2 G samples per second, an average pulse power of 0 to -10 dBm input to the photoelectric detector (PD) has been tested. The Vpp increased with increasing pulse power. And the ENOB for pulse power -9~-3 dBm was almost the same and all were greater than four. Meanwhile, the ENOB decreased either when the pulse power was more than -2 dBm due to the saturation of PD or when the pulse power was less than -10 dBm due to the non-ignorable noise relative to the converted weak signal. In addition, RF powers of -10~15 dBm were loaded into the Mach-Zehnder modulator (MZM). The Vpp increased with the increase in RF power, and the ENOB also showed an increasing trend. However, higher RF power can saturate the PD and induce greater nonlinearity in MZM, leading to a decrease in ENOB, while lower RF power will convert weak electrical signals with more noise, also resulting in lower ENOB. In addition, the back-end bandwidths of 0.2~8 GHz were studied in the experiments. The Vpp decreased as the back-end bandwidth decreased from 8 to 3 GHz, and remained nearly constant for the bandwidth between the Nyquist bandwidth and the subsampled RF signal frequency. The ENOB was almost the same and all greater than four for a bandwidth from 3 to 8 GHz, and gradually increased up to 6.5 as the back-end bandwidth decreased from the Nyquist bandwidth to 0.25 GHz. A bandwidth slightly larger than the Nyquist bandwidth was recommended for low costs and without compromising performance. In our experiment, the -3 to -5 dBm average pulse power, about 10 dBm RF power, and 3 GHz back-end bandwidth were recommended to accomplish both a high ENOB more than four and large Vpp. Our research provides a solution for selecting optical sampling pulse power, RF power, and electronic back-end bandwidth to achieve low-cost and high-performance PADC.

A statistical method of GPS spoofing detection using power spectral density for hardware

AbstractLocation and navigation information has become an important part of our lives. GPS data is vital in many areas from transportation to economy, from energy to security. The alteration of this information can have deleterious downstream effects that cannot be reversed. Today, it is possible to attack GPS data with accessible hardware and open source software. In this paper, the signal obtained with the help of a transceiver that samples the high frequency RF signals down to the intermediate frequency was compared with the RMS value by windowing over PSD. Then this data was statistically analyzed with the NIST runs test. In order to realize GPS signal spoofing in a short time that after GPS receiver lock the spoofing signal, field programmable gates connected to the transceiver were used. The results obtained varied according to the dimensions of the windowing process, and it was understood that the window size was in a correlation with the detection of deception in a short time. The suggested method's key benefit is its simplicity, which makes it adaptable to various GNSS systems.

Optical Carrier Phase Sweeping Based RF Signal Frequency Measurement System

Optical Carrier Phase Sweeping Based RF Signal Frequency Measurement System

Study of the Optoelectronic Mixer Based on Modified Uni-Traveling Carrier Photodetector for Efficient Harmonic Mixing

We report the measurements and characterizations of optoelectronic harmonic mixing in a modified uni-traveling carrier photodetector (MUTC-PD). A systematic study of the bias voltage-dependent properties of the output power under different input optical power levels has been made and the proper operating conditions for different mixing modes have been investigated. In addition to the fundamental mixing, the higher-order harmonic mixing and RF subharmonic mixing products were obtained. The harmonic mixing mechanism is mainly attributed to the nonlinearity of the dependence of the output power on the bias voltage. Flat intermediate responses over a wide RF signal frequency range were measured for the fundamental mixing and the high-order RF subharmonic mixing. The feasible harmonic optoelectronic mixing in MUTC-PD shows great application potential in the wireless multichannel communication system.

Microwave Frequency Dividers With Reconfigurable Fractional 2/N and 3/N Division Ratios

Two microwave photonic signal processing structures, which are capable to realise microwave frequency division with a tunable integer or non-integer division ratio, are presented. They are based on applying an input RF signal to a Mach Zehnder modulator, which is biased to generate a carrier-suppressed double sideband (DSB) or single sideband (SSB) optical signal. The carrier-suppressed DSB optical signal consists of the upper and lower 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> order sidebands with a separation of two times the input RF signal frequency. The carrier of the SSB optical signal is suppressed by a fibre Bragg grating (FBG) connected to the modulator output. Hence the FBG output consists of the upper 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> order and lower 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> order sidebands with a separation of three times the input RF signal frequency. The carrier-suppressed DSB or SSB optical signal is injected into a semiconductor laser. The semiconductor laser is oscillated in the period-one state that generates a number of equally spaced frequency components, which are frequency locked by the injection light wave. Beating of these optical frequency components at the photodetector produces an RF signal with a frequency of 2/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> or 3/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> times the input RF signal frequency where <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> can be between 3 and 6. Hence the 2/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> frequency divider can realise 2/3, 1/2, 2/5 and 1/3 frequency division operation, and the 3/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> frequency divider can realise 3/4, 3/5 and 1/2 frequency division operation. The proposed 2/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> and 3/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> frequency dividers have a very simple structure compared to the reported photonics-based microwave frequency divider that can realise both integer and non-integer frequency divisions. The frequency division ratio can be tuned by simply adjusting the forward bias current of the semiconductor laser subject to optical injection. Experimental results demonstrate the two proposed structures can realise microwave frequency division with a tunable integer and non-integer division ratio for different input RF signal frequencies of 10 to 18 GHz, and over 60 dB output signal-to-noise ratio performance. More than 27 dB suppression in the unwanted frequency components around the frequency divided signal is also demonstrated.

Photonics-Based Instantaneous Microwave Frequency Measurement System With Improved Resolution and Robust Performance

This paper presents a photonics-based instantaneous microwave frequency measurement system. The system operates based on the frequency-to-power mapping technique. The trade-off between frequency measurement range and resolution, which is present in the reported frequency-to-power-mapping based frequency measurement systems, is addressed by designing the system to produce two amplitude comparison functions (ACFs). Using two ACFs enables an RF signal frequency in a specified range to be determined while having an improved resolution and smaller errors compared to using only one ACF for frequency measurement. The proposed frequency measurement system is designed to have a simple structure and robust performance suitable for use in real world applications. The effect of the higher order sidebands and harmonics on the system performance are investigated. Proof-of-concept experiments are carried out with results showing frequency measurement errors can be reduced by using the proposed system that generates two ACFs for frequency measurement.

Novel Highly Flexible PCB Design Based on a Via-Less Meander Ground Structure to Transmit mm-Wave RF Signals in 5G Foldable Mobile Products

Recently, new form factors, such as foldable, have increased demand for mobile products.Moreover, mobile phones should support the RF signal frequency up to the mm-wave frequencydue to the expansion of 5G mobile products. Therefore, 5G foldable products require componentsthat facilitate both mm-wave RF transmission and ultra-high flexibility for interconnecting throughthe hinge structure of foldable products. To improve flexibility, a flexible PCB must be thin with noground vias in its bending section; in contrast, the low-loss flexible PCB for mm-wave transmissionmust be thick and have many ground vias, so there is a trade-off relationship between flexibilityand RF characteristics. This paper proposes a new flexible PCB structure that does not experienceproblems regarding signal transmission to the mm-wave band, even when folded 200,000 times.To overcome the physical limits of the trade-off relationship, an interlayer air-gap was formed; astructure with a via-less and meander ground shape is proposed. The simulated loss of the proposedstructure was 0.0254 dB/mm @ 10 GHz, and the isolation between signals ranged from 21.98 dBto 10 GHz. The simulated results of insertion loss and isolation were experimentally verified. Theproposed structure is currently being applied to the RF flexible PCB that interconnects through thehinge of a foldable phone, and is currently being mass-produced.

Photonic microwave frequency divider with a tunable division ratio and harmonic suppression capability

A photonic microwave frequency divider that is capable to realise tunable high order frequency division, is presented. It is based on injecting an RF phase modulated optical signal into an off-the-shelf DFB laser operating at period-N state. Optical frequency components with a frequency separation of 1/N times the input RF signal frequency are generated by the DFB laser. An optical bandpass filter can be employed to select two optical frequency components to be detected by a photodetector to obtain a divide-by-N RF signal without harmonic components. The proposed frequency divider can be operated over a wide frequency range and has high reconfigurability as it is free of electrical components. Experimental results demonstrate the realisation of frequency division operation with a tunable 1/2 to 1/5 division ratio for different input RF signal frequencies of 8 to 20 GHz by adjusting the DFB laser forward bias current. Over 35 dB harmonic component suppression is demonstrated. A proof-of-concept experiment is also set up to show the frequency divider based on an optically injected semiconductor laser is capable to operate at a high input RF signal frequency of 50 GHz and has a tunable high order division ratio of 1/2 to 1/8.

Wideband Photonic Microwave Frequency Divider With Large Harmonic Suppression Capability

An all-optical photonic microwave frequency divider (PMFD) is presented. It is based on injecting an RF phase modulated optical signal into a semiconductor laser oscillating in the period-two state. New optical frequency components with frequency separation of half of the input RF signal frequency are generated at the laser output. A frequency divided signal can be obtained by the beating of these optical frequency components at a photodetector. Large harmonic suppression can be achieved by using an optical filter to select only two optical frequency components to be detected by the photodetector. The proposed PMFD is free of electrical components and does not suffer from the modulator bias drift problem. It has the potential to operate over a 100 GHz frequency range. System parameters required to realise divide-by-two frequency division operation for different input RF signal frequencies are investigated. Experimental results are presented for the novel PMFD, which demonstrate the generation of a 1/2 frequency component for different input RF signal frequencies by controlling the forward bias current of an off-the-shelf semiconductor laser, and which also show the important advantages of large harmonic suppression and high signal-to-noise ratio performance. Wide input RF signal power range and high output stability performance are also demonstrated experimentally.

Characterization and alignment of acousto-optic devices using digital tailored RF waveforms.

Acousto-optic (AO) devices are used in a variety of light control and processing applications in different environments, including laboratory conditions and the field environment, as well as on air- and space-borne platforms. Accurate alignment and calibration of AO deflectors and tunable filters is crucial for correct operation and to achieve specified performances of those devices. We describe a simple routine to test and align AO devices with a programmable RF waveform generator. Using specially tailored RF waveforms as test signals enables single-shot measurement of an AO device's diffraction efficiency as a function of the applied RF signal frequency and amplitude. MATLAB code is available to implement the testing procedure and application demonstrations for an AO deflector and two types of AO tunable filters.

Microwave Phase Detector Based on Optical Frequency Translation and Downconversion

A new techniqueto detect the phase difference of two high-frequency RF signals is presented. It is based on downconverting the two high-frequency input RF signals into two low-frequency signals via Serrodyne optical frequency translation and optical frequency downconversion. The phase difference of the two low-frequency downconverted signals is the same as the two high-frequency input RF signal phase difference. Hence a low-cost low-frequency electronic phase detector can be used after photodetection to generate a DC voltage with a value depending on the two input RF signal phase difference. The technique is simple, novel, and innovative. It utilises the advantage of ultra-wide operating frequency range in microwave photonics and the input RF power independent phase detection characteristic in electronic phase detectors. It overcomes the limitations of the input RF signal power and frequency need to be known prior to phase detection in the reported microwave photonic based phase detectors. Experimental results demonstrate 0°-180° RF signal phase difference detection with 1° resolution and less than ±2.6° errors for different RF signal frequencies (10 GHz, 14 GHz and 18 GHz) and different RF modulation indexes (0.2, 0.4 and 0.7). Results also demonstrate the proposed phase detector has excellentrepeatability.

RF Signal Frequency Identification in a Direct RF Undersampling Multi-Band Real-Time Spectrum Monitor for Wireless IoT Usage

RF Signal Frequency Identification in a Direct RF Undersampling Multi-Band Real-Time Spectrum Monitor for Wireless IoT Usage

Ultra-Simple All-Optical Microwave Frequency Divider

A microwave frequency divider based on an optically injected semiconductor laser is presented. It has a very simple structure as it only requires an optical circulator and a DFB laser in a conventional microwave photonic link. Period-two oscillation in the DFB laser, introduced by injecting an RF modulated optical signal into the DFB laser, generates optical frequency components with half of the input RF signal frequency separation. Beating of these optical frequency components at the photodetector produces a divide-by-two frequency component. The proposed microwave frequency divider is free of electrical components. It can operate over a wide input RF signal frequency range without the need of altering the system physical configuration. Experimental results demonstrate generation of a 1/2 frequency component for different input RF signal frequencies, by adjusting the forward bias current of a DFB laser.

Photonic Techniques for Generating a Single RF Sideband With No Second Order Sidebands

Two structures that can realise optical single sideband modulation without generating both second order upper and lower sidebands are presented. They are based on a dual-parallel Mach Zehnder modulator (DP-MZM) with an optical filter and a dual-polarisation dual-parallel Mach Zehnder modulator (DPol-DPMZM) with a 90° hybrid coupler. The former is an all-optical structure and hence it has a very wide bandwidth. The latter uses one DP-MZM to realise single sideband suppressed carrier modulation while the other DP-MZM simply passes the optical carrier. It also suppresses the third order sideband on the same side as the wanted fundamental RF modulation sideband. Hence, in an ideal situation, there is no second order harmonic component generated after photodetection. Experimental results are presented for the novel structures, which demonstrate the realisation of optical single sideband modulation without second order sidebands. The results also show large fundamental to second order harmonic power ratio over a wide input RF signal frequency range even after inserting a long single mode fibre into the system for signal transmission. The new optical single sideband modulators find applications in improving the multioctave spurious free dynamic range in a long-haul fibre optic link and reducing measurement errors in an optical vector analyser.

Flexible Millimeter-Wave Carrier Generation up to the Sub-THz With Silicon Photonics Filters

We report on the characterization of a photonic integrated circuit (PIC) for the generation of a RF carrier in the millimeter-wave band up to the sub-THz region. The circuit is implemented in silicon waveguides through a standard multi project wafer service, and exploits an innovative filtering technique for effectively selecting two tones out of an input optical frequency comb (OFC) driven by a local oscillator at a sub-multiple frequency of the system output RF signal. Experimental results confirm a large suppression ratio of RF spurious tones up to more than 30 dB for generated carrier frequencies ranging from 20 GHz to 126 GHz with input OFC frequency spacing between 10 GHz and 20 GHz. Additionally, the unfiltered input OFC tones are recovered in the scheme with negligible loss, enabling efficient comb reuse for minimizing optical and RF equipment in future wireless communications and sensing networks systems. Design strategies for further improving the circuit performances are finally presented.

Generalized Analysis of Dual-Output Mach-Zehnder Modulator With Applications to Photonic-Assisted Instantaneous Frequency Measurement

We propose a general and exact analytical model forintensity modulation and direct detection (IM/DD) fiber optic links using dual-output dual-drive Mach-Zehnder modulator (DO-MZM). This model extends previously published results relying on the small-signal approximation and infinite extinction ratio at the modulator outputs. In our treatment the output optical powers are calculated through analytical expressions taking into account the modulator electronic driver and directional coupler characteristics. Closed-form expressions, explicitly displaying the dependence of the link output power on the directional coupler transmission coefficients, optical modulation depth, and bias point, are obtained for output optical power, frequency chirp, extinction ratio and static characteristic MZM curves. We also propose an equivalent interferometric representation of each DO-MZM output in terms of virtual dual-drive Mach-Zehnder modulators (DD-MZMs) and show that each modulator output has distinct characteristic curves. The model is applied to a photonic-assisted instantaneous frequency measurement (IFM) based on frequency-to-power mapping, in which an amplitude comparison function (ACF) is generated by the power fading functions. We have obtained a generalized ACF through which both the frequency and amplitude of the input RF signal can be determined by a search procedure.

Photonics-Based RF Signal Phase Detector With Wide Operating Frequency Range and High Resolution

A new topology that generates a DC voltage whose value is depending on the phase difference of two input RF signals is presented. It is implemented by the mixer concept. With the use of microwave photonics, the mixer-based phase detector can be operated over a very wide frequency range. It also has the advantages of high phase detection resolution, small phase detection error, and small DC offset and phase offset. The phase detector can be constructed using off-the-shelf components including a single integrated optical modulator and a low-speed balanced detector. Measured results on the phase detector demonstrate a 0° to 180° RF signal phase difference detection range for 3.5 GHz to 18 GHz input RF signal frequencies, less than ±3° phase detection error and 2° phase detection resolution.