Low Phase Noise Microwave Research Articles

Ultralow noise microwave synthesis via difference frequency division of a Brillouin resonator

Low phase noise microwave oscillators are at the center of a multitude of applications that span the gamut of photonics and electronics. Within this space, optically derived approaches to microwave frequency synthesis are particularly compelling owing to their unique combination of ultrawideband frequency access and the potential for resiliency to temperature and environmental perturbation via common-mode noise rejection. We demonstrate here an optical frequency divider that uses the 30 terahertz frequency gap between two stimulated Brillouin scattering (SBS) lasers as the basis for frequency division. The resulting microwave signal, centered at 10 GHz frequency, exhibits exceptionally low phase noise levels of −95dBc/Hz and −110dBc/Hz at 10 Hz and 100 Hz frequency offset, respectively. Moreover, the two SBS lasers, generated from a common fiber resonator, exhibit a high degree of correlated noise cancellation in their frequency difference. We measure 16.1 dB of noise rejection against intentionally applied vibrations, thus showcasing a promising pathway towards portable and robust ultralow noise photonic-microwave synthesis.

Low phase noise microwave oscillator based on gain driven polariton

Low phase noise oscillators are key building blocks of many high-end microwave systems. This work introduces a phase noise reduction mechanism through a gain driven polariton platform, where coherent coupling is used to suppress frequency distribution around the carrier, effectively reducing the phase noise. The design process for achieving low phase noise performance is outlined, and three prototypes are constructed, all of which feature key components, such as gain-embedded planar microwave cavity, yttrium iron garnet, and magnets. In particular, the first prototype is used to showcase the phase noise reduction mechanism, while the second prototype, a fixed-frequency oscillator working at 3.544 GHz, exhibits phase noise levels of −117 and −132 dBc/Hz at 10 and 100 kHz offset frequencies, respectively. The third prototype offers a tuning range from 2.1 to 2.7 GHz, while maintaining phase noise levels comparable to the second prototype.

Dual-microcomb generation via a monochromatically pumped dual-mode microresonator

Microcombs have enabled a host of cutting-edge applications from metrology to communications that have garnered significant attention in the last decade. Nevertheless, due to the thermal instability of the microresonator, additional control devices like auxiliary lasers are indispensable for single-soliton generation in some scenarios. Specifically, the increased system complexity would be too overwhelming for dual-microcomb generation. Here, we put forward a novel approach to mitigate the thermal instability and generate the dual-microcomb using a compact system. This process is akin to mode-division multiplexing, as the dual-microcombs are generated by pumping the dual-mode of a single Si3N4 microresonator with a continuous-wave laser. Both numerical simulations and experimental measurements indicate that this innovative technique could offer a straightforward way to enlarge the soliton existence range, allowing entry into the multistability regime and triggering another microcomb alongside the main soliton pulse. This outcome not only shines new light on the interaction mechanism of microresonator modes but also provides an avenue for the development of dual-microcomb-based ranging and low phase noise microwave generation.

The photodetection of ultrashort optical pulse trains for low noise microwave signal generation.

Electrical signals derived from optical sources have achieved record-low levels of phase noise, and have demonstrated the highest frequency stability yet achieved in the microwave domain. Attaining such ultrastable phase and frequency performance requires high-fidelity optical-to-electrical conversion, typically performed via a high-speed photodiode. This paper reviews characteristics of the direct photodetection of optical pulses for the intent of generating high power, low phase noise microwave signals from optical sources. The two most popular types of photodiode detectors used for low noise microwave generation are discussed in terms of electrical pulse characteristics, achievable microwave power, and photodetector nonlinearities. Noise sources inherent to photodetection, such as shot noise, flicker noise, and photocarrier scattering are reviewed, and their impact on microwave phase fidelity is discussed. General guidelines for attaining the lowest noise possible from photodetection that balances power saturation, optical amplification, and amplitude-to-phase conversion, are also presented.

Ultra low phase noise and high output power monolithic microwave integrated circuit oscillator for 5G applications

novel structure of low phase noise and high output power monolithic microwave integrated circuit (MMIC) oscillator is presented in order to use it in 5G applications. The oscillator is based on the ED02AH process which allows us to design a microwave oscillator keeping a minimum size which is impossible to have it using other technologies since microwave oscillators are sensitive components above 20 GHz. The oscillator is studied, designed, and optimized on a GaAs substrate from the OMMIC foundry using the advanced design system (ADS) simulator in order to obtain a miniaturized oscillator (1.1×1.3 mm<sup>2</sup>) generating two signals of different frequencies f<sub>o1</sub>=26 GHz and f<sub>o2</sub>=30 GHz. The objective is to design an oscillator with high output power and low phase noise while respecting its specifications. The optimization of the proposed microwave oscillator shows satisfying results. Indeed, at 26 GHz and 30 GHz, the output powers are respectively 13.33 dBm and 14.89 dBm. The oscillator produces a sinusoidal signal of 1.5 V and 1.75 V amplitude respectively at 26 GHz and 30 GHz. The oscillator phase noise at f<sub>o1</sub> and f<sub>o2</sub> resonance frequencies using the liquid crystal (LC) resonator shows respectively -109 dBc/Hz and -110 dBc/Hz at 10 MHz offset.

Highly improved side mode suppression ratio and a low phase noise optoelectronic oscillator.

By adopting self-injection locking (SIL) technology in an external injection locking (EIL) optoelectronic oscillator (OEO), a highly improved side mode suppression ratio (SMSR) and low phase noise microwave signal generator is designed. The EIL ranging is closely related to the frequency spacing ranging of the free-running OEO, which is the reverse of the oscillation loop length, and limits the phase noise performance. Here SIL technology is introduced to significantly increase the Q-factor of the OEO without degrading the SMSR by setting the longer loop without oscillation. Both the simulation and experimental results are carried out to confirm the conclusion. Additionally, an SMSR up to 86 dB and phase noises as low as -88.80dBc/Hz@100Hz and -122.83dBc/Hz@10kHz, respectively, are demonstrated. Furthermore, the frequency overlapping Allan deviation of the proposed OEO scheme is also enhanced by 103 times, which benefits from the external injection technology compared with the free-running OEO. In addition, the SMSR and phase noise modification dependence on the fiber length, the RF source quality and external injection power, as well as the frequency tunability, are detailed and discussed to reveal the compatibility combination mechanism of the EIL and the SIL.

Low phase noise microwave signal generation with microcombs (Invited)

Low phase noise microwave signal generation with microcombs (Invited)

Photonic microwave synthesizer based on optically referenced sub-sampling phase-locked optoelectronic oscillator

Wideband and low phase noise microwave synthesizers are of critical significance for various applications. Phase-locked loop (PLL) based microwave synthesizers are widely used. Normally, the phase noise of the PLL based microwave synthesizer is limited by the phase noise of the reference, phase detector, frequency divider and voltage-controlled oscillator (VCO). It is challenging to achieve a pure electronic microwave synthesizer with wide frequency coverage and low phase noise, simultaneously. Here, we propose a photonic microwave synthesizer which incorporates an ultra-low phase noise fiber mode-locked laser (MLL) based reference and a wideband frequency tunable optoelectronic oscillator (OEO) based VCO in a hybrid optoelectronic sub-sampling PLL. The frequency of the OEO can be locked to the harmonics of the repetition-rate of the fiber MLL, which combines the distinct advantages of the low phase noise of MLL and the wideband frequency tuning of OEO. A sub-sampling phase detector formed by a balanced optical microwave phase detector (BOMPD) eliminates the use of RF frequency divider and maintains low residual phase noise. We attain an X- and Ku-band photonic microwave synthesizer with over an octave frequency coverage from 8 GHz to 18 GHz, frequency synthesis step of 60.74 MHz and phase noise of -80 dBc/Hz at 100 Hz offset and -100 dBc/Hz at 1 kHz offset for 10 GHz output. The characteristics of the optoelectronic sub-sampling phase detector, the phase-locking dynamics of the hybrid optoelectronic PLL, and the phase noise of the synthesized RF signals are both theoretically and experimentally investigated. The experimental results agree well with the theoretical models. Our approach is promising to achieve wideband and low phase noise microwave synthesis.

Phase-noise suppression for the optical-heterodyne-generated microwave based on the amplitude-to-phase conversion

By reducing the noise component induced by the amplitude-to-phase conversion, a broadband phase-noise suppression of a 20.5 GHz photonic microwave, generated from an optical heterodyne of a dual-wavelength single-frequency fiber laser (DW-SFFL) is experimentally demonstrated. This is done by suppressing the intensity noise of the DW-SFFL through optoelectronic feedback, the microwave shows phase-noise suppression around the frequency range 100 Hz to 100 kHz. The phase-noise suppression of 6.1 dB at 1 kHz offset frequency and a linewidth of 2.15 kHz are obtained from this microwave system. The successful improvement indicates the feasibility of our strategy in obtaining a low phase-noise microwave.

A low phase noise microwave source for high‐performance CPT Rb atomic clock

Phase noise of the frequency synthesizer is one of the main limitations to the short-term stability of microwave atomic clocks. In this work, we demonstrated a low-noise, simple-architecture microwave frequency synthesizer for a coherent population trapping (CPT) clock. The synthesizer is mainly composed of a 100 MHz oven controlled crystal oscillator (OCXO), a microwave comb generator and a direct digital synthesizer (DDS). The absolute phase noises of 3.417 GHz signal are measured to be -55 dBc/Hz, -81 dBc/Hz, -111 dBc/Hz and -134 dBc/Hz, respectively, for 1 Hz, 10 Hz, 100 Hz and 1 kHz offset frequencies, which shows only 1 dB deterioration at the second harmonic of the modulation frequency of the atomic clock. The estimated frequency stability of intermodulation effect is 4.7*10^{-14} at 1s averaging time, which is about half order of magnitude lower than that of the state-of-the-art CPT Rb clock. Our work offers an alternative microwave synthesizer for high-performance CPT Rb atomic clock.

Low phase noise microwave oscillator with greater than 60 dB second‐harmonic suppression

This study presents a microwave oscillator in which an innovative hybrid microstrip and substrate integrated waveguide filter is proposed for phase noise reduction. By virtue of equivalent circuit, the specific topology of this filter is clearly revealed and verified by both the simulation and the experiment. It can realize a third-order elliptic bandpass response with two transmission zeros near the passband. Specific phase response can be formed, in which a high group delay can be achieved within a narrow frequency range around the centre frequency. This feature makes the filter performance very flexible for its application in microwave oscillator, and then reduces its phase noise. For demonstration, an oscillator example at 9.0 GHz was deigned, fabricated and measured. The measured phase noise at 100 kHz away from the oscillation frequency is as low as −120.06 dBc/Hz. Furthermore, the proposed oscillator is featured by greater than 60 dB second-harmonic suppression and at least the third harmonic is suppressed.

Fast automatic frequency calibration assisted phase-locked highly stable optoelectronic oscillator

Highly stable, low phase noise microwave oscillators are essential for various applications. An optoelectronic oscillator (OEO) can overcome the short-term phase noise limitation of pure electronic oscillators at high oscillation frequency. Nonetheless, the long-term frequency stability should be addressed. To stabilize the frequency of OEO, a phase-locked loop (PLL) is widely used to synchronize the OEO to a stable reference. However, due to the narrow free-spectral-range (FSR) of the oscillation cavity of the OEO, the pull-in range of the PLL is limited. It is challenging to acquire phase-locking at startup and phase-relocking when mode-hopping of OEO occurs. Here, by using an automatic frequency calibration (AFC) assisted PLL, we attain a highly stable 10 GHz phase-locked OEO with robust phase-locking at startup and phase-relocking when mode-hopping of OEO occurs, for the first time. With the use of a fast digitally-controlled frequency shifter and a real-time frequency error detection unit in the AFC loop, the phase-locking and phase-relocking time are below 120 ms. Furthermore, it shows the phase noise of -135 dBc/Hz at 10 kHz offset, side-mode suppression ratio (SMSR) of 128 dBc, and Allan deviation of 4.8×10-11 at 5000 s for the phase-locked OEO. We thoroughly investigate the dynamics of the automatic frequency calibration, the phase-locking process, the phase-relocking after OEO mode-hopping, the system under vibration, and the frequency switching. Our approach is promising to generate a highly stable, low phase noise, and determinate frequency microwave signal, which can be used as a low phase noise reference for a microwave frequency synthesizer and high performance sampling clock for a data conversion system.

Development of Integrated Low-Phase Noise Microwave Frequency Synthesizer for Cold Atomic Gravimeter

Development of Integrated Low-Phase Noise Microwave Frequency Synthesizer for Cold Atomic Gravimeter

85Rb喷泉钟的低相位噪声微波频率综合器设计

85Rb喷泉钟的低相位噪声微波频率综合器设计

Demonstration of a Sub-Sampling Phase Lock Loop Based Microwave Source for Reducing Dick Effect in Atomic Clocks**Supported by the National Key Research and Development Program of China under Grant No 2017YFA0304400, and the National Natural Science Foundation of China under Grant Nos 91336213, 11703031, U1731132 and 11774108.

We demonstrate a simple scheme of 6.835 GHz microwave source based on the sub-sampling phase lock loop (PLL). A dielectric resonant oscillator of 6.8 GHz is directly phase locked to an ultra-low phase noise 100 MHz oven controlled crystal oscillator (OCXO) utilizing the sub-sampling PLL. Then the 6.8 GHz is mixed with 35 MHz from an direct digital synthesizer (DDS) which is also referenced to the 100 MHZ OCXO to generate the final 6.835 GHz signal. Benefiting from the sub-sampling PLL, the processes of frequency multiplication, which are usually necessary in the development of a microwave source, are greatly simplified. The architecture of the microwave source is pretty simple. Correspondingly, its power consumption and cost are low. The absolute phase noises of the 6.835 GHz output signal are −47 dBc/Hz, −77 dBc/Hz, −104 dBc/Hz and −121 dBc/Hz at 1 Hz, 10 Hz, 100 Hz and 1 kHz offset frequencies, respectively. The frequency stability limited by the phase noise through the Dick effect is theoretically estimated to be better than 5.0 × 10−14τ1/2 when it is used as the local oscillator of the Rb atomic clocks. This low phase noise microwave source can also be used in other experiments of precision measurement physics.

Low phase noise microwave frequency synthesizer for cold atom clock

We present a low phase noise microwave frequency synthesizer for the integrating sphere cold atom clock. The synthesizer is based on an ultralow-phase-noise oven-controlled crystal oscillator and employs a phase-locking technique. The absolute phase noise performance of the 6834 MHz signal was measured to be −60 and −120 dBc/Hz at offset frequencies of 1 Hz and 10 kHz, respectively. For the new scheme, the direct digital synthesizer is used to tune the clock output. The frequency tuning resolution was improved to 3.4 × 10−15. This scheme for the microwave frequency synthesizer can also be used in other precision measurements.

Development of low phase noise microwave frequency synthesizers for reducing Dick effect of Cs fountain clocks

The Dick effect is one of the main factors limiting the short-term frequency stability of Cs fountain clocks, which is especially decided by the phase noise of the local oscillator at smaller offset frequency. Here we report on the development of a 9.192 GHz microwave frequency synthesizer with low phase noise to be used as the local oscillator for Cs fountain clocks. The synthesizer is based on frequency multiplication and synthesis from an ultra-low phase noise 5 MHz Oven Controlled Crystal Oscillator (OCXO). The key component of the frequency multiplication is a non-linear transmission-line (NLTL) as a frequency comb generator. Based on the principle of the NLTL, we carefully optimized the input power, the input and output impedances of the NLTL to suppress its excess phase noise. The measured results show that the absolute phase noises of 9.192 GHz signal are -64 dBc/Hz, -83 dBc/Hz, -92 dBc/Hz and -117 dBc/Hz at 1 Hz, 10 Hz, 100 Hz and 1 kHz offset frequencies, respectively. The residual phase noise of the synthesizer is − 82 dBc/Hz at 1 Hz offset frequency. The frequency stability limit due to the absolute phase noise via Dick effect is theoretically estimated to be 7.0 × 10−14τ−1/2.

Parity-time-symmetric optoelectronic oscillator.

An optoelectronic oscillator (OEO) is a hybrid microwave and photonic system incorporating an amplified positive feedback loop to enable microwave oscillation to generate a high-frequency and low-phase noise microwave signal. The low phase noise is ensured by the high Q factor of the feedback loop enabled by the use of a long and low-loss optical fiber. However, an OEO with a long fiber loop would have a small free spectral range, leading to a large number of closely spaced oscillation modes. To ensure single-mode oscillation, an ultranarrowband optical filter must be used, but such an optical filter is hard to implement and the stability is poor. Here, we use a novel concept to achieve single-mode oscillation without using an ultranarrowband optical filter. The single-mode operation is achieved based on parity-time (PT) symmetry by using two identical feedback loops, with one having a gain and the other having a loss of the same magnitude. The operation is analyzed theoretically and verified by an experiment. Stable single-mode oscillation at an ultralow phase noise is achieved without the use of an ultranarrowband optical filter. The use of PT symmetry in an OEO overcomes the long-existing mode-selection challenge that would greatly simplify the implementation of OEOs for ultralow-phase noise microwave generation.

Theoretical investigation of an ultra-low phase noise microwave oscillator based on an IF crystal resonator-amplifier and a microwave photonic frequency transposer

Resonator Q-factor and amplifier flicker noise are the most important parameters that determine the phase noise of microwave oscillators. The Q-factor of intermediate frequency (IF) crystal resonators at IF frequencies can be larger than that of microwave resonators at microwave frequencies. In addition, flicker noise of IF amplifiers can be less than that of microwave amplifiers. One of the well-known usual ways to implement low phase noise microwave oscillators is to use ultra-low phase noise IF crystal oscillators and frequency multipliers. In this paper a new method to implement such oscillators based on IF crystal resonators and amplifiers, called a transposed-frequency microwave oscillator, is proposed. In the proposed structure, a microwave photonic frequency transposer is used to downconvert RF to IF followed by a high-Q IF crystal filter and low flicker noise IF amplifier and then upconverting IF to RF frequency. This novel method in comparison to the well-known multiplier-based microwave oscillators presents significant advantages. In this paper, the proposed new structure is introduced, and a theoretical model is developed to study its performance. Oscillation frequency and amplitude, equivalent Q-factor, phase noise, etc., are theoretically studied and compared with the performances of the well-known microwave oscillators based on frequency multipliers and optoelectronic oscillators. It is shown that the phase noise of the proposed microwave oscillator can be less than −130 dBc/Hz at 100 Hz from a 10 GHz carrier with a noise floor less than −165 dBc/Hz.

Electro-optic comb based real time ultra-high sensitivity phase noise measurement system for high frequency microwaves

Recent progress in ultra low phase noise microwave generation indispensably depends on ultra low phase noise characterization systems. However, achieving high sensitivity currently relies on time consuming averaging via cross correlation, which sometimes even underestimates phase noise because of residual correlations. Moreover, extending high sensitivity phase noise measurements to microwaves beyond 10 GHz is very difficult because of the lack of suitable high frequency microwave components. In this work, we introduce a delayed self-heterodyne method in conjunction with sensitivity enhancement via the use of higher order comb modes from an electro-optic comb for ultra-high sensitivity phase noise measurements. The method obviates the need for any high frequency RF components and has a frequency measurement range limited only by the bandwidth (100 GHz) of current electro-optic modulators. The estimated noise floor is as low as −133 dBc/Hz, −155 dBc/Hz, −170 dBc/Hz and −171 dBc/Hz without cross correlation at 1 kHz, 10 kHz, 100 kHz and 1 MHz Fourier offset frequency for a 10 GHz carrier, respectively. Moreover, since no cross correlation is necessary, RF oscillator phase noise can be directly suppressed via feedback up to 100 kHz frequency offset.