Monolithically-integrated 3D printed coaxial bandpass filters and RF diplexers: single-band and dual-band

The manuscript reports on additively-manufactured (AM) coaxial-resonator-based bandpass filters (BPFs) and RF diplexers. A monolithic integration concept using stereolithography apparatus (SLA) is proposed and discussed in detail. Coupled-resonator-based synthesis alongside full-electromagnetic-based design methods is used for the design of the monolithic filters and RF diplexers. In particular, the paper discusses a new external coupling mechanism for dual-band BPFs that allow to independently control the coupling in each of the BPF passbands. Furthermore, a novel coaxial transmission line-type T-junction is proposed for the design of single- and dual-band RF diplexers. For practical validation purposes, multiple BPF and RF diplexer prototypes were designed, manufactured and tested at S- and C-band demonstrating the applicability of the proposed concept to low-cost, low-loss and low-weight RF components with complex geometrical features.

Determining the Stable Injection Locking of a Fabry-Pérot Laser by Observing the RF Spectral Components Generated by a Low-Reflectivity External Cavity

A novel approach to monitoring the laser injection-locking (IL) state is proposed and verified using the side-mode suppression ratio (SMSR). In a photonics experiment for laser IL, an optical spectrum analyzer with the conventional criterion of a 35-dB SMSR is conventionally used to detect the locking state of a Fabry–Pérot (FP) laser with multiple longitudinal modes to an external master laser with one longitudinal mode. Since the 35-dB criterion is not always a sufficient locking condition, we propose a microwave-photonic technique to determine the stable-locking regime based on the observation of the radio-frequency (RF) components. A novel approach to monitoring the generated additional spectral components uses the well-known delayed-self-homodyne technique and the RF spectrum analyzer. For the novel generation of additional longitudinal groups on each FP laser’s resonator mode in the optical spectrum and consequently the overlapping RF components in the RF spectrum, an additional external resonator with low reflectivity was connected to the slave FP laser. The novel monitoring approach was experimentally verified by connecting a 1-m-long external cavity with 0.5% reflectivity and observing the optical IL phenomenon of a 1550-nm FP semiconductor laser.

A Tracking Method in FM Broadcast-based Passive Radar Systems

Passive radars are popular because without the expensive, high-power-rated RF components, they are much cheaper than the active ones, nevertheless, they are much harder to detect from their electromagnetic emission. Passive radars produce so-called RV matrices in an intermediate signal processing step. Although accurate RV matrices are found in DVBT-based passive radars, the characteristics of the FM signals are not always suitable for this purpose. In those situations, further signal processing causes false alarms and unreliable plots, misleads the tracker, and consumes power for processing unnecessarily, which matters in portable setups. Passive radars also come with the advantage of a possible MIMO setup, when multiple signal sources (broadcast services for example) are reflected by multiple targets to the receiver unit. One common case is the stealth aircraft’s which form is designed to reflect the radar signal away from the active radar, but it could also reflect the signals of the available broadcast channels. Only one of these reflected signals could reveal the position of the target.

Novel Object Detection and Multiplexed Motion Representation in Retinal Bipolar Cells

Antagonistic interactions between the center and surround receptive field (RF) components lie at the heart of the computations performed in the visual system. Center-surround RFs are thought to enhance responses to spatial contrasts (i.e., edges), but how they contribute to motion processing is unknown. Here, we addressed this question in retinal bipolar cells, the first visual neuron with classic center-surround interactions. We found that bipolar glutamate release emphasizes objects that emerge in the RF; their responses to continuous motion are smaller, slower, and cannot be predicted by signals elicited by stationary stimuli. The alteration in signal dynamics induced by novel objects dwarfs the enhancement of spatial edges and can be explained by priming of RF surround during continuous motion. These findings echo the salience of human visual perception and demonstrate an unappreciated capacity of the center-surround architecture to facilitate novel object detection and multiplexed encoding of distinct sensory modalities.

Harmonic Dual-Band Diode Mixer for the X- and K-Bands

This paper presents a new dual-band diode mixer for the X- and K-bands. The proposed mixer consists of a pair of series-connected diodes and a frequency-dependent delay line that operates at 180° and 360° at the X-band of 10.525 GHz and at the K-band of 24.15 GHz, respectively. Without reconfigurable devices such as switches, the proposed mixer operates as a single-balanced diode mixer at the X-band and a subharmonically pumped antiparallel diode mixer at the K-band simultaneously. The designed circuit was implemented in a hybrid microwave integrated circuit using discretely packaged RF components on a microwave printed circuit board. The measurement results showed conversion losses of 6.5 dB and 16.6 dB at the X- and K-bands, respectively.

A DC Circuit Model of Distributed Diodes on Active Substrate Boards for CAD Tools

In the past few years, a new type of circuit board, named here as active substrate board (ASB), was introduced over circuit applications of diodes. Unlike a traditional printed circuit board (PCB), an ASB has its substrate made of a semiconductor. The inability of the traditional integrated circuit (IC) technology to integrate wavelength dependent radio frequency (RF) components triggered the advent of ASBs. These boards draw desirable features from IC as well as PCB technologies. Unprecedented challenges came up in modeling the different devices fabricated on an ASB owing to their large sizes and the presence of wideband microwaves. So far, modeling the effect of large sizes and ambient microwaves on DC bias of diodes have not been considered in scientific literature. Furthermore, the state of the art numerical simulators are unable to imitate the behavior of such diodes observed over measurements. Here, a semi-analytical, circuit model of distributed diodes on ASB is presented that is fairly accurate in predicting the actual behavior of the diodes. The model also opines a novel phenomenon where microwaves affect the DC characteristics of diodes with added resistances.