Patent Application
20120286899
1. Field of the Invention
This invention relates generally to a true time delay (TTD) line and, more particularly, to a TTD line circuit including one or more Archimedean spiral delay lines and components for providing electric and/or magnetic isolation between the delay lines.
2. Discussion of the Related Art
TTD lines are electrical devices that delay an electrical signal, such as an RF signal, for a defined period of time. Standard TTD technology employs digitally switched transmission line sections where weight, loss and cost increase rapidly with increased operational frequency and/or phase tuning resolution.
TTD lines have application for many electrical circuits and systems, especially wideband systems. For example, TTD lines have application for wideband pulse electronic systems, where the TTD line provides an invariance of a time delay with frequency or a linear phase progression with frequency. In this application, the TTD line allows for a wide instantaneous signal bandwidth with virtually no signal distortion, such as pulse broadening during pulsed operation.
TTD lines also have application in wideband phased array antenna systems. These types of phased arrays provide beam steering where the direction of the antenna beam can be changed or scanned for the desired application. As the beam radiation pattern changes, the phase of the received signals at the node from different antenna elements also changes, which needs to be corrected. Phase shifters can be provided for each antenna element for this purpose. The frequency and bandwidth of a conventional phased antenna array is altered or limited by the bandwidth of the array elements, where limitations are caused by the use of the phase shifters to scan the antenna beam. TTD lines can be employed in the place of phase shifters to provide a delay in the transmitted and received signals to control the phase. The use of TTD lines potentially eliminates the bandwidth restriction by providing a theoretically frequency independent time delay on each antenna element channel of the array.
The most distinct advantage of a TTD based phased array is the elimination of the beam squint effect. Compared to those phase shifter based phased arrays, TTD based phased arrays can simultaneously operate at various frequencies over a very wide bandwidth without losing precision of antenna directionality across the entire band.
There are a number of techniques and designs in the art for providing TTD lines. For example, high temperature superconductor delay line structures have been disclosed. One particular structure of this type includes two substrates having thin film strips on opposing sides that are in contact with each other to implement a single strip-line circuit, which provides an air gap between the substrates. However, this type of design provides a narrow RF line width that increases overall signal loss. If a wider strip line is used, then extra long tapered transformer sections are required to interface with 50 ohm systems, which causes extra size and loss that complicate the design. Further, there are related manufacturing issues in that only periodic contacts exist on the RF traces. Also, accumulative cross-talk and forward/backward coupling may be a problem. The design is also typically expensive to deploy and difficult to integrate with other components and systems.
Coaxial delay lines are also known in the art and have long been used in electronic systems to delay, filter or calibrate signals. Coaxial delay lines can be provided in many different sizes and formed into countless configurations. Certain front-end designs can improve cost, size, configuration and overall electrical performance of not just the delay line, but the overall system. However, coaxial delay lines are typically not suitable for planar integration, are vulnerable to mechanical forming and have a velocity factor that is higher than most commercially available coaxial cables.
Other known TTD lines include constant R delay lines, varactor non-linear transmission line (NLTL) tunable delay lines, ferro-electric substrate tunable delay lines, dielectric filled waveguide delay lines, surface acoustic wave (SAW) delay lines, air line inside a PCB three-dimensional coaxial structure delay line, micro-electro-mechanical system (MEMS) tunable transmission delay lines, meta material structure synthesized transmission delay lines, photonics delay lines, resonator structure delay lines, and digital time delay lines.
However, each of these TTD line designs suffers one or more drawbacks that make it at least somewhat undesirable for wideband applications, such as wideband phased array antenna systems. For example, constant R delay lines are typically limited to lower microwave frequency bands and are very lossy. Varactor NLTL tunable delay lines have issues with the varactors, a small time delay range, and are difficult to tune because of being continuous in a digital command world. Ferro-electric substrate tunable delay lines have problems with linearity, require very high voltages, have variable impediments and return losses, and are difficult for providing as much delay as desired. Dielectric filled waveguide delay lines are typically very heavy and bulky for practical applications. SAW delay lines are typically difficult to implement at high frequencies, provide too much signal loss and are difficult to manufacture. Air line coaxial structure delay lines are typically heavy and bulky to be practical. MEMs tunable transmission lines typically have too small of a delay time, are often unreliable and require high voltages. Meta material structure synthesized transmission lines typically are very narrow band. Photonics delay lines typically require too much power and have significant RF losses. Resonant structure delay lines are typically difficult to provide both wide bandwidth and high delay at the same time. Digital time delay lines typically have high power consumption.
What is needed is a TTD line that provides all of the desired qualities for wideband applications, such as significant delay, ease of manufacture for monolithic integration, ease for multi-bit delay implementation, low weight, low cross-talk, forward/backward coupling, low radiation level, small size, ultra-wide bandwidth, low losses, low cost, etc.
The following discussion of the embodiments of the invention directed to TTD lines is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
The microstrip line 14 is separated into a first line section 24 having an inner port 26 at a center location of the line 14 opposite to the port 18 and a second line section 28 having an inner port 30 opposite to the port 20 and adjacent to the port 26. The two line sections 24 and 28 are concentric with each other. Circuit components, such as other time delay sections, can be coupled to the ports 26 and 30 at the center of the microstrip line 14 for reasons that would be well understood by those skilled in the art. Alternately, the ports 26 and 30 can be connected together so that the line 14 is continuous.
Because the line sections 24 and 28 are basically parallel to each other as they wind to the center of the line 14, there is signal cross-talk between the line sections 24 and 28 that causes signal loss. In other words, the signal being delayed and propagating down the line sections 24 and 28 are electro-magnetically coupled between the line sections 24 and 28 so that signal intensity is lost as a result of the signal transferring from one of the line sections 24 or 28 to the other line section 24 or 28. In order to electrically isolate the line sections 24 and 28 from each other and reduce the cross-talk, the circuit 10 includes a plurality of metal vias 32 provided between the line sections 24 and 28 that extend through the substrate 12. In this embodiment, the vias 32 are ground vias that are electrically routed to a ground plane 34 deposited and formed on a backside of the substrate 12. The metal in the vias 32 disrupts the signal electro-magnetic coupling between the line sections 24 and 28 that reduces or prevents cross-talk therebetween. These vias also help to eliminate possible cavity resonances. The number of the vias 32, the size of the vias 32, the spacing between the vias 32, the material of the vias 32, etc., would typically be different for different circuits where the various parameters for the vias 32 could be designed to provide optimal performance.
In this configuration, the metal layer 54 provides magnetic isolation between the delay lines 46 and 58 to provide an ultra-wideband delay structure. The length of the delay defined by the circuit 40 is provided by a combination of the lengths of the lines 46 and 58. Thus, the combination of the delay lines 46 and 58 being connected by the line 64 is a single delay line that is compact by the Archimedean spiral configuration, where the metal layer 54 provides magnetic isolation and prevents signal cross-talk between the lines 46 and 58 as the signal propagates from the port 50 to the port 60 with reduced backward/forward coupling effects and suppressed radiation.
Each of the circuits 10, 40, 80 and 90 discussed above provide a number of advantages for true time delay lines over those known in the art. The monolithic design of the circuits 10, 40, 80 and 90 provide ease of integration with other MMIC front end circuits with no complicated transitions. Significant reduction in radiation, cross-talk and forward/backward coupling is achieved by portioning the delay line into multiple sections on different layers. Further, the circuits 10, 40, 80 and 90 provide orders of magnitude tighter tolerance and delay lines due to the MMIC design and process, and provide a much smaller size due to the configuration. Further, the circuits 10, 40, 80 and 90 provide an optimization and design methodology for trade-off wafer/circuitry configurations with various electrical performance. The wafer level packaging (WLP) available with the MMIC designs of the circuits 10, 40, 80 and 90 provides hermetic operation from close to DC into the millimeter wavebands with unprecedented bandwidth.
The foregoing discussion discloses and describes merely exemplary embodiments. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.
