by Dr. Bo Zhou, Nanjing University of Posts and Telecommunications (NJUPT)
The X-band frequency range is widely used for satellite communications (SATCOM) because it provides a number of advantages over lower-frequency systems, including resilience to interference and weather, smaller terminal size, higher data rates, and ability to provide remote coverage. SATCOMS are further enhanced through the use of active phased-array antennas, which are made possible by advances in electronic packaging technology and semiconductor device performance. By employing space-saving, high functionality system-on-chip (SoC), system-in-package (SiP), and low temperature co-fired ceramic (LTCC)-based modules, SATCOM systems with active-phased arrays offer greater flexibility and robustness, as well as the ability to detect targets more accurately through electronic beam scanning [1-6].
An active phased-array antenna with higher gain (directivity) and a narrower, steerable beam provides greater range coverage with a smaller aperture than would be possible using a conventional antenna. However, greater design complexity is the tradeoff for achieving this performance improvement because each antenna element is interconnected to a transmit/receive (T/R) module and beam-steering electronics. Consequently, RF performance, compactness, and cost of the T/R module are key design goals.
This article describes the design of a highly integrated X-band receiver module based on a 10-layer LTCC substrate using NI AWR Design Environment electronic design automation (EDA) software. The design achieved the key design goals by integrating surface-mount components with embedded distributed (passive) components built directly into the receiver module substrate.
The receiver module included two bandpass filters (BPFs) and a hybrid coupler that were constructed from distributed structures integrated into the layers of the LTCC receiver substrate. The input RF signal was filtered and amplified before being down converted to the several tens of megahertz intermediate-frequency (IF) band. This low frequency signal was then filtered and amplified for final signal processing. Due to the low frequency of operations (most IFs are below 200 MHz in super-heterodyne receivers) and longer wavelength, the IF-band components are typically among the largest components in the receive chain, constraining efforts to reduce the size of the module.
LTCC Circuit Design Flow
The X-band receiver architecture in this design downconverts signals at a center frequency of 9 GHz to the IF center frequency of 60 MHz. The receiver was designed to achieve a gain over 51 dB, a noise figure (NF) below 2.5 dB, and image rejection above 37 dB. The overall size of the receiver module was restricted to a 54 mm × 15 mm × 1 mm form factor. The 15 mm width of the receiver module was predicated on the element spacing in the phased array, which called for a half-wavelength distance between adjacent antenna array elements at the design frequency. Since size reduction was a key goal, an initial design task was to choose an appropriate RF link with a minimal number of circuit elements. The proposed diagram is shown in Figure 1.
The system was composed of two low-noise amplifiers (LNAs), two X-band BPFs, one image rejection mixer (IRM), one quadrature hybrid coupler, one IF BPF, and one IF amplifier. The LNAs were Analog Devices HMC902s, the reject mixer was an Analog Devices HMC520, and the IF amplifier was a Prewell PW112 device. The PW112 is a high-performance indium gallium phosphide (InGaP) heterojunction bipolar transistor (HBT) monolithic microwave integrated circuit (MMIC) Darlington pair amplifier with temperature compensation that offers an operating frequency range from DC to 1 GHz with 26 dB gain at 70MHz. This configuration and lineup of devices provided an overall path gain of about 51 dB.
The 10-layer LTCC structure provided an excellent medium for developing a compact, high-density module with integrated passive components constructed from multiple stacked layers of metallization and dielectric substrate forming the capacitive and inductive elements from which passive devices (BPFs/couplers) could be designed. Vias and trace metals provided the interconnects between elements in the receive module.
NI AWR Design Environment software enables users to create a stackup definition of the LTCC structure and define the various layers and their properties to be used for EM simulation and manufacturing layout (Figure 2). This material stackup, which maintains/stores these layer definitions, can also be saved as a technology or layer-process file (LPF) and reused for future design projects. Furthermore, a technology file can be incorporated into a process design kit (PDK), which also includes a component model library for simulation, layout, and manufacturing rules. The software enables design teams to create parameterized components that control the shape, size, and layer placement of these highly vertical structures.
Link Analysis with Visual System Simulator
Visual System Simulator™ (VSS) system design software helps designers establish the right system architecture and formulate suitable specifications for each of the underlying components in communications systems. The RF link analysis technology enables designers to account for all the gains and losses in a receiver (or transmitter) chain. By accounting for the signal level (and spurious emissions from device nonlinearities) as it propagates through the system, designers can determine if the level (gain) needs to be increased for proper target detection, decreased (attenuated) to avoid amplifier compression, filtered, and more. The RF link analysis also enables designers to calculate cascaded performance of the RF link to determine performance metrics such as error vector magnitude (EVM), cascaded noise figure (NF), gain, and/or output third-order intercept (OIP3). Designers can perform rapid exploration of system architecture and component specification options in order to achieve ideal cost, performance, reliability, and size tradeoffs.
The designers of this X-band receiver module used behavioral models to represent the RF properties of each component in the receiver, enabling them to investigate the individual contribution of each component, as well as the overall system performance, and to make any necessary adjustments in order to achieve the required specifications. By substituting vendor component models for the initial ideal elements, they were able to develop a bill of materials and determine which components would play a critical role in system performance and might require a more costly part, as well as discover an alternative that would reduce cost without sacrificing overall performance. Figure 3 shows the block diagram (top) and RF budget analysis (bottom) for the 9 GHz receiver, which includes the overall performance for a given measurement type such as NF, gain, or OIP3, as a function of cascaded components in the RF signal chain.
RF and IF BPF Design
While the dual-mode BPFs reported in [5-7] suppressed the upper stopbands, the orthogonal orientation of the I/O feed ports was not well-suited for making an RF connection to the adjacent components and therefore a modified dual-mode LTCC BPF with nonorthogonal I/O feed ports was developed. The center frequency of the proposed BPF was 9 GHz with a bandwidth of 1 GHz. To realize an enhanced capacitive coupling for a relatively wide fractional bandwidth (FBW) of 11%, the square ring resonator was fed by two-layer, broadside parallel-coupled lines. The bottom side of the second layer was set as the ground layer. The two-layer structure not only achieved high coupling capacitance, but also gained a size reduction using vertical broadside coupled lines instead of planar-edge coupled lines. The wide upper stopband was achieved by the stepped-impedance microstrip lines near the I/O ports, shown in Figure 4. The vertical and horizontal side strips were designed with a width of W1 and W2, respectively. The vertical and horizontal side strips had the same length, L. The characteristic impedance of the I/O microstrip lines was 50 Ω.
The AXIEM method-of-moments (MoM) planar 3D electromagnetic (EM) simulator in Microwave Office software  was used to simulate the structure’s frequency response and obtain the physical dimensions based on filter performance optimization. The parameterization of the filter dimensions allowed the filter geometry to be modified not only for passband/reject-band performance, but also for yield optimization, which was improved to over 85% for the known manufacturing tolerances. The simulated and measured results in Figure 4 show a center frequency of 9 GHz with (3 db) filter bandwidth of 11%. The filter has an insertion loss of 1.8 dB and return loss of 12 dB in the passband, 40 dB suppression at 2 f0 (18 GHz), and greater than 20 dB suppression up to 23 GHz. The BPF fit within a 7×4×0.2 mm footprint (without pads for GSG probes).
Mixer and Hybrid Design
The HMC520LC4 is a compact I/Q MMIC mixer in a leadless SMT package, configured as an image reject mixer. The device uses two standard double-balanced mixer cells and a 90-degree hybrid local-oscillator (LO) divider fabricated in a gallium arsenide (GaAs) metal-semiconductor field-effect transistor (MESFET) process. The LO is applied to this integrated coupler where it is split into in-phase and quadrature signals and applied to the two mixers. The 90-degree phase difference applied to the mixers’ LO ports allows the upper and lower sideband signals to combine or cancel at the output ports of an IF hybrid coupler.
The IF outputs of the mixers were 90 degrees out of phase with each other. To utilize the lower sideband from the mixer, the IF1 pin of the device was connected to the 0° port of a hybrid combiner and the IF2 pin was connected to the 90° port of the hybrid. The 90-degree phase delay in the hybrid resulted in additive image signals at one of the hybrid’s output ports. The two signals at the other port were equal in amplitude but 180 degrees in phase, so they canceled. The desired IF output was from the sum port of the hybrid, and the difference port was terminated in a 50 Ω load.
In the tradeoff of integration, size, and performance, the choice of the IF frequency determined how the IF 90-degree hybrid could be implemented, or whether it should use discrete chip components. Since the IF signal was centered at 60 MHz with a 20 MHz passband, a quadrature hybrid was designed based on a lumped-element design constructed from capacitors and inductors fabricated with LTCC elements. A lumped-element implementation of the 90-degree hybrid was achieved with four inductors and four capacitors, as shown in the circuit schematic on the left side in Figure 5. The four-port frequency response, detailing the insertion and return loss as well as the isolation (S41), is shown on the right side in Figure 5.
The ideal hybrid lumped-element values were derived from literature, providing the capacitance and inductance values (51pf, 126 nH) for the LTCC design. Because the LTCC substrate was a layered structure, the lumped-element components of the hybrid circuit were realized with a very compact multi-layer design using stacked capacitive plates and inductive coils. EM analysis was used to characterize the electrical behavior of the LTCC passive devices by simulating the frequency response of the individual components as a function of their physical dimensions. This information enabled the designers to select the dimensions that provided the required inductance/capacitance.
EM data can also be used to develop a more efficient design flow based on an LTCC PDK. PDK development involves building a custom library of electro-physical models supported by the specific process. In addition to processing specific models, a customized LPF must be created based upon the specific dielectric and metallization layers chosen for the design. The LPF will be utilized by both the simulation and layout tools to synchronize the placement of components within the dielectric stackup, and provides the ability to easily cut and paste layout geometries into an EM simulator for final verification.
One of the many advantages of LTCCs is that they enable engineers to create highly compact designs by leveraging the large number of available layers. To take full advantage of the LTCC process, it is essential to develop a set of custom models for structures that go beyond the standard microstrip, stripline, and coplanar waveguide elements in most standard RF EDA solutions. This design requires compact inductive and capacitive elements that maximize use of the Z-plane, especially for the larger IF-band designs. While it would be possible to create the two-layer rectangular microstrip inductors that are popular in MMICs, it is far more cost effective to build the inductor on successive vertical layers in an LTCC process, as seen in Figure 6.
Figure 7 shows the IF-band hybrid, including the 3D structure (a), the 2D layout view (b), a photograph of the fabricated device (c), and the measured S-parameters (d). Using the LTCC PDK, designers were able to determine the dimensions and number of layers required to achieve the inductance and capacitance values determined by the ideal lumped-element hybrid. By combining these elements through the appropriate interconnects, the hybrid performance was verified initially through EM analysis of the entire structure and then through measurement of the isolated device in a test fixture, as shown in the Figure 7d insert.
IF-Band Filter Design
Using the same approach, an IF-band BPF was also designed for a 10-layer LTCC substrate using the lumped-element filter design shown in Figure 8 (c). A test device with ground-signal-ground pads was also created for direct measurements using RF probes, shown in Figure 8 (b).
Figure 9 shows the simulated versus measured performance results. The narrowband filter was designed to be centered at 60 MHz with a 15 MHz bandwidth and minimum in-band return loss of 15 dB. Two finite zeros were located at the prescribed frequencies. The measured filter exhibited a higher passband insertion loss of 1.95 dB than the simulation, which was attributed to manufacturing artifacts such as the greater surface roughness of the top metal layer that introduced a higher resistive loss.
The size of the BPF was only 10 × 10 × 1 mm (without pads for GSG probes), which is equivalent to 0.004 × 0.004 × 0.0004 λg, where λg is the guided wavelength on a 1-mm thickness (10 layers) Ferro-A6 substrate at 60 MHz.
Once the passive components were designed and measured, the next step was to interconnect all active and passive components according to the system diagram described in Figure 1. The complete receiver was also assembled in a 10-layer LTCC substrate. Each LTCC layer had a post-fired thickness of 0.1 mm in Ferro-A6 material with a dielectric constant of 5.9 and loss tangent of 0.002. The planar and 3D views of the complete receiver are shown in Figure 10. The input and output ports are located at the left and right side and the LO port and DC feed ports are placed on the top side.
The size of the receiver module was only 54 mm ×15 mm ×1 mm. After the chips were mounted and the cavity assembled on the LTCC board, the receiver module needed to be heated for 20 minutes at a temperature of 130° C to guarantee the paste adhesive worked well. The interconnections between chips and microstrips were checked using a microscope to avoid a circuit short.
Simulated vs. Measured Results
The measured results are summarized in Table 1. As shown in Table 2, the measured gain is better than 51 dB, the NF is better than 2.5 dB, and the image rejection is better than 37 dB in the working band. The measured results meet the requirements of the receiver.
Three passive microwave components were proposed and integrated into a 10-layer LTCC receiver module with high integration. The overall size of the receiver module was only 54 mm × 15 mm ×1 mm. The measured gain parameter of the proposed receiver was higher than 51 dB, and NF and image rejection were better than 2.5 dB and 37 dB in the working band, respectively. The combined use of VSS software for system simulation of the link budget analysis, Microwave Office for linear/nonlinear RF component synthesis and simulation, and the AXIEM EM simulator for passive device characterization proved critical in the successful development of this compact, LTCC X-band receiver for phased-array applications. The use of 10-layer LTCC substrates for a compact design using stacked passive components was made possible with the LTCC design flow and PDKs available with NI AWR design software.
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 NI AWR software AXIEM EM simulator, El Segundo, CA.
Special thanks to Qiang Ma, Liwei Yan, Na Zhou, and Chong-hu Cheng of NJUPT for their contributions to this article. The study was supported by National Natural Science Foundation of China and Natural Science Foundation of Jiangsu Province.