Low temperature co-fired ceramic (LTCC) technology is a modern multi-layer substrate process that emerged in the mid-1980s. This innovative technique uses a unique material system, allowing it to be sintered at relatively low temperatures. As a result, LTCC can be combined with metal conductors during the co-firing process, significantly enhancing the performance of electronic devices.
This paper presents a design for a miniaturized, low-noise amplifier (LNA) tailored for wireless local area networks (WLANs), leveraging the advantages of LTCC technology. In practice, low-noise microwave amplifiers (LNAs) are widely used in various applications such as microwave communications, GPS receivers, remote sensing and control, radar systems, electronic warfare, radio astronomy, geodetic mapping, television, and high-precision microwave measurement systems. With the advancement of industrial technology, the demand for miniaturization has grown significantly, making LNAs more compact and efficient.
1. Technical Advantages of LTCC
Compared to conventional FR4 substrates, LTCC offers significant advantages in high-frequency performance. Due to its three-dimensional multi-layer integration capability, LTCC provides a much smaller footprint compared to traditional microwave substrates like Teflon PTFE. Additionally, LTCC materials offer a wide range of dielectric constants, making them adaptable to different frequency bands.
When compared to HTCC, LTCC's lower sintering temperature allows the use of low-melting-point, low-loss conductor pastes such as silver and gold, which reduces signal loss in the final product. Moreover, its thermal expansion coefficient closely matches that of semiconductor processes, making it ideal for active and passive integration.
In summary, LTCC technology has several key advantages:
- Excellent high-frequency and high-Q characteristics, supporting frequencies up to several tens of GHz, meeting the demands of advanced RF and microwave applications.
- High-conductivity metals like silver, gold, and copper are used as conductors, improving the quality factor and reducing circuit losses.
- High integration capability, enabling boards with dozens or even hundreds of layers, allowing embedded passive components and various types of elements, including EMI suppressors and protection circuits.
- Ability to withstand high current and temperature, with better thermal conductivity than standard PCBs.
- High reliability, suitable for harsh environments such as military, aerospace, and automotive electronics.
- Lower production costs due to a discontinuous manufacturing process that improves yield and reduces waste.
2. Overall Design
2.1 Low Noise Amplifier Design Principle
A low-noise amplifier differs from a general-purpose amplifier in that it prioritizes noise matching over maximum gain. As a result, the gain may be slightly reduced. The gain achieved under optimal noise matching is referred to as the correlation gain, typically about 2 to 4 dB less than the maximum gain.
2.2 Performance Specifications, Device Selection, and Single-Stage Circuit Simulation
This design employs a two-stage amplification structure. The performance requirements include a noise figure ≤ 0.8 dB, gain ≥ 24 dB, gain flatness ±0.5 dB, input and output VSWR ≤ 1.3, and stability across DC to 14 GHz.
For the first stage, the ATF55143 transistor is selected for its high gain, low noise figure, small size, and stable operating point. The board is fabricated using Ferro’s A6 diaphragm, taking into account built-in inductance, capacitance, and mechanical strength. The board thickness is 0.6 mm.
Figure 1 shows the schematic of the first-stage biasing and matching circuit. Proper selection of components such as C1, L1, C2, and L2 is crucial for achieving the desired noise performance, S11, S22, and gain. These LC high-pass filters also help suppress low-frequency oscillations. The choice of R1 and R2 helps reduce leakage current and stabilize low-frequency signals.
The bypass capacitors C3 and C6 provide filtering and stabilization, while R3 and R4 ensure an appropriate gate bias voltage. Source-level series negative feedback plays a vital role in maintaining full-band stability. These microstrip lines act as small inductors, simplifying the design and reducing costs.
The second stage utilizes the RFMD SPF-5043Z monolithic amplifier, known for its compact size, low noise, simple power supply, and good stability. Both stages are powered by a single supply, with low voltage and current for easy adjustment.
2.3 Debugging and Optimization of Two-Stage Circuit
During the assembly and debugging of the two-stage circuit, challenges often arise related to stability and input VSWR not meeting specifications. Even after simulation, many components may not perform as expected, such as mismatched capacitor, inductor, or resistor values, or overly narrow trace widths. Simply connecting single-stage circuits without proper tuning often leads to failures in meeting the design criteria.
Figure 2 illustrates the overall circuit simulation results, showing the performance before final optimization. Through iterative adjustments and fine-tuning, the design was improved to meet all required specifications.
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