With the rapid advancement of GNSS (Global Navigation Satellite System) and the ongoing expansion of China's Beidou second-generation satellite navigation system, the application of satellite navigation and positioning technology is becoming increasingly widespread and deeply integrated into various industries. As a result, there is a growing demand for satellite receiver chips that are more energy-efficient, produce less noise, and have lower costs. Among the key components in RF front-end circuits, the mixer plays a crucial role in frequency conversion, making it a focal point of current research. The main objective of this paper is to develop a low-noise mixer suitable for wideband GNSS applications.
1. Design Specifications
The design aims to create a low-noise mixer capable of operating across the entire GNSS frequency band, from 1 GHz to 1.6 GHz. The specific performance requirements include: 1) RF input frequency range of 1–1.6 GHz; 2) IF output frequency of 46 MHz; 3) RF input power between -120 dBm and -30 dBm; 4) Local oscillator (LO) power of -10 dBm; 5) Operating voltage of 3.3 V; 6) Conversion gain of at least 10 dB; 7) Noise figure below 4 dB; and 8) A 1 dB compression point of -17 dBm.
2. Overall Mixer Design
The circuit layout of the mixer is illustrated in Figure 1. The load circuit includes R1, C1, R2, and C2, which form two first-order RC low-pass filters to suppress high-frequency leakage signals such as LO and RF from reaching the output port. At the transconductance stage, Q8, R9, Q5, R7, Q6, and R8 form a current mirror configuration to ensure stable biasing and signal amplification. Proper resistor values are selected to allow efficient RF signal injection while minimizing interference. The switching stage uses Q7 as a diode-connected transistor to provide bias voltage for the switching transistors Q1–Q4. R5 and R6 are chosen to be large enough to facilitate LO signal injection, while R3 and R4 set the bias current and voltage for the switch stage. Capacitors C1 and C2 are both 2 pF, and smaller-sized transistors like Q7 and Q8 are used to reduce power consumption. Their model is N05005011SH. The resistor values are listed in Table 1.
3. Simulation Results
Using the "MixConvGainNF Schematic Template" provided by ADS, we simulated the mixer’s performance. When the RF input frequency was 1.575 GHz with an input power of -85 dBm, and the LO power was -10 dBm, the measured conversion gain was 15.79 dB, and the single-sideband noise figure was 4 dB, as shown in Figure 2. The mixer was also tested over the full 1–1.6 GHz range, and the results showed a gain variation of less than 2 dB, maintaining a gain above 15 dB. The noise figure remained stable, confirming the broadband low-noise performance of the design. Additionally, the linearity of the mixer was evaluated, and the 1 dB compression point was found to be -17 dBm, meeting the project requirements.
4. Results Analysis
From the simulation results in Figure 3, it is evident that within the 600 MHz bandwidth of 1–1.6 GHz, the gain remains above 15 dB with a variation of less than 2 dB, satisfying the design specifications. The noise figure also shows minimal fluctuation, demonstrating the mixer’s effectiveness in maintaining low noise across the entire frequency range. Furthermore, the 1 dB compression point of -17 dBm, as shown in Figure 4, confirms that the mixer operates linearly within the expected input power range, ensuring reliable performance in real-world GNSS applications.
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