## 3 Implementation of a High-Linearity Down-Conversion Active Mixer  
The mixer was manufactured using a 130 nm SiGe BiCMOS process based on Gilbert-cell. Figure 9a shows that the basic circuit is composed of bipolar junction transistors. Q1 performs the amplification of the fundamental RF signal and the generation of its harmonics, four Q2 transistors form a double-balanced switching core for frequency conversion, and a Marchand balun is used as the output load. At the same time, using phase-adjusting inductors L1, with proper choice of voltage bias and size of Q1/Q2, will make the \"reverse uplift\" possible. According to the previous analysis, the base voltage of Q1 is set to about 0.8 V to obtain stronger second harmonic energy. The emitter length of Q2 is 40 \(\upmu\)m for low-input impedance, and the value of 80 pH is selected for L1 to meet the phase condition.  
When the harmonics appear at node 1, where the second harmonic is stronger than normal due to the choice of bias voltage, the mixing of the fundamental and the second harmonic leads to additional fundamental and third harmonic components at node 2. Low-impedance load at node 3 and phase-adjusting inductors are used to meet the conditions for \"reverse uplift\"; finally, a better IP1 dB is achieved. On the other hand, when the RF input signal is large enough, the mode of the Q1 could be regarded as a base-pumped mixer, not just with the principle of the Gilbert-cell mixer, which can also explain the \"reverse uplift\" in another way.  
Figure 8: Design process of “reverse uplift”.  
As shown in Figure 9b, like the way of source degeneration, with the increasing value of emitter inductors, the IP1 dB increases continuously and the gain drops. However, in this work, before the 1 dB compression point, the gain has a proper rise to \"delay\" the gain compression. Compared with the traditional source-degenerating method, low-load impedance will not reduce the conversion gain significantly. The simulation results show that if the source degeneration is adopted, the exchange ratio of the gain and the IP1 dB is almost 1:1. If the high linearity is to be achieved, gain will be greatly sacrificed. In contrast, this method improves IP1 dB more and only sacrifices less gain. Therefore, OP1 dB can be synchronously enhanced, which benefits the system. Compared with the MGTR method, the implementation is less sensitive to bias and can be used in engineering applications. Additionally, in addition to the phase-adjusting inductors, no additional circuits are added, which is easier to be implemented and is more efficient.  
Figure 10 shows the bias circuits of the transconductance stage and the switching stage. The current mirrors are used to provide required bias voltage and current of the transistors. The values of some main components are listed in Table 1.  
Figure 10: Schematic of the bias circuits.  
Figure 9: (**a**) The proposed mixer. (**a**) Schematic of the main circuit, (**b**) comparison of the proposed method and source degeneration.  
Transformer-based matching networks are used in the RF port and LO port for single-ended to differential conversion and perform impedance matching to 50 ohms simultaneously. This process provides two thick metal layers with a thickness of 3 \(\upmu\)m and the detailed parameters of the passive components are shown in Figure 11a,b. The two coils use the M5 and M6 layers, respectively, while using the M1 layer as the ground. In order to obtain a wide IF bandwidth, the IF port uses a Marchand-balun-based matching network, as shown in Figure 12. R1 reduces the Q value of the matching network and makes the input impedance of the Marchand balun much closer to the output impedance of the switching core. R2 enables the circuit to maintain proper gain flatness in the entire IF band while matching to a 50-ohm load. The influence of R1 and R2 is shown in Figure 13.