Abstract: This paper introduces the design concept and principle block diagram of the transmit upconverter in a radar system, and focuses on the design of the mixer and several design methods of the amplifier circuit. Keywords: mixer; amplifier; monolithic microwave integrated circuit (MMIC) 1. Overview The transmit upconverter component is an important part of the frequency synthesizer in the radar system, and its performance directly affects the overall system performance. The main function of the transmit upconverter component is to convert an intermediate frequency (IF) linear frequency modulated signal to the frequency of the transmit signal and amplify it to a suitable power level. To achieve this, the IF signal needs to undergo two upconversion processes: the first upconverter, called the IF upconverter, mixes the IF signal with an 800 MHz local oscillator signal to obtain a modulation signal with a center frequency of 940 MHz. Then, in the second upconverter (called the transmit upconverter), it is mixed with a local oscillator signal with a frequency of 4.26–4.86 GHz to finally obtain the desired transmit signal. 2. Technical Specifications and Requirements of Transmitter Upconverter Components (1) Input Signals ① Local Oscillator Signal of Intermediate Frequency Upconverter: fLO1=800 MHz, Pin=12 dBm ② Local Oscillator Signal of Transmitter Upconverter: fLO2=4.26～4.86 GHz, Pin=12dBm ③ Intermediate Frequency Signal: f=140 MHz, Δf=20 MHz FM pulse, Pin=3 dBm Source: http://www.tede.cn (2) Output Signals ① fout=5.20～5.80 GHz ② Pout≥17 dBm, in-band power fluctuation ≤2.5 dB (including temperature-induced changes) (3) Output Signal Spectrum ① Phase noise degradation ≤3 dB (input signal phase noise ≤-110 dBc/Hz@1 kHz) ② In-band spurious level ≤-60 dBc ③ Out-of-band spurious level ≤-50 dBc (4) The output path contains a high-isolation modulation switch with an isolation ≥60 dB. A "manual switch" is added to put it in the detection or working state. (5) Control signal of the modulation switch: TTL level. When the level is high, the transmission channel is unobstructed. A shaping circuit is set at the input end to shape the control signal. (6) Provide an output power detection port. (7) Provide a fault detection signal: TTL level. High level is normal, low level is fault. (8) Operating temperature: -25 °C～+55 °C 3 Design considerations for the mixer circuit According to the working principle of the mixer, two input signals to be mixed are simultaneously applied to one nonlinear device to obtain the required sum frequency or difference frequency. However, due to the nonlinearity of the device, many other unwanted combination frequencies will also be generated. The intermediate frequency product of the mixer is fIF=mfLO±nfRF. If expressed in dB, the interference signal will change by 1dB for every 1dB change in signal strength. The ratio of the strength of the n×m combined interference signal to the required n×m=1×1 signal strength changes by (n-1) dB. If the signal drops from -20dB to -30dB, then the combined interference of n×m=3×5 will decrease by 30dB, and the corresponding ratio change is: (3-1)×［-30-（-20）］=-20dB. Here, according to the requirements of this scheme, the intermediate frequency product components of the local oscillator signal at low, medium, and high frequencies were calculated and analyzed, as shown in Tables 1 to 3. Analysis revealed that the intermediate frequency components falling within the frequency band (5.20～5.80 GHz) are all fifth- or sixth-order combined components. Therefore, in the design, we use a double-balanced mixer and reduce the RF signal level, setting a high-suppression bandpass filter to reduce spurious emissions and ensure that the spectral requirements are met. 4. Transmitter Upconverter Implementation Block Diagram and Explanation 5. Amplifier Circuit Design Considerations Since this upconverter component has high requirements for spurious levels and output power fluctuations, to ensure low spurious levels, the scheme uses a method of reducing the intermediate frequency signal level for mixing. This increases the overall gain of the component (by about 60 dB), making output power fluctuations difficult to control, especially the power variation problem at high and low temperatures. Therefore, the design of the amplifier circuit needs to be carefully considered. Based on the current device availability, the amplifier circuit can adopt the following methods. The first method is to use a multi-transistor cascade method. For example, using Agilent's ATF-10136 MESFET transistors, it is estimated that at least 5 stages are needed to achieve the required power output. This method requires adding matching networks at the input and output terminals of each stage, so the circuit design must rely on microwave circuit design software to optimize the circuit. At the same time, the circuit debugging is also relatively complex and difficult. The second method is to use a monolithic microwave integrated circuit (MMIC) for the pre-amplifier stage. The input and output impedances of the monolithic microwave integrated circuit are matched to 50 Ω, thus simplifying the amplifier circuit. The final stage uses a medium-power gallium arsenide FET amplifier to amplify the power to the rated value. Since the μ-value of the amplifying transistor used is <0.12, a unidirectional design is possible. The input and output matching networks employ a conjugate matching method, with the input matching network using a parallel open-circuit branch configuration and the output matching network using a series microstrip line structure. By properly designing the amplifier gain, the final stage transistor and the preamplifier transistor are kept in saturation, ensuring that the power fluctuations of the components meet requirements during high and low temperature operation. This method is a relatively simple and commonly used approach to address the power variation of the amplifier with temperature. The third method involves using a monolithic microwave integrated circuit (MMIC) for the preamplifier stage and a temperature compensation circuit for the final stage amplifier to compensate for power fluctuations caused by temperature, ultimately ensuring that the power and power fluctuations meet requirements. This method is more difficult to implement and more expensive. In conclusion, the second method is the most reasonable approach for this design. 6. Design Considerations for the Control Circuit Based on the characteristics of the FET amplifier, we adopted a dual power supply and designed a negative voltage protection circuit, so that the positive power supply is controlled by the negative power supply to protect the amplifier from normal operation and prevent damage to the components due to power supply problems. At the same time, to ensure high isolation between the transmitting and receiving components, the drain voltage of the power amplifier tube is also controlled by the TTL level. 7. Conclusion The transmitter upconverter designed according to this scheme meets all performance requirements and has entered the practical application stage. References 1. Liao Chengen. Fundamentals of Microwave Technology. Beijing: National Defense Industry Press, 1984 2. Wu Guoji. Microwave Electronic Circuits. Xi'an: Northwestern Polytechnical University Press, 1994