Simulation analysis of high-frequency electronic circuits based on OrCAD circuit design software

OrCAD/Pspice is a general-purpose electronic circuit computer-aided analysis and design software, and is an extremely excellent software in the circuit computer simulation program. It has powerful circuit design and simulation capabilities, and can easily realize DC analysis, AC analysis, transient analysis, noise analysis, sensitivity analysis, Fourier analysis, harmonic distortion analysis, and circuit performance analysis at different temperatures of electronic circuits, and complete the optimization of component parameters of electronic circuits. It provides a wealth of electronic component models, which can realize the testing, analysis functions of various circuit parameters and the construction function of device libraries. With the rapid development of OrCAD/Pspice, the operation becomes simpler when realizing various functions, the programming process is less restricted, and the calculation and simulation of circuits are more accurate. On the basis of mastering the circuit principles, it is convenient to use the electronic-aided simulation design software Pspice to complete the design analysis and device characteristics analysis of the required circuit. The author will analyze and discuss the simulation process of variable capacitor frequency modulation and power amplifier and transmitting circuits.

1 Advantages of OrCAD/Pspice in high-frequency electronic circuit simulation

The oscillation circuit, amplitude modulation circuit, mixing circuit, frequency modulation circuit, and demodulation circuit in high-frequency electronic circuits are widely used in life. In design and production, OrCAD/Pspice is used to assist in analyzing the functions and characteristic indicators of the required high-frequency circuits, which can easily realize various design needs of high-frequency electronic circuits. Moreover, the application of OrCAD/layout phus can quickly complete the design of actual printed circuit boards that meet the circuit performance requirements. The most important thing is that OrCAD/Pspice software calculates accurately, making the design simulation indicators more consistent with the actual effect of the circuit. The complexity of high-frequency electronic circuit design and the efficiency of OrCAD circuit software make OrCAD/Pspice more fully reflect the advantages of OrCAD-assisted design technology when designing, simulating, dividing, and manufacturing high-frequency electronic circuits: shortening the design cycle, improving the overall efficiency of design and manufacturing projects, and saving design costs; using the sensitivity analysis, tolerance analysis, noise analysis, worst-case analysis, and optimization parameter analysis functions in OrCAD to improve and ensure quality; OrCAD's large number of unit designs and rich component models and easily adjustable model parameters provide convenience for complex design analysis. Although OrCAD provides an efficient platform, when designing and analyzing high-frequency integrated circuits, you still need to learn and master the methods of OrCAD analysis, debugging, and optimization of circuits to better reflect the performance of OrCAD. For example, when OrCAD/Pspice electronic circuit computer-aided analysis and design software performs integrated circuit analysis and simulation of high-frequency electronic circuits, due to the high frequency of the loop signal and the complexity of the circuit, the simulation operation is large, the simulation debugging process is complex, and the optimization of parameters is difficult. If the design efficiency is not consciously improved, many designers will waste a lot of energy in the debugging and parameter optimization process.

2 Simulation electronic schematic diagram

As shown in Figure 1, the practical variable capacitance diode frequency, power amplification and transmission circuit, the left end IN interface is the modulation signal input. The right OUT antenna is the FM signal after power amplification and transmission output. The transistor Q1 and capacitors C1, C2, C3, C4, inductor L1 and variable capacitance diode D10 in the circuit form the carrier signal and the FM working stage of the modulation signal. The FM signal is coupled to the small signal resonant amplifier stage composed of transistor Q2, capacitor C11 and transformer TX1 through capacitor C12. This amplifier circuit works in the Class A amplification state. The FM signal is input to the power amplifier Q3 after resonant amplification of the Q2 stage. As the power output stage, the Q3 stage requires both high power and high efficiency output, so it is amplified in Class C and is required to work in the critical weak overvoltage state. Finally, it is transmitted from OUT after reasonable amplification.

To ideally complete the design requirements of the variable capacitance diode frequency modulation, power amplification and transmission integrated circuit, the design needs to carefully analyze the performance indicators of each level of functional circuits and reasonably calculate the parameters of each component, otherwise, it will be difficult to debug successfully. Even if the parameters are initially set, if the debugging and analysis is performed in the integrated circuit, the simulation calculation amount will be large due to the high frequency of the loop signal and the complexity of the circuit, and the optimization of the parameters will be difficult. When debugging, many designers repeatedly optimize the parameters of each component because the Probe module provided by OrCAD can easily determine whether the signal waveform of the measurement point is distorted. When the waveform of a certain measurement point is distorted, they fail to consider that debugging in the integrated circuit is extremely time-consuming. More importantly, the non-compliance of the performance of a certain measurement point in the integrated circuit also causes the circuit to affect each other due to the connection between the front and rear stages of the circuit. As shown in Figure 1, a buffer isolation stage is often added in the middle of the variable capacitance diode frequency modulation, power amplification and transmission circuit. Generally, a non-resonant ordinary Class A amplifier stage is used. The purpose is to isolate the oscillation stage from the power amplifier stage to reduce the influence of the power amplifier stage on the stability of the oscillation stage. Because of the mutual influence between the front and back stages in the circuit, and the superposition of small distortions at each stage, even if it is easy to determine the waveform distortion at a certain point, it is still difficult to optimize. In order to improve the design efficiency, we should start with the independent design of each functional discrete stage circuit, and then optimize the stages one by one. After all, the amount of calculation of high-frequency circuits is much higher than that of low-frequency circuits, and the distributed impedance of components and wiring should be fully considered in high frequencies.

3 Simulation analysis of key functional circuits

Figure 1 In the design of variable capacitance diode frequency modulation, power amplification and transmission circuits, the designer should first complete the design of Q1 oscillation stage, and then add the design of variable capacitance diode and modulation signal. Otherwise, when no oscillation occurs, it cannot be judged whether the oscillation stage design is not good or the parameters of the variable capacitance diode are not determined. The variable capacitance diode has a series of key parameters that need to be calculated and set. In the frequency modulation circuit, the oscillation stage does not use the ordinary capacitor three-point oscillation circuit and the Clapp oscillation circuit with low stability, but uses the Siler oscillation circuit with high stability, as shown in Figure 2.

In the Siler oscillation circuit, the oscillation frequency of the circuit can be adjusted by changing the capacitance value of C4 connected in parallel with the inductor L1. C3 uses a fixed capacitance. When C1>>C3, C2>>C3, the oscillation frequency is approximately

When C3 is selected as 40 pF, C4 is 40 pF, and other components are set according to the design requirements, the oscillator simulation waveform is shown in Figure 3. The oscillation signal frequency generated by the simulation is almost equal to the calculated design frequency, both of which are about 4MHz.

For the Q3 power amplifier stage, as shown in Figure 4, the amplifier output power is required to be large and the efficiency is high, that is, the resonant power amplifier generally works in a critical state, because the resonant power amplifier in the critical state has the largest output power and high efficiency, which best meets the design requirements, and the overvoltage state has a higher efficiency, so the working point can be close to the overvoltage state, which is better than the undervoltage state. When designing, the working state of the Q3 power amplifier circuit should be adjusted independently first, otherwise, it will be difficult to optimize the parameters because the current waveform of the working state is affected by many factors of the previous and next stage circuits. Figure 5 shows the waveform of the current Ie passing through the emitter when the load value, Vct and Vbb parameters are set, and the critical state of weak overvoltage (the upper waveform shown in Figure 5) and the strong overvoltage state (the lower waveform shown in Figure 5) are simulated. From this waveform, it can be judged that the working state of the power amplifier stage has been adjusted.

On the basis of the successful design and analysis of each independent functional circuit, the previous stage begins to gradually associate the next stage for debugging and analysis, considering the influence of related stages, so as to complete the design requirements of the entire integrated circuit, which will greatly improve the efficiency of circuit parameter optimization.

4 Conclusion

Through the simulation process analysis of the design of high-frequency electronic circuits-comprehensive circuit diagrams-varicap diode frequency modulation, power amplification and transmission circuits by OrCAD/Pspice, designers should fully consider the mutual influence between the previous and next stage circuits and the influence of the simulation process calculation when debugging and optimizing high-frequency integrated circuits. The use of associated optimization methods can efficiently realize the optimization design of high-frequency electronic circuits.