Design of 220GHz Passive Frequency Tripler

　　1 Introduction

　　Frequency multipliers are important nonlinear circuits in high-frequency circuits of radio technology. As basic electronic devices, they are widely used in various electronic devices such as transmitters, frequency synthesizers, and receiver local oscillator sources. Submillimeter wave frequency multipliers can reduce the main oscillation frequency of the equipment and expand the operating frequency band. At the same time, since its output frequency can be selected at the nth harmonic of the input frequency, the required input signal source can be selected to be made in the relatively mature millimeter wave frequency band in terms of technology, thus providing conditions for ensuring the required frequency stability and phase noise characteristics. At the same time, solid-state frequency multipliers are small in size, easy to integrate, and have a long service life. Therefore, currently low-power submillimeter wave solid-state sources are mainly realized by frequency multiplication methods.

　　Submillimeter waves overlap with millimeter waves in the long-wave band, and overlap with infrared rays in the short-wave band. It can be seen that submillimeter waves occupy a very special position in the electromagnetic spectrum. Due to its special position, submillimeter waves have a series of special properties. In the frequency domain, submillimeter waves are in the transition zone from macroscopic classical theory to microscopic quantum theory, and in the transition from electronics to photonics. Its quantum energy is very low, the signal-to-noise ratio is very high, and the frequency is extremely wide. It covers the rotation and oscillation frequencies of macromolecules including various proteins. Therefore, it has very important academic value in academia, and has many attractive applications in science, technology and industry: such as ultra-high-speed imaging signal processing in information science, large-capacity data transmission; material processing, layered imaging technology, biological imaging; diagnosis of plasma fusion; astronomy and environmental science, etc. And it also has very important application prospects in national defense.

　　2 Design of frequency tripler

　　2.1 Overall plan

　　This solution uses a standard waveguide input, transitions through a suspended microstrip probe, connects to a low-pass filter, connects to an input matching section at the end of the low-pass filter, and then connects to a pair of diodes connected in parallel in the same direction. The output structure then transitions from a suspended microstrip to a standard waveguide. The solution block diagram is as follows:

　　Figure 1 Overall plan

　　2.2 Selection of transmission lines and dielectric substrates

　　Since the frequency of this frequency multiplier reaches 220GHz, the transmission line uses a suspended microstrip line, and most of its electromagnetic field is concentrated in the air, so its effective dielectric constant is close to 1, making its electrical parameters close to those of the air line, close to the dispersion-free characteristics; and the dielectric loss is greatly reduced, so it has a higher Q value (500~1500) than the microstrip line, and this transmission line can achieve a wide range of impedance values, which is conducive to impedance matching. [2] In addition, in order to suppress the high-order modes caused by discontinuities, the size of the cavity should be carefully selected.

　　Generally speaking, the requirements for the substrate are low microwave loss, high surface smoothness, strong hardness, good toughness and low price. The dielectric substrates that can be used in the millimeter wave band mainly include: alumina ceramics, RT/duroid5880, sapphire, quartz, etc. In addition, there are gallium arsenide and indium phosphide mainly used for MMIC. For this article, considering the factors of dielectric loss, processing accuracy, surface finish and cost, this article uses quartz as the substrate. In the design process, this article takes into account the limitations of domestic process level. The specific parameters are as follows:

　　Relative dielectric constant = 3.78, substrate thickness h = 0.1mm, loss tangent tan = 0.0027.

　　2.3 Transition structure between suspended microstrip and waveguide

　　There are two main forms of the transition from suspended microstrip line to waveguide: the normal direction of the suspended microstrip probe is parallel to the waveguide propagation direction; the normal direction of the suspended microstrip probe is perpendicular to the waveguide propagation direction. The principle of selection is: convenient processing. Based on this principle, this paper selects the scheme in which the normal direction of the suspended microstrip surface is perpendicular to the waveguide propagation direction. The short-circuit length of the waveguide terminal is 1/4 of the waveguide wavelength to ensure that the probe is at the maximum voltage in the waveguide, that is, the position with the strongest electric field strength, so as to achieve the highest possible coupling efficiency to reduce insertion loss and return loss.

　　For the frequency multiplier in this paper, two probe transitions need to be designed: a 73GHz input probe and a 220GHz output probe. After comprehensively considering the suppression of high-order modes, the reduction of discontinuities from the suspension microstrip to the filter, and the processing accuracy capability, this paper selected the most direct probe transition structure in the input transition part: no suppression cavity is used, the high-order modes are suppressed by reducing the size of the suspension microstrip cavity, and a gradient probe is used to reduce discontinuities. The following is the simulation model and results of the 73GHz input probe. In the range of 70GHz~80GHz, the insertion loss is less than 0.05dB, and the return loss is greater than 20dB.

　　The 220 GHz output probe transition also uses a similar structure, achieving S parameter indicators equivalent to the above performance in the 210 GHz to 227 GHz frequency band.

　　Figure 2 Input probe simulation model and results

　　2.4 Design of Low-Pass Filter

　　The microstrip line low-pass filter is used to pass the fundamental wave (71.7~75GHz) and prevent the third harmonic (215~225GHz) signal generated by the parallel diode pair from returning to the input circuit from the microstrip line. Here, the low-pass filter is realized by using a high-low impedance line structure. Since the operating frequency of the filter is relatively high, it was found during the design that the high-resistance line is extremely thin (0.02mm), which has high requirements for processing accuracy. The quartz substrate selected in this design can meet the accuracy requirements. The results of the HFSS simulation are shown in the figure below. From the simulation results, it can be seen that the loss of the filter to the fundamental wave is less than 0.15dB, the suppression of the third harmonic is above 40dB, and the suppression of the second harmonic is above 25dB. This shows that the harmonic signal generated by the diode pair after the low-pass filter will basically not leak to the input end.

　　Figure 3 LPF simulation model and results

　　2.5 Input and Output Matching

　　For the design of output waveguide matching and input microstrip matching, the output impedance and input impedance are first determined using the harmonic balance method in ADS, and then the waveguide circuit and the suspended microstrip line are matched with the help of the simulation function of HFSS. The load resistance size and microstrip line width after the diode pair are scanned in ADS, and the third harmonic power is observed. It is found that the third harmonic output is the strongest when the suspended microstrip line width is about 0.2mm. Then a model is established in HFSS, and a lumped parameter port is set at the location where the balanced diode pair is installed. The port impedance is set to the output impedance of the balanced diode pair calculated in ADS, which is used as the embedded impedance of the balanced diode pair in this complex structure.

　　It should be noted that since the width of the diode is larger than the previously selected width of the suspended microstrip cavity, it is necessary to expand the cavity where the diode is placed, and the performance degradation caused by this discontinuity needs to be minimized.

　　In addition, the output of the low-pass filter before the diode pair is equivalent to a section of road for the third harmonic. The third harmonic will be reflected back and forth between the output of the filter and the input of the balanced diode pair. In this way, the power of the third harmonic will overlap and cancel each other at certain frequency points, which is very likely to cause low power points in the final output frequency band. Therefore, the length from the low-pass filter to the diode pair needs to be designed during simulation to avoid the above situation.

　　The following figure shows the simulation model and results from the diode placement to the output section:

　　Figure 4 Output of part of the simulation model and results

　　From the results, we can see that the return loss of the 220GHz signal from the diode to the output end is greater than 11dB, and the insertion loss is less than 0.5dB.

　　It should be noted that the cavity widened to accommodate the diode only shows the half connected to the output probe in the above figure, and the half connected to the low-pass filter is not shown. The return loss of the cavity where the diode is placed for the fundamental signal is greater than 15dB, and the insertion loss is less than 0.2dB.

　　2.6 Overall system simulation

　　The input and output probes and filters simulated in HFSS are brought into ADS in S2P format for overall simulation. The diode used is DBES105a. The input power is 15dBm, and the output power is 0.6dBm in the frequency band of 215~225GHz. The simulation model and simulation results are shown in the following figure:

　　Figure 5 Overall simulation structure and results

　　3 Conclusion

　　This paper discusses the design of a 220GHz tripler using HFSS and ADS simulation software for simulation and optimization. The structure of the frequency doubler is relatively simple, but due to the high frequency, the size of the suspended microstrip and the cavity size are very small, and the processing accuracy and error issues need to be considered during the design. Some compromises need to be made during the design process. I will continue to analyze and minimize this effect in future research.