Design and implementation of EHF frequency upconverter

The EHF band is the preferred operating band for the next generation satellite communication system, and the development of equipment is becoming more and more urgent. The upconverter is a key device in the system. Through the application of simulation software, the frequency configuration, spurious and other indicators were simulated and analyzed, and the upconverter was developed. The equipment realizes the frequency conversion from the L band to the EHF band with a bandwidth of 2 GHz. The 1 dB output power of the EHF band is greater than + 16 dBm, and the amplitude-frequency characteristic within the 2 GHz bandwidth is less than 315 dB. By adding compensation measures, a smaller in-band amplitude-frequency characteristic can be achieved.

introduction

The upconverter is the main device in the uplink of the satellite communication system. Its main function is to complete the frequency conversion from intermediate frequency to radio frequency, provide appropriate gain and adjust the link gain.

Compared with other frequency bands, the EHF band (30-300 GHz) has the characteristics of wide available bandwidth, less interference, and small equipment size. The EHF band has a large communication capacity, which can greatly improve the performance of anti-interference and anti-interception measures such as spread spectrum and frequency hopping. At present, most of the EHF band military communication satellites in the world work in the 44 GHz/20 GHz band.

The difficulties in developing EHF band upconverters are: high operating frequency, complex processing and manufacturing processes; relatively wide intermediate frequency bandwidth, difficult to achieve in-band amplitude-frequency characteristic indicators; and difficult to control output spurious of the equipment's tertiary frequency conversion.

1. Solution selection

Main design technical indicators of the upconverter in the EHF band:

①Gain:> 25 dB;

②Amplitude-frequency characteristics: < 5 dB/ 2 GHz;

③Spurious suppression: < - 50 dB.

The key to the design of the frequency converter is the intermediate frequency selection (i.e. frequency configuration) and level distribution in the device. From the perspective of the frequency conversion process, the frequency converter can be divided into primary frequency conversion, secondary frequency conversion and multiple frequency conversion.

If a single frequency conversion method is used in the up-converter, the local oscillator frequency will leak into the RF output working bandwidth because the intermediate frequency is lower than the RF output frequency and the RF output working bandwidth is larger than the intermediate frequency input bandwidth. In order to avoid the leakage of the local oscillator frequency, a bandpass filter that suppresses the local oscillator frequency must be added to the output end of the device, and the center frequency of the filter must be synchronized and adjustable with the bandpass filter and frequency synthesizer, which undoubtedly increases the cost and design difficulty of the device.

In frequency converters that use 2 or more frequency conversions, the desired signal can be selected by simply changing the frequency synthesizer, and each filter can be designed as a filter with fixed frequency and bandwidth. The scheme has the strongest adaptability to frequency changes and the best frequency flexibility. However, as the number of frequency conversions increases, combined frequency interference, local oscillator harmonic interference, etc. will occur when the frequency configuration is not appropriate.

According to the technical indicators of the equipment and the requirements of the frequency interface, and considering the technical indicators of components such as the mixer, the equipment adopts a three-fold frequency conversion solution. For specific analysis, see the first intermediate frequency selection. The block diagram of the equipment frequency conversion process is shown in Figure 1.

For the convenience of the following description, the frequency after the first-stage mixer is defined as the first intermediate frequency, abbreviated as IF1; the frequency after the second-stage mixer is defined as the second intermediate frequency, abbreviated as IF2; and the intermediate frequency input of the device is referred to as IF.

2. Intermediate frequency selection and spurious calculation

The key to inverter design is the control of spurious signals. Spurious signals are generally caused by harmonics generated by the mixer during the mixing process. The mixer is one of the most important components in the inverter. The basic purpose of the mixer is to use the local oscillator signal to change the signal from one frequency to another.

The mixer uses nonlinear elements to implement the addition and subtraction operations of the input signal, the local oscillator signal and the higher harmonics of the two signals in terms of frequency. In order to extract the useful signal, an appropriate filter should be added after the mixing to filter out the useless spurious components. However, some spurious components (such as in-band spurious) cannot be filtered out by the filter. The only way is to adjust the signal frequency to prevent the spurious frequency in the mixing process from falling into the working band.

When the mixer is used in an up-converter, the input frequency, output frequency and local oscillator frequency have the following relationship:

frf = ±mflo ±nfif . (1)

Where frf is the mixer output frequency; flo is the mixer local oscillator frequency; fif is the mixer input frequency; m and n represent the harmonic orders of the local oscillator frequency and the input frequency respectively.

If the device uses multiple mixing, the output frequency of the previous mixer is used as the input frequency of the next mixer, resulting in a lot of spurious frequencies in the output of the final device. Therefore, a filter needs to be added after each mixing to filter out the spurious signals generated by this mixing and prevent them from entering the next mixer. As mentioned earlier, some in-band spurious signals cannot be filtered out by filters. For some in-band spurious signals, the level of the spurious signals can be reduced by adjusting the mixer input level.

If the frequency and level of the spurious signal can be calculated in the design stage, the intermediate frequency can be adjusted according to the calculation results so that the spurious signal of the final equipment can meet the requirements and achieve the optimal design of the equipment. Choosing a good intermediate frequency can reduce the design complexity of the system and improve the spurious signal and other indicators of the equipment.

In order to determine the optimal intermediate frequency, the mixing output spurious frequency can be calculated by formula, the frequency can be adjusted according to the result, and then calculated again to find the appropriate intermediate frequency with a longer period. Another method is to use microwave simulation tools to find the appropriate intermediate frequency more intuitively and quickly.

The frequency planning (What IF) integrated design tool in Agilent's microwave design software GENESYS can automatically find the area with no spurious or the lowest spurious according to the set frequency relationship and select the optimal intermediate frequency point. The following two intermediate frequency selections are achieved using this software.

2. 1. Intermediate frequency selection

(1) Selection of the second intermediate frequency IF2

By using the frequency planning tool to set the RF output frequency of the mixer, as well as the RF bandwidth, intermediate frequency bandwidth and local oscillator power, you can find an area with no spurious or the lowest spurious. The simulation results of intermediate frequency selection are shown in Figure 2. In the figure, area ① is an area without spurious; area ② is an area with spurious. It can be seen that the intermediate frequency with a spurious level less than -60 dBc is available in an area of ​​1.0 to 20.5 GHz. The frequency of intermediate frequency IF2 can be determined according to the following two principles: ① There are not many commercial devices available for mixers working in the millimeter wave band, and the intermediate frequency of mixers working in the EHF band is generally not greater than 4.5 GHz, so the frequency of IF2 must be less than 4.5 GHz; ② At the same time, because the bandwidth of the intermediate frequency IF2 of the device is 2 GHz, in order to avoid the 2 × IF2 + LO3 signal from entering the working band, the lowest frequency of the intermediate frequency must be selected above 2 GHz. Comprehensive consideration is made to select intermediate frequency IF2 to be above 2 GHz and below 4.5 GHz.

(2) First IF frequency selection

When IF2 is selected, assuming that the device uses 2 frequency conversion, the IF1 frequency is the intermediate frequency input IF (the device input frequency is the L-band signal). At this time, the IF and IF1 are close to each other, resulting in the inability to suppress the local oscillator frequency, so the solution must use 3 frequency conversion. Only when IF1 is selected as a higher intermediate frequency can the above problems be avoided.

The simulation analysis results of IF1 intermediate frequency selection are shown in Figure 3. In the figure, area ① is the area without spurious signals; area ② is the area with spurious signals. In the range of 0 to 17 GHz, only the range of 0 to 450 MHz has no spurious signals, and there are spurious components of different combinations in other frequencies. When the frequency exceeds 12.35 GHz, the only spurious component is -2IF1 +3LO1, which can be reduced to the required value of the equipment by reducing the level of IF1.

Because the L/Ku module solution is already very mature, the final intermediate frequency IF1 frequency was selected in the Ku band, shortening the equipment design cycle.

2.2 Spurious analysis

After determining the intermediate frequency, perform spurious analysis. After the first mixing, there is no spurious frequency in the IF1 band, and the main spurious outside the band is f (2, 1). By adding a bandpass filter after mixing, the spurious level can be completely suppressed to a very low level and can be ignored. It should be noted that the LO1 local oscillator frequency is mostly implemented by frequency doubling. In order to prevent multiple harmonics of the fundamental frequency of the LO1 local oscillator from entering the mixer with the LO1 local oscillator, a filter should be added after the LO1 local oscillator output to filter out useless LO1 local oscillator harmonics.

The second mixing spurious analysis shows that within the IF2 frequency range, the main spurious are the combination frequencies of f (-2, -2), f (3, -2), f (-4, 3), etc. The minimum number of combined spurious in the band is 4. Since the second mixing adopts a double-balanced mixer, the even number of m is cancelled. Although the 7th product falls into the band, the amplitude of the combined interference can be controlled by reducing the input IF1 level, which can fully meet the requirements of the general indicators of the frequency conversion link.

The spurious signal analysis after the third mixing shows that the main spurious signal in the EHF frequency range is f (1, 2). By adjusting the intermediate frequency input level, the spurious signal level amplitude can be reduced. At the same time, adding a cavity filter after mixing can also suppress the spurious signal to a certain extent. Through the above analysis, as long as the input level of each mixer is controlled well, the spurious signal can be controlled within the required range.

2. 3. Level distribution

According to the above spurious calculation, in order to reduce the spurious generated by the second mixing, the input level of the mixer must be reduced to below -20 dBm. On this basis, the gain of each stage is reasonably allocated to ensure that the various indicators of the equipment meet the requirements.

3. Key technologies

3.1, intermediate frequency selection

The EHF band upconverter adopts 3-time frequency conversion technology. The selection of intermediate frequency is particularly important. A suitable intermediate frequency can not only reduce the spurious level of the equipment, but also reduce the design difficulty of the equipment.

3. 2. High-precision manufacturing and assembly process

In the EHF band, the wavelength is very short, and the actual size of circuit components with the same electrical length is much smaller than that of components in the low-frequency band. The amplifier and mixer components used can only use unpackaged die, which places high demands on the processing accuracy of printed circuit boards, component installation accuracy, and welding accuracy. In the EHF band, the processing accuracy of some key dimensions is required to be less than 0.01 mm.

For the same reason, the requirements for the assembly process of equipment in this frequency band are more stringent. The assembly process mainly includes: cutting, bonding, bonding and other processes. In particular, problems are more likely to occur in the bonding and bonding processes. Through the trial and exploration of manufacturing and assembly processes, the high-precision manufacturing and assembly process problems in the millimeter wave frequency band have been well solved.

3.3 EHF frequency conversion technology

The frequency converter includes multiple links such as mixing, filtering, and amplification. In order to meet the requirements of equipment indicators, the following technical measures are mainly taken: 1. Use a new harmonic mixer to reduce the working frequency of the local oscillator and reduce the difficulty of realizing the local oscillator; 2. Use a microstrip probe in the form of waveguide-microstrip transition, insert it into the waveguide cavity through the hole on the waveguide H surface, reduce the insertion loss, and improve the port standing wave; 3. Use a new circuit, the circuit has a frequency response of less than 2 dB within the IF2 frequency range, ensuring the amplitude-frequency characteristic indicators of the whole machine; 4. Add a new structure of the equalization compensation circuit to compensate for the influence of the distributed parameters of the EHF frequency band. The amplitude-frequency characteristic of the whole machine within the 2 GHz bandwidth is less than 3. 5 dB, meeting the system indicator requirements.

3.4 Waveguide filter design

During the equipment development process, it is necessary to develop an EHF band filter that can suppress the local oscillator signal by more than 60 dB. According to the simulation results, it is difficult for a general microstrip filter to meet the requirements.

Waveguide filters have the characteristics of high Q value and low loss, and they can meet the index requirements. According to the index requirements, a waveguide filter was developed, whose in-band loss is less than 2 dB, the in-band amplitude-frequency characteristic is 0.5 dB/2 GHz, and the suppression of the local oscillation frequency reaches more than 64dB. After the whole machine test, the suppression of stray signals meets the design requirements.

4. Test results

After the equipment was debugged, the index test was carried out. The test curve shown in Figure 4 is the curve before and after the compensation of the equipment's amplitude-frequency characteristic. It can be seen from the figure that the amplitude-frequency characteristic before compensation is about 8dB, and after compensation it is 3.5dB, and the gain also meets the design requirements. The maximum spurious output of the equipment is -55dBc, and the 1 dB output power is greater than +16dBm.

5. Conclusion

With the continuous increase of system frequency, the operating frequency of microwave radio frequency equipment is getting higher and higher, which brings great challenges to the development of equipment. The three-time frequency conversion scheme is adopted to realize the upconverter from L band to EHF band. The 1dB output power of EHF band is greater than +16dBm, and the amplitude-frequency characteristic within 2 GHz bandwidth is less than 3.5 dB. The index test meets the design requirements, and the key indicators reach the level of similar foreign products. In the process of equipment development, key process technologies such as EHF band processing and assembly were solved, providing technical guarantee for the development of similar products.