Design of receiving channel for Ka-band satellite ground station

1 Introduction

Entering the 21st century, with the comprehensiveness of information and the huge global demand for personal multimedia communication traffic and seamless coverage, satellite communication has become one of the most important means of communication in the world today, and it plays an increasingly important role. With the shortage and congestion of low-end frequency spectrum resources and GEO satellite orbit resources, it can no longer meet the needs of many applications such as high speed and broadband. Since satellite communication systems above the Ka band have the characteristics of wide available bandwidth, less interference, and small equipment size, they are increasingly used in satellite communications and various forms of satellite ground stations. At present, in domestic ground relay communication systems, the technology of Ka-band transmitting and receiving equipment is relatively mature. However, there is still a certain gap in the development technology of the receiving and transmitting equipment of satellite communication ground stations. Therefore, the development of key technologies and key equipment of the radio frequency unit of Ka-band satellite communication ground stations is urgent.

2 Composition and working principle of ground station receiving unit

In the research on the key technologies of Ka-band ground station radio frequency units, the development direction is to provide small, light, low-cost and highly reliable outdoor units for fixed and mobile stations according to the needs. The ground station radio frequency unit consists of a transmitting unit and a receiving unit. The ground station receiving unit introduced in this article is the downlink channel part that matches the Ka-band radio frequency transmitting unit. Figure 1 is a block diagram of the receiving unit channel composition. The 20.3 GHz to 20.8 GHz radio frequency signal from the receiving output end of the Ka-band ground station antenna duplexer is amplified by a low noise amplifier (LNA) and then connected to a bandpass filter with a passband of 20.3~20.8 GHz. This bandpass filter has a strong suppression characteristic for image frequencies and other out-of-band clutter. Then, it is mixed with the 19.35 GHz local oscillator signal output by the Ka-band phase-locked local oscillator source in a single balance. The difference frequency signal is the L-band intermediate frequency signal of 950 MHz~1 450 MHz. It is filtered out by a bandpass filter with a passband of 950 MHz~1 450 MHz to remove various combined interference frequency signals, and then amplified by the L-band intermediate frequency amplifier and output to the indoor unit (IDU).

Figure 1 Block diagram of the receiving channel of a Ka-band satellite communication ground station

According to the design scheme of the Ka-band receiving unit, the main technical indicators of the known system are:

1) Input frequency range: 20.3 GHz to 20.8 GHz

2) Output frequency range: 950 MHz to 1 450 MHz

3)LNA noise factor: ≤2.5 dB

4) Image frequency suppression: ≥60 dB

5) Spurious output: £-50 dBc (500 MHz in-band) £-60 dBc (500 MHz out-band)

3. Design Concept of Receiving Unit

With the rapid development of semiconductor and integration technology, the integration degree of equipment is getting higher and higher. In the Ka band, there are already integrated modules such as low-noise amplifiers and mixers that meet the requirements, which can be directly applied to the system. Therefore, the design ideas of miniaturization, modularization, and generalization are implemented throughout the design process of the Ka-band receiving unit. In the system design, the key components are all selected from the corresponding monolithic integrated (MMIC) modules, so that the development of the system saves time and ensures the maintainability of the equipment, while improving reliability. The key component in the receiving unit is the low-noise amplifier, which must have the characteristics of low noise figure, high gain, and large dynamic range. After comprehensive consideration, Triqunt's low-noise integrated amplifier TGA1319A-EPU was selected; secondly, the single-sideband down-conversion mixer uses Hittite's monolithic integrated balanced mixer module HMC260; the intermediate frequency amplifier also uses the corresponding integrated module. In order to suppress the image frequency and other out-of-band spurious generated by the mixer, the input bandpass filter uses an E-plane waveguide metal diaphragm bandpass filter with strong out-of-band suppression. The circuit structure of the Ka-band receiving unit is shown in Figure 2.

Figure 2 Receiving channel circuit diagram

4. Receiver unit simulation design and index analysis

4.1 Receiving unit topology circuit

Before the equipment enters into substantial design and development, EDA software is used to simulate and optimize the system design, referring to the main technical indicators of the receiving unit and the various components selected. ADS microwave design software is used to perform system simulation and optimization design on the channel part of the receiving unit. The simulation topology circuit of the receiving unit is shown in Figure 3. According to the topology circuit, the corresponding Simulation Controller is selected to simulate and optimize the main indicators of the receiving channel, and the simulation results are obtained.

Figure 3 Receive channel simulation topology circuit

4.2 Noise Figure Simulation

The noise factor is the total measure of the deterioration of the signal-to-noise ratio at the input of the receiving system between the input and a certain point of the intermediate frequency amplifier. It is also a quality factor to measure the quality of the receiving unit design. The design of a high-quality receiving unit is to obtain the best signal-to-noise ratio at its output as much as possible. It directly affects the communication quality of the receiving system. The receiving unit is composed of multiple components such as amplifiers, mixers, filters and intermediate frequency amplifiers. For the cascade network, the noise factor calculation formula is:

It can be seen that the noise coefficient of the receiving unit mainly depends on the noise coefficient of the front low-noise amplifier. Since the integrated amplifiers APM1 and APM2 (see Figure 2) have a gain of about 40 decibels, the noise influence of other components in the system can be ignored. The noise coefficient of the receiving unit is simulated, and the simulation curve in Figure 4 shows that the noise coefficient is ≤2.16 dB within the working frequency band, which meets the system requirements.

Figure 4 Noise figure curve

4.3 Image rejection and spurious output simulation

The input signal of the receiving unit is amplified by various amplifiers and then enters the mixing module. After mixing, several parasitic frequencies will be generated, but the most noteworthy frequencies are two: one is the sum frequency ω+=ωL+ωs; the other is the image frequency ωk=2ωL-ωs. This frequency is in the mirror position of the signal relative to the local oscillator, but the image frequency is relatively close to the signal frequency, so it is easy to fall within the signal frequency band and affect the signal.

In microwave receiving systems, image frequency suppression is a very important technical issue. Good image frequency suppression can avoid noise deterioration and reduce external image frequency interference. A common method to achieve single-sideband reception is to connect a bandpass filter to the mixer signal input to suppress the image input signal. The use of a waveguide-type E-surface metal diaphragm bandpass filter with strong out-of-band suppression can effectively suppress the interference of image frequency and external clutter. The simulation optimization shows that the image frequency suppression of the receiving unit is ≥70 dB and the spurious output is <-68 dB. The simulation results are shown in Figures 5 and 6 respectively.

Figure 5 Image frequency suppression curve

Figure 6 Spurious output curve

5 Conclusion

Through simulation design, it can be seen that the technical indicators of each component selected in the design of the receiving unit match the requirements of the receiving channel. Various technical indicators can meet the system requirements, and the feasibility of the simulation results is also verified during the development of the equipment. Therefore, by using EDA software to simulate the system, the rationality and advantages of the technical indicators of the communication system can be judged, so that the communication system can achieve the maximum benefit, and at the same time, some processes that were originally impossible to test can be verified in the simulation. It provides an important basis for the successful development of equipment, thereby effectively avoiding detours and time-consuming in the development of equipment. In the Ka-band ground station receiving channel, a variety of amplification and frequency conversion integrated devices are reasonably selected to realize the modularization, miniaturization and generalization of the channel. For ground stations with different requirements, the requirements can be met by appropriately replacing the corresponding devices.