Design and application of microstrip Ku-band power synthesis circuit

With the continuous development of semiconductor materials and processes, the output power of microwave/millimeter wave power semiconductor devices is getting larger and larger. The pulse power of L-band power transistors has reached the kilowatt level; the continuous wave of X-band power GaAs field effect tubes reaches tens of watts, and the pulse power reaches 500W. However, due to the physical properties of semiconductors, the output power of a single solid-state device is still limited. Using power synthesis technologies such as chip synthesis, circuit synthesis and space synthesis to superimpose the output power of multiple solid-state devices in phase is one of the effective ways to obtain higher output power.

In 1968, Josenhans first proposed the concept of chip-level power synthesis. Later, in the late 1970s, Rucker first realized multi-chip circuit power synthesis in the X-band and then extended it to 40 GHz. In 1999, Kohji Matsunag, Ikuo Miura and Naotaka Iwata used MM IC multi-chip synthesis technology to design and manufacture Ka-band power amplifier chips through four independent MM ICs, and obtained 3 W continuous wave output power in the frequency range of 26.5 to 28.5 GHz.

This paper studies the power synthesis circuit based on the microstrip Wilkinson power divider and realizes a 1 W power amplifier in the Ku band. In applications such as satellite communications, the power level of the required power amplifier ranges from tens of watts to hundreds of watts. Obviously, the amplifier with this power level cannot be directly used as a power amplifier for satellite communications, but it can be widely used as a driver for high-power amplifiers such as traveling wave tubes. As the basis of 2n-way power synthesis, the power synthesis technology involved in this paper can provide important reference value for related technical fields.

1 Overall structure and design objectives

The principle block diagram of the power synthesis circuit used in this paper is shown in Figure 1. WPD1 in Figure 1 is a Wilkinson power divider as an input power divider, and WPD2 is another Wilkinson power divider as an output power synthesizer. Their structures use the new structure first proposed in this paper and are designed using the same design method described below. The SPS in Figure 1 is a Schiffman orthogonal phase shifter. The TGF2508-SM in Figure 1 is a Ku-band power amplifier chip from Triquint, USA, with a 1 dB power compression point power of 28 dBm, a small signal gain of 25 dB, and an operating bandwidth of 12 to 17 GHz. This device is selected as the basic component for two-way power synthesis in this paper. The theoretical maximum synthesis power is 31 dBm. The design goal of this paper is to make full use of the bandwidth of TGF2508-SM to achieve the widest possible frequency band, with a synthesis efficiency within the frequency band greater than 70%. For this purpose, both the Wilkinson power divider and the Schiffman orthogonal phase shifter must have a bandwidth equivalent to that of TGF2508-SM. The subordinate research in this paper achieves this goal.

Figure 1 Schematic diagram of the power synthesis circuit implemented in this paper

2 Improvement of Wilkinson power divider

FIG. 2 generally shows a modification of a Wilkinson power divider.

Figure 2a is the basic form of the Wilkinson power divider. Since an isolation resistor needs to be connected between the two output ends, and the size of this resistor is very small, the distance between the two λ/4 transmission lines in the circuit is required to be very close, resulting in mutual coupling, which affects the bandwidth properties of the circuit. Due to these inherent shortcomings of the basic form of the Wilkinson power divider, when it works at a frequency higher than the X-band, its bandwidth and other performance can no longer meet the requirements. The improved version shown in Figure 2b is proposed to avoid the above shortcomings of the basic type. However, due to the isolation resistor, the distance between its two output ports is still very close and mutual coupling cannot be avoided. The circuit shown in Figure 2c overcomes the above shortcomings, but due to the introduction of a longer transmission line segment, the bandwidth performance is reduced.

Figure 2 Evolution of WPD structure

This paper improves the circuit shown in Figure 2b into the circuit shown in Figure 3a.

Thus, the above shortcomings of the basic type and improved type are overcome, while retaining good broadband characteristics. The microstrip layout of the circuit is shown in Figure 3b. The results of the ADS simulation of the above circuit are shown in Figure 4. The results show that the circuit has good 3 dB divider performance in the range of 12 to 18 GHz, meeting the design goals mentioned above.

Figure 3 Schematic diagram and microstrip structure of the Wilkinson power divider used in this paper

Figure 4 ADS co-simulation results of Wilkinson power divider

3 Schiffman Quadrature Phase Shifter

This paper also implements the Ku-band orthogonal phase shifter required for power synthesis. The phase shifter type used in this paper is a dual Schiffman orthogonal phase shifter. The dual Schiffman phase shifter has a slightly smaller bandwidth than the standard Schiffman phase shifter, but the requirement for the coupling coefficient is greatly reduced. The results of the simulation of the dual Schiffman orthogonal phase shifter using ADS are shown in Figure 5. As can be seen from Figure 5, the phase difference between the two output sections is between 70° and 95°. Substituting the maximum phase difference into the formula in reference [9]:

It can be calculated that the theoretical synthesis efficiency is greater than 90%, which meets the design goals proposed above.

Figure 5 Ku-band dual Schiffman phase shifter simulation results

4 Circuit assembly and testing

After simulating and designing the above components, we designed and prepared a power synthesis circuit based on microstrip. The microstrip in this paper selected RTDuriod 6002 as the substrate. The circuit board after production is shown in Figure 6. The substrate dielectric constant is 2.94, the loss tangent is 0.0012, and the board thickness is 0.254 mm (10 mil). In addition to Rogers' RT Duriod 5880 and 6002 series, the commonly used boards also include Arlon's DiClad, CuClad, AD and other series.

Figure 6 Ku-band dual-path power synthesis circuit board

After the shielding is matched and the microstrip-coaxial connector with SMA connector is assembled, a Ku-band amplifier is obtained. We tested the amplifier. During the test, the amplifier was mounted on a heat sink and a high-power attenuator was connected to the output of the amplifier, as shown in Figure 7. The small signal gain of the amplifier measured by the Agilent 8510C vector network analyzer is shown in Figure 8. The curve in the figure includes a 30 dB attenuator connected to the output, so the actual gain should be the corresponding value of the line in the figure plus 30 dB. It can be seen from Figure 8 that between 13 and 16 GHz, the gain of the amplifier is greater than 20 dB and is relatively flat.

Figure 7 Amplifier under test and test equipment

Figure 8 Small signal gain of the amplifier under test

In addition, this paper uses the Agilent E8257D signal source and the Agilent E4418B EPM power meter to measure the saturation power of the amplifier, and the results are shown in Table 1. The second row in the table is the measured value of the amplifier saturation power, and the data in the third row is obtained from the manufacturer's datasheet of the single-chip TGF2508-SM chip, and the attenuation value of the output connector is subtracted. The combined efficiency in the fourth row is calculated from the data in the second and third rows.

Table 1 Saturation power of the amplifier under test

5 Conclusion

This paper proposes and studies in detail a new type of Wilkinson power divider. On this basis, the dual Schiffman orthogonal phase shifter and MM IC chip are further combined to design a Ku-band balanced power synthesis circuit. This paper also completes the processing and assembly of the circuit, and obtains a Ku-band 1 W power amplifier with a saturated power greater than 1 W at 13-16 GHz, a small signal gain greater than 20 dB, and a synthesis efficiency greater than 80%. In short, this paper realizes a Ku-band power amplifier based on 2-way power synthesis. The related technologies involved have important reference value in the relevant technical fields, and the obtained devices have certain application prospects.