Advantages of using a differential-to-single-ended RF amplifier in a transmit signal chain design

传统的 射频 (RF) 发送信号链通常使用数模转换器 (DAC) 来生成基带信号。然后，使用射频混频器和本地振荡器将此信号上变频为所需的射频频率。射频 DAC 技术取得进步，现在允许直接以所需的射频频率生成信号，从而显著简化射频发送信号链的设计和复杂性。

High-frequency RF DACs have balanced differential outputs, while the RF transmit chain and antenna are single-ended. In the past, RF engineers used two devices, passive baluns and intermediate RF gain blocks, to perform differential-to-single-ended (D2S) conversion and boost the power of RF signals. However, passive baluns have several limitations, including large printed circuit board (PCB) size, high insertion loss, poor matching, gain, and phase imbalance when required to operate over a wide bandwidth. In addition, RF passive baluns cannot support DC or near-DC operation.

The D2S RF amplifier is a monolithic device that converts a differential signal to a single-ended signal and provides gain over a wide bandwidth. This article outlines the advantages of using a D2S RF amplifier over the traditional passive balun and RF gain block approach.

Figure 1 shows the TRF1108 D2S RF amplifier used as a DAC buffer and power amplifier (PA) pre-driver.

Figure 1: Simplified RF transmitter signal chain showing the TRF1108 differential-to-single-ended RF amplifier used as a DAC buffer and PA pre-driver

Differential to single-ended conversion and gain in a 4mm2 package

The passive baluns that perform the D2S conversion at the output of the RF DAC are typically bulky and costly, especially when wideband is required. The large size of the passive balun increases the PCB area and results in long PCB traces, which limits RF performance, especially when used with multi-channel RF DACs. In addition, wideband passive baluns also have high insertion losses, so high-performance RF gain blocks are required to compensate for the signal power loss.

The TRF1108 D2S RF amplifier is a monolithic device that performs D2S conversion and provides gain. The bandwidth of the D2S RF amplifier covers the range of DC to 12GHz and can be used in wideband DAC buffer applications from DC to several gigahertz. With a tiny PCB size of only 2mm x 2mm, the TRF1108 reduces PCB area, shortens routing, and improves RF performance.

Figure 2 shows the 2mm x 2mm PCB footprint of the TRF1108, which reduces the required PCB area, thereby shortening routing and improving RF performance on the TRF1108 DAC39RF10 evaluation module.

Figure 2: TRF1108 DAC39RF10 evaluation module (TRF1108-DAC39RFEVM)

High-density use case example

Radar system designers select the operating frequency based on the required range, resolution, and antenna size. The combination of RF DACs with wide bandwidth coverage and D2S RF amplifiers allows hardware design reuse for different frequency band applications with minimal changes to the RF transmit signal chain.

Combining RF DACs and D2S RF amplifiers can bring many advantages to high-density phased array radar applications using digital beamforming. In these applications, multiple DAC outputs are connected to a large number of antennas, each transmitting a phase-shifted RF signal relative to each other. Multichannel RF sampling DACs and transceivers integrate multiple DACs within a single die and package. This integration helps simplify system design, reduce hardware size and complexity. However, a small, high-performance D2S RF amplifier is required to effectively utilize the highest possible density achieved by these multichannel RF DACs.

Matching input and output

Wideband passive baluns used in the past with RF DACs have difficulty maintaining good input and output return loss, and the return loss is also sensitive to the input and output terminal impedances. This sensitivity causes impedance variations across the RF band of interest, which can produce unwanted gain variations in the transmitted signal. The TRF1108 has a 100Ω impedance match at its differential inputs. The TRF1108 has a 50Ω wideband match at its single-ended outputs, which improves return loss and produces a very flat passband response over a wide RF bandwidth (see Figure 3).

Figure 4 highlights how matching the input and output of the TRF1108 when used in conjunction with an RF DAC produces a flat passband response from 100MHz to 8GHz.

Figure 3: TRF1108 input and output S-parameters on a Smith chart

Figure 4: TRF1108 DAC39RF10 frequency response from 100MHz to 8GHz

Performance Optimization

Wideband passive RF baluns have high insertion losses, thus reducing the maximum signal power level of the RF DAC. To compensate for the insertion loss and increase the power level of the RF signal, a single-ended high-performance RF gain block is required after the passive balun. Single-ended RF gain blocks typically have poor second-order nonlinear performance and are unable to filter out the resulting distortion when the signal bandwidth covers multiple octaves. In addition, the poor gain and phase imbalance of the wideband balun will also cause further imbalance, thereby degrading the second-order nonlinear performance of the RF signal.

D2S RF amplifiers such as the TRF1108 use feedback techniques to help improve gain and phase imbalance performance. The differential nature of the input improves second-order distortion compared to single-ended RF gain blocks. The TRF1108 D2S RF amplifier provides improved second-order nonlinear performance for multi-octave RF transmit applications.

Conclusion

Advances in RF DAC technology have enabled flexible, wideband RF applications in radar, software-defined radio, and RF test and measurement equipment. Integrating multiple RF DACs in multi-channel DACs and RF sampling transceivers simplifies transmit signal chain design and reduces the need for large PCB areas in multi-transmit RF and phased array applications.

D2S RF amplifiers like the TRF1108 can provide RF signal bandwidths in the DC to 12GHz range. They can compensate for the wide RF bandwidth and performance of RF DACs. The TRF1108 is a single-chip D2S RF amplifier that improves on the traditional passive balun and RF gain block. It has a smaller PCB area, shorter RF wiring length, better matching, and stronger performance. As a result, higher density, better performance, and flexible RF transmit designs can be achieved.

Additional Resources

• For more technical information on the D2S RF amplifiers, see the application note, TRF1208/TRF1108 Active Balun Interface with Xilinx RFSoC Data Converters.

• Read our Analog Design Journal article, “The Impact of Baluns on Second Harmonics in RF DACs.”

• Order the TRF1108EVM on TI.com and get started today.

• View Texas Instruments' RF and microwave products.