Direct-RF DACs for high-speed communications
Article By : Bonnie Baker
5G基础设施和其他更高带宽通信系统的设计人员可以查看直接到RF架构，以满足性能要求并降低成本。
消费者通过电缆，卫星和地面导管对更高视频，数据和语音带宽具有难以置信的需求。对于下一个进化步骤，消费者将以较低的成本想要更多的带宽。为了满足这一挑战，OEM设计人员需要提供以较低的解决方案成本支持高容量的通信设备。
现代无线无线电发射机设计包括真实（中频）发射器，复杂的IF发射器，以万博投注网址及零IF发射器。目前，这些变送器继续通过模拟路径进行抽搐。然而，模拟域存在局限性，影响系统的性能，容量和成本。
为了满足更高带宽通信的需求，IC制造商开发了直接射频架构，可提供优异的杂散，低噪声性能，并在每秒GIGA样品（GSP）范围内的输出更新速率。
In this article, we will compare the direct-RF transmitter to the analog-RF structure. We will examine the evolution of the direct-RF, digital-to-analog converter (DAC) transmitter and see how it simplifies RF design and achieves higher capacity at a lower solution cost.
模拟complex IF transmitter
Traditional transmitter architectures use the superheterodyne principle, where a local oscillator (LO) and a mixer generate IF.图1提供模拟复杂IF发射器的基本图。
图1模拟complex-IF to RF-transmitter multi-device solution
复杂的基带数字输入信号通常使用跨两个通道的LVDS（低压差分信号）接口：同相或“I”数据信道和正交或“Q”数据信道。一些系统在复杂的基带I和Q信号上使用插值，因子R.插值减轻了模拟过滤器的要求，同时还降低了带内噪声。数字复制调制器和数控振荡器（NCO）将信号混合在频率（“异差”）中混合。然后，双DAC将数字I和Q转换为模拟信号。
Now in the analog domain, the two parallel signals flow through lowpass filters to their respective I and Q mixers. These mixers are fed by an LO, which has a straight-through I path and a 90° phase-shifted Q path. Finally, the two signals are combined through a summation block, resulting in a complex modulated signal at the desired frequency.
The use of this conventional transmitter architecture produces an LO “image” artifact. Before the final voltage-gain amplifier (VGA) stage, a bandpass or SAW filter is used to reduce the magnitude of the unwanted image. The filter rolloff must be sharp and the LO frequency stable enough to reduce the unwanted sideband image (at fLO – fIF) without adversely affecting the desired signal (图2.a）。
图2.模拟RF（I / Q）发射器（A）与理想的直接RF（RF DAC）发射器（B）的非理想伪像
作为图2.a示出，I和Q路径之间的任何模拟不匹配（相位或增益误差）导致边带图像。此外，LO可以促进混合阶段并出现在RF输出频谱中作为LO泄漏。这些非理想的工件限制了模拟系统的性能，需要额外的滤波器和校准电路，这反过来增加了设计复杂性和成本。
该架构具有有限的输出信号带宽，因为双基带内插DAC的输入采样率受到在相对慢的LVDS或CMOS接口上传输的数据量的约束。这种带宽限制依次需要不同的AQMS（模拟正交调制器）或多组硬件，每个硬件具有不同的LO频率，以支持不同的RF频带。
Evolving to an RF-DAC solution
The baseband signal generated by the input is digitally upconverted using the I/Q interpolators, digital quadrature modulator (DQM), and NCO. The I/Q data paths in the DQM are perfectly matched (as a result of the digital implementation), which prevents the development of a sideband image (图2.b）。The absence of the sideband image and LO carrier frequency eliminates the need for costly and complex SAW filters. The system then presents the signal to the RF DAC core, which produces the RF output.
At the JESD204B input of the RF-DAC transmitter shown in图3.，内插器（↑r）相对于输入数据采样率增加DAC采样率。
图3.JESD204B input to RF-DAC transmitter enables higher input data sample rates. JESD204B is a high-speed serial interface that allows Gbps data rates.
The RF-DAC transmitter replaces the analog LO with a digital NCO, eliminating the LO feedthrough or leakage to the analog RF output. The output bandwidth of the RF DAC and the Nyquist bandwidth (fDAC/2) determine the maximum RF frequency.
RF-DAC发射器的输入结构可容纳跨JESD204B串行接口的高速5G信号，创建具有比模拟复合 - IF RF发射器更高的信号带宽的系统。
与模拟复杂的IF实现相比，RFDAC发射机架构简化并降低了本申请的成本，同时增加了带宽性能并减少了PCB占用空间。
建筑比较
RF-DAC提供三次降低系统总成本的机会，包括PCB空间，组件数量和简化设计（Table 1）。
Table 1RF-DAC to complex-IF transmitter comparison
The wave of the future in RF transmitters is before us. Although popular analog transmitter topologies are effective at lower frequencies with higher noise and cost, the speed of wireless data transmission is becoming beyond their capability. The next evolutionary step is the direct-to-RF transmitter. These interpolating and modulating 16-bit RF DACs provide improved spurious low-noise performance and simplified design. Their input data rates in the Gsps region provides the high bandwidth requirements needed for 5G technology at a lower cost.
Bonnie Baker.一直在使用模拟和数字设计和系统，超过30年，并代表Maxim集成写作。万博投注网址