关于5G毫米波滤波器，你要了解的5件事

今天我们接着分享一篇关于5G滤波器的文章，作者是Robert Hammond和Mike Eddy。Robert是Resonant 的CTO，Mike是Resonant 的负责市场的VP。我们看一下这两位业界大佬对5G滤波器有什么建议？

原文发表于www.analogictips.com，语言为英语。射频学堂负责整理翻译，以供大家学习参考。

我们知道人们最初认识的5G是基于毫米波频段，所以在滤波器领域，人们就需要开发毫米波滤波器。因此这篇文章主要是关于毫米波滤波器的几个问题。

No.1 什么是滤波器？它是怎么工作的？

Radio frequencies occupy the range of electromagnetic spectrum designated for broadcasting over distance. Using the piezoelectric effect, RF signals are converted into electrical energy that causes minute fluctuations in a substrate. These fluctuations determine which frequencies are processed. Filters can select designated signals from specific frequency bands while rejecting unwanted signals that interfere with the reception of the intended frequency. In a modern smartphone, there are currently more than 50 RF filters allowing operation anywhere in the world. As the number of filters increases then so does the design challenge for the filter and the phone PC board. Good filter performance is important because bad filter design increases power consumption and allows interference.

无线电频率占据指定用于远距离广播的电磁频谱范围。利用压电效应，RF 信号被转换成电能，从而在基板中引起微小的波动。这些波动决定了处理哪些频率。滤波器可以从特定频段中选择指定信号，同时拒绝干扰预期频率接收的无用信号。在现代智能手机中，目前有超过 50 多个射频滤波器。随着滤波器数量的增加，滤波器和电话 PC 板的设计挑战也随之增加。良好的滤波器性能很重要，因为糟糕的滤波器设计会增加功耗并允许干扰。

No.2 What are the frequency ranges for 5G filters?

The 3GPP is the standards body for cellular networks. In 3GPP Release 15, it designated two ranges of radio frequency bands for 5G applications. Frequency range one (FR1) is between 450MHz – 6000MHz (6 GHz) and FR2 is from 24250 MHz (24 GHz) to 52600 MHz (52.6 GHz).

Within FR1, there are 34 frequency bands available for 5G, but from an RF filter perspective, the interesting bands are n77 (3.7 GHz), n78 (3.5 GHz) and n79 (4.7 GHz) because these bands offer channel bandwidths of more than 500 MHz, which is essential for the data throughput promised by 5G. However, this represents operation at the very high end of the performance range of acoustic wave resonator structures that are popular in 4G filters.

Within FR2 there are four frequency bands specified: n257 (26 GHz), n258 (24 GHz), n260 (39 GHz) and n261 (28 GHz). These are all designated for 5G millimeter wave (mmWave) services with channel bandwidths over 800 MHz.

3GPP 是蜂窝网络的标准机构。在 3GPP 第 15 版中，它为 5G 应用指定了两个无线电频段范围。频率范围一 (FR1) 介于 450MHz – 6000MHz (6 GHz) 之间，FR2 介于 24250 MHz (24 GHz) 至 52600 MHz (52.6 GHz) 之间。

在 FR1 中，有 34 个频段可用于 5G，但从 RF 滤波器的角度来看，有趣的频段是 n77 (3.7 GHz)、n78 (3.5 GHz) 和 n79 (4.7 GHz)，因为这些频段提供了超过 500 MHz，这对于 5G 承诺的数据吞吐量至关重要。然而，这代表了在 4G 滤波器中流行的声波谐振器结构的性能范围的非常高端的操作。

在 FR2 中指定了四个频段：n257 (26 GHz)、n258 (24 GHz)、n260 (39 GHz) 和 n261 (28 GHz)。这些都指定用于信道带宽超过 800 MHz 的 5G 毫米波 (mmWave) 服务。

No.3 Why can’t current resonator structures be used in 5G filters?

Resonator structures support resonance oscillation at defined frequencies. The most popular structures are acoustic wave because of performance and low cost. Surface acoustic wave (SAW) and bulk acoustic wave (BAW) have been practical up to about 2.7 GHz, which is lower frequency than the high bandwidth 5G bands of the FR1 bands. As operating frequencies increase, the physical dimensions decrease, making it more demanding on the processing technologies. Higher frequency signals have higher signal attenuation, which means filters need to support high-power RF signals to boost signal strength. So with the challenges presented by 5G for existing technologies, filter manufacturers are looking for new solutions to meet the demands of the handset manufacturers, which includes new and innovative structures, like the new acoustic wave structure called XBAR, which meets the demands for high frequency, high bandwidth and high power at the band edge as well as other options with more limited performance such as electromagnetic (EM), dielectric, cavity waveguide, on-chip filtering and microstrip (or planar thin film) filters.

谐振器结构支持定义频率的谐振振荡。由于性能和成本低，最流行的结构是声波。表面声波 (SAW) 和体声波 (BAW) 最高可达 2.7 GHz，其频率低于 FR1 频段的高带宽 5G 频段。随着工作频率的增加，物理尺寸减小，从而对处理技术提出了更高的要求。更高频率的信号具有更高的信号衰减，这意味着滤波器需要支持高功率射频信号以增强信号强度。因此，面对 5G 给现有技术带来的挑战，滤波器制造商正在寻找新的解决方案来满足手机制造商的需求，其中包括新的创新结构。

No.4 What are the RF signal power concerns for 5G filters?

It’s generally true that higher frequency radio signals will need more power to maintain the same coverage as 4G due to higher signal attenuation. This has been born out in early trials where 5G signals operating at 28GHz demonstrate a signal range of up to 2,000 feet from the nearest base station with no obstructions.[1] Because of this performance, it is anticipated that indoor coverage – where 80% of usage occurs – will be very poor. Filters need to have high signal power capability – as much as 1 Watt (30dBm) at the band edge – to achieve maximum transmit distance.

通常情况下，由于更高的信号衰减，更高频率的无线电信号需要更大的功率才能保持与 4G 相同的覆盖范围。这已经在早期试验中得到证实，其中以 28GHz 运行的 5G 信号显示距离最近的基站长达 2,000 英尺的信号范围，没有障碍物。由于这种性能，预计室内覆盖（80% 的使用发生）将非常差。滤波器需要具有高信号功率能力——在频带边缘高达 1 瓦 (30dBm)——以实现最大传输距离。

No.5 What are the interference and coexistence challenges for 5G filters?

In many use cases, particularly as the full potential of 5G for high data-rate, video applications is realized, the coexistence of 5G and Wi-Fi in the 3-6GHz frequency range will be essential. Bands n77, n79 and 5GHz Wi-Fi are adjacent in frequency with little guard band to separate these bands (see figure below).  New Wi-Fi6 standards operate adjacent to the n79 frequency band, which, in turn, neighbors the n77. A high-performance filter, such as an XBAR filter, is essential to mitigate interference between the 5G and Wi-Fi bands, enabling maximum bandwidth operation, preventing Wi-Fi signals from bleeding into the n79 data path and vice versa.

在许多用例中，尤其是在实现 5G 在高数据速率视频应用方面的全部潜力时，5G 和 Wi-Fi 在 3-6GHz 频率范围内的共存将是必不可少的。频段 n77、n79 和 5GHz Wi-Fi 在频率上相邻，几乎没有保护频段来分隔这些频段（见下图）。新的 Wi-Fi6 标准在 n79 频段附近运行，而 n79 频段又与 n77 相邻。XBAR 滤波器等高性能滤波器对于减轻 5G 和 Wi-Fi 频段之间的干扰至关重要，可实现最大带宽操作，防止 Wi-Fi 信号渗入 n79 数据路径，反之亦然。