Addressing Phase Noise Challenges in Radar and Communication Systems: Part 2

本博文首次由自定义MMIC发布谁加入了Qorvo家族in February 2020. Custom MMIC is known for its best-in-class die and packaged components, which augment our power amplifiers to enable multi-chip modules for a broad range of defense, aerospace and commercial applications.

介绍

在第1部分在这个博客系列中，我们讨论了相位噪声以及如何影响直接转换接收器和雷达系统的性能。在第2部分中，我们将讨论可以使用的是使用自定义MMIC的解决方案来减少微波系统中相位噪声的影响。

Amplifiers and Phase Noise

One obvious place to limit phase noise is in the choice of oscillator. This problem can be addressed by spending considerable time and money to design or procure a low-noise oscillator. However, most oscillators do not generate enough output power, and indeed let us assume that for a particular application, the oscillator output of +5 dBm needs to be amplified to a level of +15 to +17 dBm in order to drive the LO port of a mixer. The question then becomes - Does the amplifier affect the phase noise of the LO signal?

在an ideal situation the answer would be “no”, as the amplifier would simply raise desired LO signal and the skirts by the same level. However, in reality, microwave amplifiers add noise of their own to any signal, and herein lies the problem. All electronic devices exhibit a phenomenon called 1/f noise or “pink noise”, which is noise power that is added to an input signal spectrum but falls off proportionally to the inverse of the offset frequency. In Figure 4, we present the phase noise of theCMD307P3，覆盖10到17 GHz范围的低噪声放大器与偏移频率远离所需信号。输入信号的相位噪声已被取消，因此该曲线表示放大器产生的噪声。

图4：CMD307P3 LNA相位噪声

在图4中，我们注意到相位噪声随着频率偏移的增加，相位噪声在对数尺度上线性下降，这是1 / f噪声的特性。如果这种噪声水平高于输入信号的相位噪声，则放大器噪声将主导输出噪声谱。在我们的示例中，这意味着振荡器的低相位噪声将被放大器的较高相位噪声替换，从而击败低相位噪声振荡器的目的。这种现象的图示表示如图5所示。

图5：放大器引起的相位噪声的降解。
通过放大器后，左侧的输入信号的裙子增加，右侧输出频谱。

一个明显的问题是 - 可以做任何事情来降低放大器的相位噪声吗？答案位于设备物理学中。1 / f噪声是由有源装置的通道中的随机和热电荷移动引起的。例如，CMD307P3在砷化镓（GaAs）PHEMT工艺上制造，栅极长度为0.13μm。由于其高电子迁移率，该过程上的FET器件通常具有高1 / F角。另一方面，GaAs双极器件往往具有较低的电子迁移率，这意味着噪音的1 / f噪音得多。因此，它们比其FET反零件相当更好。因此，降低附加相位噪声的一种解决方案是使用GaAs HBT过程。

在Custom MMIC，我们利用我们对放大器设计技术的广泛了解创建了一个family of new Low Phase Noise Amplifiers（LPNAS）在6到40 GHz的GaAs HBT过程中。

在Figure 6, we present the phase noise versus offset frequency for theCMD245放大器，如前所述的4 mm QFN式封装包装，相对于先前的CMD307P3 HEMT LNA。我们注意到的相位噪声CMD245C4is 15 to 20 dB lower than the CMD307P3 pHEMT LNA.

图6：CMD245C4（蓝色）低相噪声放大器与CMD307P3（绿色）LNA的相位噪声。

Other components besides oscillators and amplifiers can contribute to phase noise, including frequency multipliers. Many microwave systems utilize a lower-frequency oscillator that is then multiplied to produce a higher frequency. One common approach for multiplication is to use a harmonically-terminated amplifier to generate the required output frequency. Unfortunately, such an approach will then add the amplifier’s phase noise to the multiplied signal, which will degrade the phase noise of the original oscillator.

A second approach is to use passive multiplication, which has the potential to add minimal additional phase noise to the multiplier's signal (aka doublers). Custom MMIC has also created a family of passive HBT-style frequency multipliers which do not add to the phase noise of the input signal. In the table below, we present a summary of Custom MMIC’s passive multiplier die family (SMT packaged versions are also available).

Product	输出频率（GHz）
CMD225	8 - 16
CMD226	14 - 22
CMD227	16 - 30

结论

在这两个部分博客系列中，我们讨论了相位噪声如何与振荡器的拓扑和构造直接相关的二次效果。我们已经分解了相位噪声如何影响直接转换接收器和雷达的性能。最后，我们已经提出了特定的自定义MMIC解决方案来解决这些挑战。

To review additional data, and to view the complete findings together, download our tech brief:如何排除雷达和通信系统中的阶段噪声。