在FPGA中从线性转化为circular polarization
Conversion from linear to circular polarization in FPGA
摘要：
内容：射电天文学接收器现在正在扩大其频率范围，以覆盖大（倍频程）分数带宽的灵敏度和频谱的灵活性，这使得良好的模拟圆偏振器的设计具有挑战性。更好的偏振纯度要求在越来越宽的带宽上具有更平坦的相位响应，这最容易通过数字技术实现。由于易于引入完美的正交相移，它们能够在任意宽的分数带宽上形成具有完美偏振纯度的圆偏振。此外，现场可编程门阵列的快速改进提供了使数字地形成圆偏振的实现实际化所需的高处理能力、低成本、便携性和可重构性。目的：在这里，我们探讨了与数字技术实现的圆偏振器的性能。研究方法：我们设计了一个数字圆极化器，它对接收机输出的中频信号进行采样并将其转换为圆极化。使用注入的噪声信号来确定频率相关的仪器相位差和增益缩放因子，并将其应用于两个线性偏振以均衡两个偏振通道的传输特性。该均衡在512 MHz带宽上的512个频率信道中执行。通过正交相移和求和均衡的线偏振信号形成圆偏振。结果：我们获得了-25 dB的偏振纯度对应于整个带宽上的D项为0.06。结论：该技术使宽带射电天文接收机的建设与本地线极化形成圆极化VLBI。
英文摘要：
Context: Radio astronomical receivers are now expanding their frequency range to cover large (octave) fractional bandwidths for sensitivity and spectral flexibility, which makes the design of good analogue circular polarizers challenging. Better polarization purity requires a flatter phase response over increasingly wide bandwidth, which is most easily achieved with digital techniques. They offer the ability to form circular polarization with perfect polarization purity over arbitrarily wide fractional bandwidths, due to the ease of introducing a perfect quadrature phase shift. Further, the rapid improvements in field programmable gate arrays provide the high processing power, low cost, portability and reconfigurability needed to make practical the implementation of the formation of circular polarization digitally. Aims: Here we explore the performance of a circular polarizer implemented with digital techniques. Methods: We designed a digital circular polarizer in which the intermediate frequency signals from a receiver with native linear polarizations were sampled and converted to circular polarization. The frequency-dependent instrumental phase difference and gain scaling factors were determined using an injected noise signal and applied to the two linear polarizations to equalize the transfer characteristics of the two polarization channels. This equalization was performed in 512 frequency channels over a 512 MHz bandwidth. Circular polarization was formed by quadrature phase shifting and summing the equalized linear polarization signals. Results: We obtained polarization purity of -25 dB corresponding to a D-term of 0.06 over the whole bandwidth. Conclusions: This technique enables construction of broad-band radio astronomy receivers with native linear polarization to form circular polarization for VLBI.
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