Development of digital self-excited loop in vertical tests of superconducting cavity
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摘要: 介绍北京大学垂直测试系统的数字化自激励环路系统，重点分析了实际测试中避免多单元（cell）超导腔模式串扰的方法以及偏离四倍频采样对信号幅度和相位的影响。该系统运行稳定可靠，可有效区分1.3 GHz 9-cell超导腔
$ {\rm{{\text{π}}}} $ 模与$ 8{\rm{{\text{π}}}}/9 $ 模，解决了多cell超导腔测试中模式串扰问题。分析了超导腔自激励环路在垂直测试中的应用，介绍了北京大学垂直测试系统的数字化自激励环路，采用上下变频方案的射频前端和包括有限脉冲响应滤波器的数字算法，系统简洁扩展性强。重点分析了实际测试中避免多cell超导腔模式串扰的方法以及偏离四倍频采样对信号幅度和相位的影响。在多种不同频率超导腔的垂直测试中该系统运行稳定可靠，可有效区分1.3 GHz 9-cell超导腔$ {\text{π}} $ 模与$ 8{\rm{{\text{π}}}}/9 $ 模，解决了多cell超导腔测试中模式串扰问题。Abstract: Vertical tests are very important for superconducting cavity after its post-processing. The aim is to obtain the quality factor versus accelerating gradient curve to evaluate a cavity’s performance. Because of its narrow bandwidth, the superconducting cavity should work stably in the resonant state during the vertical tests. The digital self-excited loop system for the vertical test stand of Peking University is introduced in this paper. The methods of avoiding crosstalk during multi-cell superconducting cavity test were brought out. The influence of deviation from quadruple frequency sampling on amplitude and phase was analyzed. The system is stable and reliable, can effectively distinguish$ {\text{π}} $ mode from$ 8{\rm{{\text{π}}}}/9 $ mode of the 1.3 GHz 9-cell superconducting cavity, and solve the problem of mode crosstalk in multi-cell superconducting cavity test.-
Key words:
- superconducting cavity /
- vertical test /
- digital /
- self-excited loop
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图 2
$ {Q}_{{\rm{L}}} $ ＝1×1010的1.3 GHz超导腔谐振曲线Figure 2. Resonance curve of a 1.3 GHz cavity with
$ {Q}_{{\rm{L}}} $ ＝1×1010
图 3 数字化自激励系统中的射频前端与时钟示意图
Figure 3. Schematic of digital SEL RF front-end and timing system
图 5 不同频率偏差时被采样信号的幅度和相位随采样点数
$ N $ 的变化Figure 5. Normalized amplitude and phase of the IF signal with different frequency shift versus number of samples
$ N $ 
图 6 在1.3 GHz 单cell超导腔测试中
$ {A}_{{\rm{d}}} $ 归一化幅度随着环路相移的变化Figure 6. Normalized amplitude
$ {A}_{{\rm{d}}} $ versus phase shift in the vertical test of a 1.3 GHz 1-cell cavity
图 7 超导腔谐振时环路相移和数字域幅度
$ {A}_{{\rm{d}}} $ 随着频率偏移的变化Figure 7. Phase shift
${\rm{set}}\,\varphi$ and normalized amplitude$ {A}_{{\rm{d}}} $ versus frequency shift when the cavity is in resonance
图 8 不同本振信号频率时幅度信号
$ {A}_{{\rm{d}}} $ 的长时间稳定性Figure 8. Long term stability of normalized amplitude
$ { A}_{{\rm{d}}} $ with different$ {f}_{{\rm{LO}}} $ 
图 9 不同本振频率时环路中激励信号的频率和提取信号功率随着环路相移的变化
Figure 9. Resonant frequency and pickup signal power versus phase shift with different
$ {f}_{{\rm{LO}}} $ -
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