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图 1所示的是基于光子技术的测频仿真结构图。该系统采用的是波长为1550nm的连续波(conti-nuous wave,CW)光源,经过耦合器将入射光分成上下两部分,把未知频率的微波信号分别通过相位调制器(phase modulator,PM)和马赫-曾德尔调制器(Mach-Zehnder modulator,MZM),之后将调制后的信号经过两段长距离的单模光纤(single mode fiber,SMF),其中光纤中的色散值为34ps/nm,根据色散引起的功率损耗的特点,可以得到单调变化的频率-幅度的映射关系,最后通过光电探测器(photo diode,PD)的输出信号功率比来得到ACF,进而实现微波测频。
对于上臂中强度调制(intensity modulation, IM)所产生的双边带(double sideband,DSB)光信号,频率响应是低通的,而对于下臂中相位调制所产生的DSB光信号,频率响应是带通的。因此,两个PD输出处的输出功率分别如下式所示[25]:
$ {P_{{\rm{IM}}}} = {R_1}{\cos ^2}\left( {\frac{{\pi D{L_1}{\lambda _{\rm{c}}}^2{f^2}}}{c}} \right) $
(1) $ {P_{{\rm{PM}}}} = {R_2}{\sin ^2}\left( {\frac{{\pi D{L_2}{\lambda _{\rm{c}}}^2{f^2}}}{c}} \right) $
(2) 式中,f是指未知微波信号的频率,Li(i=1,2)是指方案中上下部分的SMF长度,D是指SMF的色散系数,λc和c分别是指光载波的波长和光速常量,Ri(i=1,2)是指方案上下臂中包括耦合器、调制器、SMF和PD的总损耗。
根据PD探测所得的上下臂功率的比值为rACF,表达式如下所示[25]:
$ {r_{{\rm{ACF}}}} = \frac{{{R_2}{{\sin }^2}\left( {\frac{{\pi D{L_2}{\lambda _{\rm{c}}}^2{f^2}}}{c}} \right)}}{{{R_1}{{\cos }^2}\left( {\frac{{\pi D{L_1}{\lambda _{\rm{c}}}^2{f^2}}}{c}} \right)}} $
(3) 可以看出,该函数所反映的频率与功率的映射关系是非线性的,这对构建ACF有较大的难度。为了解决这一难题,可以进一步简化该非线性函数,通过校准和处理,将R1=R2,并且在方案中将上下臂的SMF长度保持一致,同时设置为L=2km,从而将ACF转换成下式[25]:
$ {r_{{\rm{ACF}}}} = {\tan ^2}\left( {\frac{{\pi DL{\lambda _{\rm{c}}}^2{f^2}}}{c}} \right) $
(4) 这样就可以构建出更为简化的ACF函数,从而方便通过计算来得到未知微波信号的频率。
基于相位和强度调制的微波测频技术研究
Research of microwave frequency measurement based on phase modulation and intensity modulation
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摘要: 为了更好地对微波信号进行频率测量,采用了一种基于相位调制和强度调制相结合的瞬时测频方法。一束连续波光源通过耦合器被分成两路,未知微波信号分别同时经过相位调制器和强度调制器从而对载波进行调制,之后进入两段长距离的单模光纤中。在光纤中由于色散引起的微波功率损耗的特点,可以获得单调变化的频率-幅度的映射关系,继而通过光电探测的微波信号输出功率比得到幅度比较函数;另外还分析与实现了测频范围与测频精确度的优化。结果表明,该方案结构简易,能够快速精准地测量出未知信号的频率,测量范围可以达到0.5GHz~53GHz,测量误差小于±200MHz。该方法可以有效地测量微波信号频率,可靠性强,适用范围广。Abstract: In order to measure the frequency of microwave signal better, an instantaneous frequency measurement method based on phase modulation and intensity modulation was adopted. A continuous wave light source was divided into two channels by a coupler. Unknown microwave signals passed through both phase modulator and intensity modulator respectively. Accordingly, the carrier was modulated. Then it entered into two long-distance single-mode fibers. Because of the characteristics of microwave power loss caused by dispersion in optical fibers, the mapping relation between frequency and amplitude of monotone change was obtained. Then the amplitude comparison function was obtained by output power ratio of microwave signal detected by photoelectric method. In addition, the range and accuracy of frequency measurement were optimized. The results show that, the structure of the scheme is simple. It can measure the frequency of unknown signal quickly and accurately. The measurement range can reach 0.5GHz~53GHz. The measurement error is less than ±200MHz. This method can effectively measure the frequency of microwave signal and has strong reliability and wide application range.
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