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雷达接收链路的组成如图1所示,其中回波信号通过天线接收,首先经过前级低噪声放大器,在图中用Pre-LNA表示,其增益为Gpre(dB),噪声系数是NFpre(dB)。随后进入ROF链路,包括电光变换、传输光纤和光电变换部分,ROF链路的增益为Gop(dB),噪声系数是NFop(dB)。最后再经过后级放大器,在图中用Post-LNA表示,其增益为Gpost(dB),噪声系数NFpost(dB)。后级放大后,回波信号可以进入采样系统或进行其他变频、去斜变换,与传统电链路类似。
CDR是噪声功率谱密度(PSD)与输出P−1之间的幅度差,而接收链路的PSD既包括光链路也包括电链路。其中光链路的噪声功率谱密度Nout-op由多方面构成,如热噪声、散粒噪声、激光器RIN噪声等。通常而言,微波光子技术普遍应用在X波段及以上,激光器的RIN噪声较低,射频光链路为散粒噪声受限,系统的光噪声为−164 dBm/Hz。另一方面,前级放大后的电链路噪声功率谱密度为−174+Gpre(dB)+NFpre(dB),在经过光链路后,电噪声也相应发生变化,而接收链路噪声功率谱密度Nout为光链路与电链路之和。
此外,为最大化链路动态范围,设计需要前一级器件的输出P−1与后一级器件的输入P−1相等,当二者不相等时,则取其中的较小值作为计算接收链路CDR的边界条件。系统中前级放大的输出P−1为15 dBm。
另一方面,由于雷达系统对动态范围和噪声系数要求较高,当前仍主要基于外强度调制链路,由激光器、调制器、探测器组成。其链路的增益和输入P−1分别为[13]:
$${G_{{\rm{op}}}}[{\rm dB}] = - 22.1 + 20\log \left( {{{{I_{\rm dc}}[\rm mA]} / {{V_\pi }[{\rm V}]}}} \right)$$ (1) $$ {P}_{-1}[{\rm dBm}]=-0.4+20{\rm{log}}\left({V}_{\pi }[{\rm V}]\right)$$ (2) 式中:Idc为探测器产生的光电流。系统中探测器产生的光电流为10 mA。Vπ为调制器的半波电压,半波电压较小时,光链路的输入P−1较小,当其小于15 dBm时,将调制器的输入P−1作为计算CDR的边界条件。而随着半波电压的增加,光链路的输入P−1增加,当其超过15 dBm时,将前级放大的输出P−1(15 dBm)作为计算CDR的边界条件。由于后级放大噪声系数小,对接收链路压缩动态范围的影响可忽略。则接收链路的CDR1dB为:
$$\begin{split} {CD}{{R}_{{\rm{1dB}}}} =& {\rm{min}}({\text{前级放大输出}}{{P}_{ - 1}},{\text{光链路输入}}{{P}_{ - 1}}) +\\ &{{G}_{{\rm{op}}}} - {{N}_{{\rm{out}}}} + 1\left( {{\rm{dB}}\cdot{\rm{Hz}}} \right) \end{split}$$ (3) 对噪声系数而言,在引入前级放大器后,前级放大、射频光链路和后级放大的整体噪声系数NF与前级放大器的噪声系数NFpre密切相关,接收链路整体噪声系数NF不低于前级放大器的噪声系数NFpre。根据噪声系数级联公式,接收传输链路的总噪声系数NF为:
$$F = {F_{\rm pre}} + \frac{{{F_{\rm op}} - 1}}{{{g_{\rm pre}}}} + \frac{{{F_{\rm post}} - 1}}{{{g_{\rm pre}}{g_{\rm op}}}}$$ (4) 式中:F、Fpre、Fop、Fpost是将接收传输链路总噪声系数NF、前级放大噪声系数NFpre、光链路噪声系数NFop、后级放大噪声系数NFpost由分贝转为倍数后的值,gpre、gop是将前级放大增益Gpre、光链路增益Gop由分贝转为倍数后的值。系统中前级放大噪声系数NFpre和后级放大噪声系数NFpost均为4 dB。
对于光链路而言,其噪声功率谱密度Nout-op、噪声系数和光链路增益之间的关系为[13]:
$${NF_{{\rm{op}}}}\left( {{\rm{dB}}} \right) = 174 + {{N}_{{\rm{out - op}}}}\left( {{\rm{dBm}}/{\rm{Hz}}} \right) - {{G}_{{\rm{op}}}}\left( {{\rm{dB}}} \right)$$ (5) 根据前文所述,光链路噪声功率谱密度Nout-op=−164 dBm/Hz,则光链路噪声系数与增益之间的关系为NFop(dB)=10-Gop(dB),并可将此关系式带入并简化公式(4)。由此,接收链路NF、CDR1dB与前级放大增益Gpre、光链路增益Gop之间的关系如图2所示。
图 2 (a) 不同前级放大增益Gpre与光链路增益Gop下,接收链路的NF变化;(b) 不同前级放大增益Gpre与光链路增益Gop下,接收链路的CDR1dB变化
Figure 2. (a) NF of receiving link under different pre-amplifier gain Gpre and optical gain Gop; (b) CDR1dB of receiving link under different pre-amplifier gain Gpre and optical gain Gop
图2(a)的结果显示,在相同Gop下,前级放大增益越大,则接收链路的噪声系数越小。因此为降低接收链路噪声系数,应尽可能增加前级放大增益。而从图2(b)中可以看出,在光链路增益一定的前提下,随着前级放大增益的增加,CDR1dB会减小。例如当Gop为−20 dB时,为达到最大链路CDR1dB(158 dB·Hz),Gpre可设计为16 dB,而此时接收链路NF约为15 dB左右,系统的噪声温度较大,将影响探测距离。因此,需合理增加前级放大增益,同步优化系统整体指标。
将接收链路实际的CDR1dB与其理论最大值CDR1dB之差称为压缩动态范围的容忍度,当设定容忍度为1 dB时,接收链路噪声系数NF能够从15 dB降至10 dB,其对噪声系数带来明显改善。图3展示了在容忍度为5 dB和15 dB两种情况下,不同光链路增益、优化后前级放大增益与噪声系数的关系。例如,由于光链路增益在−20 dB时的最大CDR1dB为158 dB·Hz,当接收链路CDR1dB的容忍度为5 dB时,可将其CDR1dB约束在153 dB·Hz以上,则接收链路的前级放大Gpre为30 dB,NF为5.67 dB。同样的,若接收链路CDR1dB的容忍度为15 dB,则将CDR1dB约束在143 dB·Hz以上,经过优化,接收链路的前级放大Gpre设定为41 dB,NF降低到4.15 dB。
图 3 (a) CDR1dB容忍度为5 dB时,不同光链路增益Gop下,前级放大增益Gpre和NF的关系;(b) CDR1dB容忍度为15 dB时,不同光链路增益Gop下,前级放大增益Gpre和NF的关系
Figure 3. (a) Pre-amplifier Gpre and NF of receiving link under different optical gain Gop if CDR1dB tolerance is 5 dB; (b) Pre-amplifier Gpre and NF of receiving link under different optical gain Gop if CDR1dB tolerance is 15 dB
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通过以上分析可以看出,在雷达接收链路中引入ROF链路后,尽管接收链路的增益和噪声系数特性有所改变,但通过优化前级放大增益,CDR1dB仍可以达到150 dB·Hz甚至更高,接收链路噪声系数可降低到4.15 dB甚至更低。
事实上,雷达接收机动态范围在130 dB·Hz的条件下,已经可以满足常规雷达的应用需求。但面临近程、远程目标同时探测任务,大目标与小目标同时探测任务时,需在复杂干扰条件下探测,需抑制地杂波、海杂波时,雷达系统需具备更大的动态范围[14]。因此,需在对接收链路CDR和NF耦合分析的基础上,对系统的探测距离和目标分辨能力等系统指标进行探讨。
从系统角度看,接收链路噪声系数NF影响系统噪声温度,与雷达探测距离密切相关,通常系统噪声温度与接收噪声系数关系如下:
$${{T}_{\rm{s}}} = 0.876·{{T}_{\rm{a}}} + 36 + {{T}_{\rm{0}}}·\left( {{{L}_{\rm{r}}}·{F} - 1} \right)$$ (6) 式中:Ta为天线温度,在天线不同频率、不同仰角时均不同,一般取典型值30 K;T0为环境温度,常温下为290 K;Lr为天线绕线层损耗,设定为1.5 dB;F即为接收链路噪声系数。在雷达方程中,雷达的探测距离与系统噪声温度的四次根号成反比[1]。图4给出了光链路增益为−20 dB时,系统动态范围CDR1dB和探测距离的关系。随着动态范围的减小,目标分辨能力受限,而与此同时探测距离会相对增加。将系统CDR1dB为130 dB·Hz时的探测距离设为100%。若特定应用场景下,需增加系统动态范围,则适当牺牲探测距离。从图4中看出,随着动态范围的提升,最大探测距离减少。当动态在130~140 dB·Hz时,对探测距离影响不大。当动态大于145 dB·Hz时,系统探测距离降低明显。设定雷达探测距离需保持在CDR1dB为130 dB·Hz时的99%,如图中虚线所示。而此时链路CDR1dB可达到143 dB·Hz,噪声系数为4.15 dB,前级放大器增益为41 dB。此时,雷达系统的探测距离和动态范围均得到了优化,通过牺牲1%的探测距离换来了动态范围13 dB的得益,此时系统能力得到均衡提升。
System optimization for radio-over-fiber radar receiving link
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摘要: 在雷达指标体系中,探测距离和目标分辨能力是其中的重要参数。而噪声系数(NF)和接收链路的压缩动态范围(CDR)则影响着这两个指标。随着射频光传输(ROF)在雷达接收链路中应用的推进,除了对光链路本身的探讨外,还需扩展到接收链路中微波和光波的协同分析。因此,将其中的微波前级放大、射频光传输(ROF)、微波后级放大进行耦合,普适性地探讨接收链路CDR和NF。例如,当光链路噪声功率谱密度为−164 dBm/Hz,光链路增益−20 dB时,可设计前级放大41 dB。在这种情况下,接收链路CDR1dB达143 dB·Hz,噪声系数为4.15 dB,能够同时满足探测距离和目标分辨的要求。对外调制光链路而言,调制器的半波电压可选择在2.0~5.8 V之间,实现性能和成本的平衡。分析从系统角度出发,探讨了基于ROF的接收链路,能够满足雷达功能要求,同时,也为接收链路中电器件和光器件的设计提供了依据。Abstract: Among parameter system of radar, detection distance and target distinguishable ability are important ones, which are related to compression dynamic range(CDR) and noise figure(NF) of receiving link. Nowadays, applications of radio-over-fiber(ROF) in radar receiving link are moved forward. Besides studying on optical link itself, cooperation analysis on both microwave and optical wave should be given. Therefore, microwave pre-amplifier, ROF link and microwave post-amplifier were coupled together. CDR and NF of receiving link based on ROF were discussed universally. For example, if power spectrum density(PSD) noise of ROF was −164 dBm/Hz and optical gain was −20 dB, the CDR1dB and NF of receiving link could be 143 dB·Hz and 4.15 dB with pre-amplifier of 41 dB. In this case, detection distance and target distinguishable ability could both match requirements of system. Moreover, parameters of optical devices were also discussed. For ROF based on external modulated optical links, half wave voltage of modulators could be among 2.0 to 5.8 V to balance performances and costs. Therefore, studies from the aspects of system proof that receiving link based on ROF could meet the requirements of radar functions. Design principles for electronical and optical devices were provided in the meantime.
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Key words:
- radar receiving link /
- radio-over-fiber /
- noise figure /
- compression dynamic range /
- detection distance
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图 3 (a) CDR1dB容忍度为5 dB时,不同光链路增益Gop下,前级放大增益Gpre和NF的关系;(b) CDR1dB容忍度为15 dB时,不同光链路增益Gop下,前级放大增益Gpre和NF的关系
Figure 3. (a) Pre-amplifier Gpre and NF of receiving link under different optical gain Gop if CDR1dB tolerance is 5 dB; (b) Pre-amplifier Gpre and NF of receiving link under different optical gain Gop if CDR1dB tolerance is 15 dB
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