图1.9指出了对高质量接收机设计的几个要求。
Figure 1.9 implies several requirements ona high-quality receiver design.
例如，本地振荡器与发射机频率必须是相同的。
For example, the local oscillator and thetransmitter frequencies must be identical.
通常通过在雷达系统中设计一个稳定的本地振荡器(STALO)来确保，该振荡器为发射机和接收机提供频率基准。
This is usually ensured by having a singlestable local oscillator (STALO) in the radar system that provides a frequencyreference for both the transmitter and the receiver.
此外，许多类型的雷达处理都需要进行相干操作。
Furthermore, many types of radar processingrequire coherent operation.
IEEE标准的雷达定义中将“相干信号处理”定义为“利用称为相干振荡器的信号幅度和相位进行回波积累、滤波或检测”(重点补充)(IEEE，1982)。
The IEEE Standard Radar Definitions defines"coherent signal processing" as “echo integration, filtering, ordetection using amplitude and phase of the signal referred to a coherentoscillator” (emphasis added) (IEEE, 1982).
相干性比频率稳定性的要求更高。
Coherency is a stronger requirement thanfrequency stability.
在实际的设计中，对于多个连续的脉冲来说，相干性意味着雷达发射的这些载波信号脉冲的相位基准总是保持不变的。
In practice, it means that the transmittedcarrier signal must have a fixed phase reference for several, perhaps many,consecutive pulses.
考虑时刻t1发射的脉冲信号a(t – t1) sin[Ω(t– t1) + ϕ]，其中a(t)为脉冲包络。
Consider a pulse transmitted at time t1of the form a(t – t1) sin[Ω(t – t1) + ϕ], where a(t) isthe pulse shape.
在一个相干系统中，t2时刻发射的脉冲表达式为a(t – t2)sin[Ω(t – t1) + ϕ]。
In a coherent system, a pulse transmittedat time t2 will be of the form a(t – t2) sin[Ω(t – t1)+ ϕ].
注意：两个时刻的sine项具有相同的相位Ω(t – t1) + ϕ，只是信号包络随时间轴发生变化。
Note that both pulses have the sameargument Ω(t – t1) + ϕ for their sine term; only the envelope termchanges location on the time axis.
这样，两个正弦信号的参考基准都是相同的绝对起始时间和相位。
Thus, both sinusoids are referenced to thesame absolute starting time and phase.
This is as opposed to the second pulsebeing of the form a(t – t2) sin[Ω(t – t2) + ϕ], which isnonzero over the same time interval as the coherent pulse a(t – t2)sin[Ω(t – t1) + ϕ] and has the same frequency, but has a differentphase at any instant in time.（似乎本句无翻译必要！）
图1.12直观地描述了相干与非相干信号的差别。
Figure 1.12 illustrates the differencevisually.
图1.12 相干信号中的固定相位基准描述。（a）由基准正弦波产生的相干脉冲对；（b）参考正弦波；（c）非相干脉冲对。Illustration of the conceptof a fixed phase reference in coherent signals. (a) Coherent pulse pairgenerated from the reference sinusoid. (b) Reference sinusoid. © Noncoherentpulse pair.
在相干情况下，两个脉冲看起来像是从同一连续的、稳定的正弦波中截取的；在非相干情况下，第二个脉冲与第一个脉冲的延拓相位是不一致的。
In the coherent case, the two pulses appearas if they were excised from the same continuous, stable sinusoid; in thenoncoherent case, the second pulse is not in phase with the extension of thefirst pulse.
根据前面讨论的相位模糊问题，相干性也意味着该系统必须包含I、Q两个通道，否则无法提取信号的相位特征。
Because of the phase ambiguity discussedearlier, coherency also implies a system having both I and Q channels.
另一个要求是I、Q通道在信号带宽上必须具有完全匹配的传递函数。
Another requirement is that the I and Qchannels have perfectly matched transfer functions over the signal bandwidth.
这样，两个信号通道的增益和相位延迟（电长度）都必须是一致的。
Thus, the gain through each of the twosignal paths must be identical, as must be the phase delay (electrical length)of the two channels.
当然，实际中的接收机是不可能具有完全匹配的传输信道。
Of course, real receivers do not haveperfectly matched channels.
第三章中将考虑接收机中增益和相位不平衡的影响。
The effect of gain and phase imbalanceswill be considered in Chap. 3.
——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》
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