定义：
在频域内测量信号的频率分量，以获得信号的多种参数和信号所通过的网络的参数。频谱测量虽属电子测量范围，但它除了对电信号进行分析研究以外，还可以借助各种传感器或转换器对各种非电量信号（水声、振动、生物、医学、各种随机过程和瞬态过程如爆炸、导弹发射、水声混响、舰船和鱼雷噪声等）进行分析研究，从而改进其设计。

工作原理
       频谱指组成信号的全部频率分量的总集。在一般的频谱测量中,往往把幅度谱称为频谱。从图形来看,频谱可分为两种基本类型。①离散频谱：又称线状频谱，图形呈线状，各条谱线（代表某频率分量幅度的线）之间有一定间隔。周期信号的频谱都是离散频谱，各条谱线之间的间隔相等，等于周期信号的基频或整数倍。②连续频谱：各条谱线之间的间隔为无穷小，谱线连成一片。非周期信号和各种无规则噪声的频谱都是连续频谱，即在所观测的频率范围内的全部频率上都有信号谱线存在。实际的信号频谱往往都是混合频谱，被测量的连续信号或周期信号，除了它的基频、各次谐波和寄生信号所呈现的离散频谱外，往往不可避免地伴有随机热噪声所呈现的连续频谱作为基底。

　　频谱测量的基础是傅里叶变换。它可以将一个随时间变化的信号变换成与该信号相关连的频率的函数。因此任意一个时变信号可以分解成不同频率、不同相位、不同幅值的正弦波。

测量方式
       频谱测量有扫频式和实时式两种方法。①扫频式：利用扫频超外差接收的原理，通过多次变频过程完成重复信号的频谱测量。这种方法的特点是本振在宽频段内扫频而接收机是窄带的，所以在任一瞬间信号中只有一个频率分量被测量（接收机滤波器有一定带宽,电路需要有一定的响应时间,所以每条谱线实际上占有一定频带），其余频率分量被抑止。随着本振的扫频，按顺序测量信号中的其余频率分量。这种方法只适用于连续信号和周期信号的频谱测量，测不出信号的相位。②实时式：能在被测信号发生的实际时间内取得所需要的全部频谱信息，并显示测量结果。这种方法的特点是利用现代数字电路技术和计算机技术，对信号进行快速取样和模数变换，然后与产生正弦、余弦信号的正交本振在数字滤波器中作相关处理，经积分运算后存储并显示测量结果。这种方法特别适用于非重复性信号和持续时间很短的平稳随机过程及瞬态过程的频谱测量，也可用于周期信号的频谱测量，并能测量信号的相位。但由于数字电路技术中取样时间和数模变换速度的限制，现阶段这种方法只能测量40兆赫以下的信号的频谱。

Definition: the field of technology dealing with precise frequency measurements

More general term: optical metrology

This article is focused on optical frequency metrology, even though the connection from optical frequency to microwave frequencies is also of central importance. For a range of applications it is necessary to determine accurately the absolute frequencies of optical signals. For the highest precision, it is not sufficient to measure a wavelength and convert it into a frequency by using the vacuum velocity of light as defined within the International System of Units (SI system): the accuracy of wavelength measurements (e.g. with wavemeters) is limited by effects such as wavefront distortions. Much higher accuracy can be achieved with real frequency measurements, where an optical frequency, or a difference between two optical frequencies, is related to a microwave reference. Note that the second as the basic time unit is currently defined in the SI system of units via a 9.192631770-GHz microwave frequency of a certain transition between hyperfine levels of the cesium-133 atom.

Frequency Differences and Absolute Frequencies

A relatively easy task is to compare the difference between two optical frequencies with a microwave reference, using a beat note, if that frequency difference is of the order of some tens of gigahertz or less. One simply superimposes the two beams on a fast photodetector and obtains an electronic beat signal. The latter can then be compared with a microwave reference either by counting the cycles or by monitoring an electronic beat between the two microwave signals.

A much more difficult task is to measure absolute optical frequencies. An early approach, taken by several metrology laboratories in the world, was based on a frequency chain which started with a stable microwave reference (linked to a cesium atomic clock) and generated exactly known higher frequencies with a number of other oscillators. The frequencies of the latter were connected to the lower frequencies by recording beat signals with harmonics of the lower-frequency signals, and automatically adjusting the oscillator frequencies so as to maintain not only given beat frequencies, but a phase-coherent connection. Various kinds of nonlinear devices (Schottky diodes, metal–insulator–metal diodes, and nonlinear crystals) were used for generating harmonics in different spectral regions. By operating the intermediate oscillators as so-called fly-wheel oscillators, good temporal coherence was achieved all the way from the RF region to microwaves, the far- and mid-infrared regions and further to visible light. However, the technology was very demanding, and having the whole system perfectly working over long times was a substantial challenge.

Frequency Comb Techniques

In the late 1990s, a new technique based on frequency combs from mode-locked lasers revolutionized optical frequency metrology. It is based on the fact that the optical spectrum of the output of a mode-locked laser consists of a comb of exactly equidistant lines (disregarding noise influences). This means that such a frequency comb is determined by only two parameters: the frequency spacing (which equals the laser's pulse repetition rate) and the absolute position, specified as the carrier–envelope offset frequency. If these two parameters can be related to a microwave reference, all optical frequencies of the comb are known. Subsequently, any optical frequency within the range of the comb can be measured by determining beat frequencies with comb lines.

Obviously, the frequency comb technique is much simpler than that based on a traditional frequency chain, and it makes it possible to construct very compact frequency reference sources and frequency measurement devices. Moreover, it delivers closely spaced lines of known frequencies in a wide spectral range, allowing for frequency measurements in this wide range, rather than only around a single optical frequency as for a frequency chain. Nowadays, frequency comb laser sources are commercially available and are beginning to be widely used for metrology purposes.

Technological and Scientific Applications

There can be no doubt about the technological and scientific importance of frequency metrology. The next generation of atomic clocks will be based on optical frequency standards, combined with optical clockworks. Such optical clocks would allow time or frequency measurements with a precision beyond that of the currently used cesium atomic clocks, which already let the frequency be the physical quantity which can be measured with by far the highest precision.

Extreme precision in time measurements has many technological implications, as is obvious even considering only the many existing or envisaged applications of the American GPS system and the European Galileo system.

The measurement of other physical quantities such as electrical voltages and currents and also magnetic field strengths can also strongly profit from accurate and precise frequency standards. Furthermore, the clarification of fundamental scientific questions depends on ultraprecise time or frequency measurements; for example, such measurements are vital for checking whether there might be any time dependence of certain quantities (e.g. the fine structure constant α) which are so far considered as physical constants. If any changes of such quantities could be detected, this would have a profound impact on future theoretical descriptions of most fundamental phenomena.

Also, some kinds of scientific devices, such as free electron lasers for the generation of ultrashort pulses and arrays of radio telescopes for astronomy, require extremely precise timing synchronization of different parts, which can e.g. be done with actively stabilized fiber-based timing links.

Bibliography