电子测量技术夏哲雷第7章节.ppt

扫频仪的应用 ②带宽测量。 对于宽带电路,可以直接使用扫频仪上的频标,方便地显示和读出频率特性曲线的带宽。 测量带宽时,先调节扫频仪输出衰减和调整Y增益,使频率特性曲线的顶部与屏幕上某一水平刻度线相切,如图中与AB线相切，然后保持Y增益不变，将扫频仪输出衰减减小3dB，则此时屏幕上的曲线将上移而与AB线相交，两交点处的频率就分别为下限截止频率fL和上限截止频率fH。因而被测电路的带宽为 ?BW＝fH－fL 扫频仪的应用 扫频仪的应用 ③回路Q值测量。 测量时电路连接和测量方法与测回路带宽相同，在用外接频标测出回路的谐振频率f0以及上、下截止频率fH和fL后，按下面的公式即可计算出回路的Q值。 扫频信号与调频信号的区别 扫频时,发生器输出频率的变化范围(扫频宽度)可能很宽可能达到发生器输出频率的整个覆盖范围 调频时发生器输出频率变化范围一般在100kHz内 扫频时的重复频率较低,一般在100Hz以下 调频时的重复频率较高,一般在20Hz~ 20kHz 扫频时发生器输出频率的变化规律一般是线性的或指数形 调频时,频率的变化规律一般是正弦形的 调频时,人们感兴趣的是已调信号的中心频率和频偏 扫频时人们感兴趣的是已扫频信号的瞬时频率和扫频范围,一般利用频率标记来确定瞬时频率 本章小结 频域测量的主要内容包括两个方面：其一是对信号进行频谱分析；其二是研究电子系统对不同频率的激励所产生的响应。 模拟式频谱仪是以模拟滤波器为基础的，其中外差式频谱分析仪是实施频谱分析的传统途径，它借用超外差接收机的原理，通过改变本地振荡器的频率来捕获欲接收信号的不同频率分量。 数字式频谱仪是以数字滤波器或快速傅里叶变换为基础的，它将输入信号数字化，并对时域数字信息进行FFT以获得频域表征。 * The most common spectrum analyzer measurements are: modulation, distortion, and noise. Measuring the quality of the modulation is important for making sure your system is working properly and that the information is being transmitted correctly. Understanding the spectral content is important, especially in communications where there is very limited bandwidth. The amount of power being transmitted (for example, to overcome the channel impairments in wireless systems) is another key measurement in communications. Tests such as modulation degree, sideband amplitude, modulation quality, occupied bandwidth are examples of common modulation measurements. In communications, measuring distortion is critical for both the receiver and transmitter. Excessive harmonic distortion at the output of a transmitter can interfere with other communication bands. The pre-amplification stages in a receiver must be free of intermodulation distortion to prevent signal crosstalk. An example is the intermodulation of cable TV carriers that moves down the trunk of the distribution system and distorts other channels on the same cable. Common distortion measurements include intermodulation, harmonics, and spurious emissions. Noise is ofte