频谱测量
信号的时域观测：幅度 vs 时间
简单例子: 正弦波
- 纵轴为幅度
- 横轴为时间
- 频率为周期的倒数 (F = 1/T)
信号的频域观测：幅度 vs 频率
简单例子: 正弦波
- 纵轴为RMS幅度
- 横轴为频率
频谱分析仪实际上为指定频率的功率计
同一个信号不同观测维度
- 注意 – 电子不知道他们正在被什么设备进行观测
- 简单的正弦波例子
更多的例子 —— 方波
方波由许多正弦波构建而成
所有信号都可由一组正弦波构建而成
更多的例子
为什么要在频域观测信号？
噪声
- 任何东西都会产生噪声，它对整体设计的影响如何?
- 噪声的来源? (EMI)
- 信号对噪声的测量
失真
- 原则上在示波器上应该是纯净的正弦波可能包含谐波成分，但是在时域上可能并不明显，但是在频域上却能清晰的分辨
通信
- 现代无线通信技术自身源于频域
- 频段分配
- 定义通信信道
- 需要确认诸如占用带宽、调制质量等方面的性能
重要的频谱分析概念
中心频率和分析范围 Center Frequency and Span
示波器FFT典型的分析方式是DC到to(采样率的一半)Hz
频谱分析仪的典型方法是设定一个中心频率CF(因为它在屏幕显示的中心)
- 典型的说法中心频率
在中心频率两侧可以观测的频率区域称作频率范围SPAN
基本频谱分析仪界面
射频核心测量
| Measurements | Description |
|---|---|
| Channel Power | Is the device transmitting? On the correct channel? At the correct power? |
| Adjacent Channel Leakage Ratio (ACLR) | Is the channel transmission spreading into an adjacent channels ? |
| Spectral Emissions Mask (SEM) | A broader check to ensure power stays within the intended channel |
| Spurious, Harmonics, Intermodulation | Is the device creating miscellaneous noise which may interfere with itself or others? |
| EMI Testing | What levels must the device fall within for radiated and conducted emissions so that it doesn’t interfere with others and can pass compliance? |
| Modulation Quality | Is the device talking clearly so that other receivers can hear it well? |
| Receiver Sensitivity & Blocking | Can the device receive its desired signals sufficiently and ignore others? |
| Impedance / Return Loss /VSWR | Are the modules impedance-matched so as to optimize operation ? |
Channel power, aclr
Spectral emissions mask
Spurious, harmonics, intermodulation
EMI Testing types and limits
EMI Pre-Compliance measurements are an approximation of the EMI signature of a product. Almost all products that use energy must pass some form of EMI testing. Products must be certified by an accredited test house before them may be sold in the market.
CISPR - an international body who specify EMI regulations. SVPC includes CISPR limit lines for many radiated and conducted specifications.
| Concern | Radiated | Wired (AC power) |
|---|---|---|
| Am I affecting others? | - Intentional radiated emissions - Unintentional radiated emissions |
Conducted emissions |
| Will I be affected by others? | electromagnetic susceptibility (EMS) |
Modulation Quality – Tx Fidelity
Symbol Table - the data being transmitted, as determined by the demodulated signal.
Error Vector Magnitude (EVM) – a numerical calculation of modulation fidelity (how accurate the modulation is); zero is best.
Constellation Diagram – a visual representation of modulation fidelity (how accurate the modulation is); the clearer the diagriam the better.
Receiver Sensitivity and blocking test
Receiver sensitivity – how well your receiver can pick up small signals, which affects its range (distance) of operation. Usually measured as bit-error rate (BER) as a function of received power, or EVM as approximation.
Blocking - how well your receiver can receive the desired signal in the presence of unwanted signals.
Impedance
- Impedance is a combination value which includes resistance and reactance
- Problem: Impedance is a dynamic value that can change based on frequency
- If there is a miss-match, some of the RF power that is being fed into a component (or cable, or antenna) will be reflected back to the source
Voltage standing wave ratio (vswr)
- We want all the signal energy transferred from the transceiver to the antenna (or component) or vice versa (perfect impedance match at the antenna junction, i.e. a VSWR of 1:1)
- A short or open would reflect all energy back, creating standing waves in the signal
- Reality is somewhere in between, but hopefully closer to perfect energy transfer! VSWR tells us the amount of energy reflected
