1 Spectrum Analyzer RBW (Resolution Bandwidth) and VBW (Video Bandwidth)#
1.1 Definitions of RBW and VBW#
1.1.1 RBW#
RBW is the 3dB bandwidth of the intermediate frequency filter (IF filter) in the spectrum analyzer, determining the minimum separation required to distinguish between two adjacent frequency signals.
1.1.1.1 Functions#
- Frequency Resolution: The smaller the RBW, the stronger the ability to distinguish adjacent signals.
- Noise Floor Impact: Increasing RBW by 10 times raises the noise floor by 10dB. The calculation formula is: $Noise\ Floor=-174dBm/Hz+NF_{sa}+10\lg(RBW)$, reducing RBW can enhance the ability to detect weak signals.
- Scan Time: Inversely proportional to $RBW^2$.
1.1.2 VBW#
VBW refers to the bandwidth of the smoothing filter in the spectrum analyzer, used to smooth the display of the spectrum graph to eliminate interference from noise and spurious signals, making the signal easier to observe and analyze. VBW is usually larger than RBW to ensure a smooth display of the spectrum graph.
1.1.2.1 Functions#
- Noise Smoothing: Reducing VBW can decrease the fluctuations of displayed noise (for example, reducing VBW by √10 times can reduce noise fluctuations by about 1dB), making the curve smoother.
- Response Speed: VBW does not affect RBW, but if too small, it may obscure details of rapidly changing signals.
2 SMD LED Forward Voltage Drop#
The forward voltage drop and operating current of SMD LEDs vary by color, package size, and manufacturing process. Below is a comprehensive summary of common color parameters:
2.1 Forward Voltage Drop and Operating Current of Different Color SMD LEDs#
| LED Color | Voltage Drop Range (V) | Operating Current Range (mA) | Typical Application Scenarios |
|---|---|---|---|
| Red | 1.82–1.88 | 5–8 | Indicator lights, low-power displays |
| Green | 1.75–1.82 | 3–5 | Low-brightness indicators, energy-saving devices |
| Orange | 1.7–1.8 | 3–5 | Warning lights, status indicators |
| Blue | 3.1–3.3 | 8–10 | Backlighting, high-brightness displays |
| White | 3.0–3.2 | 10–15 | Lighting, display backlighting |
2.2 Key Parameter Explanation#
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Forward Voltage Drop
- Refers to the voltage difference across the LED when it is conducting, determined by the energy gap of the semiconductor material.
- Rule: Red/Yellow light has the lowest forward voltage drop (about 1.8V), while Blue/White light has the highest (about 3.2V).
- The forward voltage drop of LEDs of the same color may have a ±10% deviation due to manufacturing process differences.
-
Operating Current
- Standard Range: Low-power SMD LEDs (such as 0603, 0805 packages) typically range from 3–15mA.
- Current and Brightness: The higher the current, the higher the brightness, but exceeding the rated value will shorten the lifespan (e.g., Red light >20mA may burn out).
- Minimum Starting Current: 0603 package LEDs can go as low as 0.1mA, but require 2–5mA to achieve visible brightness.
2.3 Practical Design Considerations#
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Current Limiting Resistor Calculation
- Formula: $R = \frac{(V_{supply} - V_{LED})}{I_{LED}}$
- Example: 5V supply driving a Red LED ($V_{LED}=1.8V$, $I_{LED}=5mA$), resistor value $R = \frac{(5-1.8)}{0.005} = 640Ω$ (commonly used 680Ω).
-
Package and Power
- Small Size Packages (such as 0402, 0603): Current should be ≤5mA to avoid overheating.
- High Power LEDs (such as 3030 package): Current can reach 220mA, requiring heat dissipation design.
2.4 Summary#
The selection of parameters for SMD LEDs needs to balance brightness, power consumption, and lifespan. Low current designs (e.g., Red light 5mA) can extend lifespan, while high brightness scenarios (e.g., White light 15mA) require attention to heat dissipation. In practical applications, it is recommended to refer to the specific model's datasheet and rely on actual measurements.