Variable Attenuator Board Design

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Below are some example schematics and guidance on designing passive Pi-pad attenuators with selectable jumpers using only passive components. These examples cover different attenuation levels, and you can adapt them for your 0–60 dB (10 dB steps) and 0–10 dB (1 dB steps) requirements.
Example 1: Basic Pi-pad Attenuator Schematic
A typical Pi-pad attenuator consists of two series resistors and one shunt resistor. The schematic structure looks like this:
       R1            R2
Input ----/\/\/\----+----/\/\/\---- Output
                     |
                     R3
                     |
                   GND
  • R1 and R2: These series resistors provide the main signal path.
  • R3: The shunt resistor from the node between R1 and R2 to ground.
Example 2: Using Selectable Jumpers
For selectable attenuation, you can use multiple resistor sets (each set corresponding to a specific attenuation level) and deploy jumpers to choose the resistor set that creates the desired attenuation. For example:
  • Attenuation Level A (e.g., 10 dB):
    • R1_A, R2_A, R3_A are connected.
  • Attenuation Level B (e.g., 20 dB):
    • R1_B, R2_B, R3_B are connected.
  • Jumper Network:
    • Use jumpers to switch between the resistor sets. Each jumper connects the corresponding resistor nodes according to the desired configuration.
Schematic Concept: 
                    Jumper A
          +-----[R1_A]-----+
          |                |
Input ----+                +-----[R2_A]----- Output
          |                |
          +-----[R3_A]-----+
                     |
                    GND

                    Jumper B
          +-----[R1_B]-----+
          |                |
Input ----+                +-----[R2_B]----- Output
          |                |
          +-----[R3_B]-----+
                     |
                    GND
Note: Only one set is connected at a time via the jumper configuration.
Calculating Resistor Values
For a desired attenuation ( A ) (in dB) in a system with characteristic impedance ( Z_0 ) (typically 50 Ω), the equations for a Pi-pad attenuator are:
R1=R2=Z0K−1K+1\Large R1 = R2 = Z_0 \frac{K-1}{K+1}
R3=Z02(K2−1K)\Large R3 = \frac{Z_0}{2} \left(\frac{K^2 - 1}{K}\right)
Where:
K=10A20\Large K = 10^{\frac{A}{20}}
For example, for 10 dB attenuation:
  • Compute ( K = 10^{10/20} \approx 3.16 )
  • Then:
    • R1=R2=50×3.16−13.16+1≈50×0.523≈26.15 Ω\Large R1 = R2 = 50 \times \frac{3.16 - 1}{3.16 + 1} \approx 50 \times 0.523 \approx 26.15\,\Omega
    • R3=502((3.16)2−13.16)≈25×10−13.16≈25×2.85≈71.25 Ω\Large R3 = \frac{50}{2} \left(\frac{(3.16)^2 - 1}{3.16}\right) \approx 25 \times \frac{10 - 1}{3.16} \approx 25 \times 2.85 \approx 71.25\,\Omega
Note: Round to standard resistor values using your favorite series (e.g., E12 or E24).
Detailed Schematic Example Resources
For additional information and detailed circuit examples, please refer to these resources from the web:
You can review these examples for various attenuation levels and adjust the resistor values to achieve your specific 0–60 dB and 0–10 dB step designs. This approach ensures the design remains entirely passive and maintains the desired 50 Ω impedance throughout the board.
If you have any questions on adding specific components or connecting nets within Flux, feel free to ask!
Review Pi-pad resources
Calculate resistor values
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J1
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