Optical Rx Margin
Enter Tx power and loss (or Rx power directly) and the receiver sensitivity/overload, and I compute the margin over sensitivity, headroom to overload and dynamic range, with a verdict.
Receiver operating window
—
Sensitivity / overload of common optics
| Optic | Sensitivity | Overload | Dyn. range |
|---|
Typical datasheet values; always confirm with your module's vendor.
How it is computed · optical margin
1. Optical power is in dBm
(logarithmic scale). Received power is
Prx = Ptx − loss, or you measure it directly with a power meter.
2. The receiver has a window:
sensitivity (minimum) and overload (maximum).
The dynamic range = overload − sensitivity.
3. The margin = Prx − sensitivity
tells how much is left above the threshold; the
headroom = overload − Prx how much is left before saturating.
4. Verdict: if Prx < sensitivity
there is no link; if Prx > overload
it is overloaded (add an attenuator of
Prx − overload dB); in between, OK.
Leave ≥ 3 dB of margin.
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How it works
The calculator compares the power arriving at the optical receiver against its operating window. The received power comes from Prx = Ptx − loss (transmitter power minus the total attenuation of fiber, connectors and splices) or is entered directly from a power meter reading. With the receiver sensitivity (minimum detectable) and overload (maximum before saturating) it computes the margin = Prx − sensitivity, the headroom = overload − Prx and the dynamic range = overload − sensitivity, all in dBm/dB.
The verdict follows real optical budget logic: if Prx falls below the sensitivity there is no link; if it exceeds the overload the receiver saturates and the tool tells you how many dB of attenuator to add; in between, the link is good, with the design recommendation of keeping at least a 3 dB margin for aging and repairs. It includes presets with typical datasheet values for common optics (1G SX/LX, 10G SR/LR, 25G LR, aligned with IEEE 802.3).
Example: 10G LR link with -3 dBm Tx and 8 dB of loss
- Received power:
Prx = −3 − 8 = −11 dBm. - With −14 dBm sensitivity and −1 dBm overload, the margin is
−11 − (−14) = +3 dB. - Headroom before saturation:
−1 − (−11) = +10 dB; receiver dynamic range: 13 dB. - Verdict: OK, right at the recommended 3 dB margin: the link works but has no slack for future degradation.