In the Cable TV network, the attenuation of fiber link requires comprehensive calculation of fiber transmission, splitter distribution, and connection point losses. The specific calculation steps are as follows:
Formula for Fiber Link Attenuation Composition
Total attenuation (dB)=fiber optic transmission loss + splitter loss + active connector loss + fusion point loss
Calculation basis for each sub item:
Fiber optic transmission loss
- Formula: Fiber optic cable attenuation coefficient (dB/km) x transmission distance (km)
Standard attenuation coefficient of single-mode fiber:
- 1310nm wavelength: ≈ 0.35dB/km
- 1550nm wavelength: ≈ 0.20dB/km
Fiber Optic splitter loss
- Theoretical formula: Insertion loss (IL)=-10lg (splitting ratio) (for example, when the splitting ratio is 1:16, IL=-10lg (1/16) ≈12.04dB)
- Engineering experience formula: Loss=n x 3+1 (n is the power of 2 of the denominator of the splitting ratio, for example, 16=2 ⁴in 1:16, n=4, Loss=4 x 3+1=13dB).
- Actual losses need to be added with additional losses (average 13.5dB for PLC type 1:16 spectrometers).
Loss of activity connector
- Each activity connection point (such as FC/SC/LC interface) has a loss of ≤ 0.5dB.
Splicing point loss
- Each fusion point loss is≤ 0.08dB (single-mode fiber).
Selection and loss control of splitters
| Split ratio | Theoretical loss (dB) | Empirical loss (dB) | Actual application range |
| 1:2 | 3.0 | 1.4 | Short distance branch |
| 1:8 | 9.03 | 10 | Community level allocation |
| 1:16 | 12.04 | 13~13.5 | Administrative village coverage |
| 1:32 | 15.05 | 16~17.5 | Metropolitan area node |
Selection principle:
For short distance distribution, choose the fusion cone type (FBT Splitter), and for long distance high-density distribution, choose the planar waveguide type (PLC Splitter);
The total attenuation throughout the process should be controlled within -15~-20dBm (the optimal range for optical receivers).
Attenuation acceptance and fault location
Acceptance criteria
- Full attenuation ≤ design value, with a 10% margin reserved to cope with aging;
- At a wavelength of 1550nm, the received power for gigabit services needs to be between -15dBm and -20dBm.
OTDR fault location
- Set the wavelength to 1550nm (sensitive to bending), and select the pulse width based on distance (long pulse width for 10km);
Abnormal judgment:
- Curve sudden drop → breakpoint (accuracy ± 1m);
- Minor decrease → Welding point loss>0.1dB needs to be optimized.
Design Case (10km Fiber Link)
Fiber link structure: Optical transmitter → 1:8 splitter → 2 active connectors → 3 fusion points → Optical receiver
Attenuation calculation:
- Fiber optic transmission: 0.20dB/km × 10km=2dB
- Optical splitter: 10dB (1:8 empirical value)
- Activity connector: 0.5dB × 2=1dB
- Fusion point: 0.08dB × 3 ≈ 0.24dB
- Total attenuation ≈ 2+10+1+0.24=13.24dB
→ Meet the receiving requirements of -15~-20dBm.
Key Reminder:
Deviation in the splitting ratio can cause fluctuations in the received power at the remote end. It is recommended to prioritize using a splitter with an additional loss of ≤ 0.2dB; It is recommended to use PLC splitter+OTDR bidirectional testing calibration for new network construction.
