Long spans rarely fail because the transmitter is bad. They fail because the budget was signed off before anyone asked what the launch power was measured at, over which fiber, at what linewidth. Here is how to build a 1550 nm budget that survives a 60 km span — and defend every number to an EU or North American reviewer.
The Compliance File Decides the Hardware First
In the EU, CATV transmission equipment reaches market under CE marking, with EMC assessed to EN 50083-2, material compliance to RoHS 2 (Directive 2011/65/EU), WEEE take-back and REACH declarations, plus a laser classification declared to EN 60825-1. In North America the equivalent set is FCC Part 15 (Class A for headend and hub, Class B in residential settings), NRTL listing to UL or ETL, SCTE practices for the RF interface, and California Prop 65 disclosure — what those markets require of the equipment.
HENSURE is CE certified and RoHS compliant; a DoC and the RoHS report are available on request.
[待用户补充：CE 证书编号 / 签发机构 / DoC 引用的协调标准版本；RoHS 报告编号与出具实验室。]
Why first: the DoC and the EMC report name a model, so solving reach with a higher-launch-power transmitter means new paperwork.
What a Link Budget Actually Is
P_launch − (fiber + splice + connector + splitter + margin) ≥ receiver input window
For analog CATV the right side is an input window, not a sensitivity figure, and it means nothing without the performance across it. A receiver quoted "−6 to +2 dBm" says where it will not clip or go dark, not the CNR, CSO and CTB at −6 dBm. Ask for both, at a stated channel count, OMI and optical input power.
Wall One: SBS Caps Launch Power
Stimulated Brillouin scattering turns launched power into backscatter once power density in the core passes a threshold. Past it, the extra dB does not arrive; it returns. Threshold belongs to the link, not the box: effective area, linewidth, modulation format and length all move it. Narrow-linewidth DFB sources sit in the high teens of dBm; externally modulated units with SBS suppression are quoted higher, and honest datasheets name the linewidth and fiber type behind the figure. A launch power without those two conditions is a marketing number.
Wall Two: Dispersion at 1550 nm
ITU-T G.652.D carries about 17 ps/(nm·km) at 1550 nm — near 1,000 ps/nm over 60 km. Harmless for digital, not for an analog 1550 nm RF video overlay, where chirp converts to second-order distortion: CSO moves first, then MER on the DOCSIS channels. Three fixes, ascending cost: low-chirp externally modulated transmitter, a dispersion compensation module at the receive end, or a shorter analog path.
Wall Three: The EDFA Cascade
In a booster / in-line / preamp chain the noise figure that matters is not the booster's: the first amplifier after the longest loss sets the floor for everything downstream. Cascade noise follows the Friis relation: two 5 dB amplifiers with 20 dB gain in the first stage land near 5.0 dB; reverse the order so the first stage has 10 dB and the pair sits near 5.3 dB. Gain ahead of loss buys CNR; gain after it does not.
Two consequences: every extra stage costs CNR roughly in step with its noise figure, and in long chains double Rayleigh backscatter and multipath interference accumulate into a CNR floor no launch power removes.
Wall Four: The Losses Nobody Writes Down
| Element | Reference value | Depends on |
|---|---|---|
| Fiber at 1550 nm | 0.19–0.22 dB/km | Cable type, bending, temperature |
| Fusion splice | 0.05–0.1 dB each | Quality, splices per km |
| Connector pair | 0.3–0.5 dB each | Polish, cleanliness, mating cycles |
| 1×2 / 1×4 / 1×8 splitter | 3.5 / 7.2 / 10.5 dB | Ratio plus excess loss |
| Aging and repair margin | 1–2 dB | Design life, closures, re-routes |
The connector line is where budgets die. A 60 km route with four patch panels, two in field cabinets, carries eight pairs — up to 4 dB absent from the route drawing and usually found during commissioning, in the rain. Splitter loss is the other silent term: a 1×8 sized for today's take-rate is 10.5 dB gone, where a 1×2 feeding two 1×4 units costs the same, deferring half.
Five Ways It Fails in Service
- Launch power accepted without SBS conditions.
- Patch-panel and connector count understated.
- Splitter ratio sized for year one.
- Noise figure quoted at rated input and 25 °C, not at real input in a hot cabinet.
- Margin set to zero — then a repair splice arrives.
What Redundancy Costs in dB
A 50/50 split and combine for a protected route costs about 3.5 dB each way; 1+1 with an optical switch loses less but adds a single point of failure. On a span near its SBS ceiling, decide before the transmitter.
Where This Sits in an FTTH or DOCSIS Programme
On an HFC supertrunk the span carries the analog and DOCSIS 3.1 spectrum to a hub, and every dB is shared by every subscriber behind it. On GPON or XGS-PON feeder routes the same fiber often carries a 1550 nm RF video overlay alongside the PON wavelengths, and the analog overlay — not the PON — sets the launch limit. In RFoG the optical segment reaches further in — MDU risers, hospitality — and one long span becomes many short ones, changing the wall you hit first. Where the plant is being rebuilt for 1.2 GHz HFC and a path to DOCSIS 4.0, budget the spectrum five years out.
The Numbers to Demand Before You Sign
- Launch power at a stated linewidth and fiber type, with the SBS suppression method named
- Receiver window together with CNR / CSO / CTB across it, at a stated channel count and OMI, plus the AGC or MGC range in dB
- EDFA noise figure at the input power the link actually presents, at cabinet ambient
- A per-unit test report, not a family datasheet
- Splice and connector loss as counts on a drawing, not as an allowance
- A stated aging margin, and who owns it when commissioning comes in short
Spares and RMA
A long span is a service commitment, not a box. Ask for RMA turnaround, whether replacement ships before the faulty unit returns, and how many spare amplifier stages to hold — one per three in-line sites is a common starting point.
[待用户补充：HENSURE 质保期与 RMA 条款（响应/周转时效）。]
The One-Paragraph Version
A long 1550 nm span is a compliance document, a launch-power limit and a noise budget, in that order. Get the SBS figure with its conditions, the receiver window with its CNR, the connector count onto the drawing, and hold 1–2 dB of margin nobody may spend — so it still works in year five.
If you are budgeting a supertrunk, a long PON feeder or a regional backbone, send route length, splice and connector counts, splitter plan and intended receiver; our engineering team will return the launch power, amplifier spacing and margin needed — test conditions included.