A Compliant Receiver Is Not a Flat Receiver
A CATV or FTTH optical receiver entering the EU arrives under CE marking, with the EMC assessment commonly referenced to EN 50083-2, plus RoHS 2 (2011/65/EU), WEEE and REACH duties. North America is a separate scope: FCC Part 15 limits (Class A hub, Class B residential), NRTL listing to UL or ETL, SCTE practices and California Prop 65 are what those markets require. HENSURE equipment is CE certified and RoHS compliant, DoC and reports on request.
None of that tells you whether the RF output holds flat when the optical input falls. That is a control-loop question, and where most "level dropped" complaints begin.
[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。]
The Failure Mode Nobody Measures: Level Flat, CNR Falling
An AGC (automatic gain control) receiver adjusts its own gain as optical input changes, holding RF output at a reference. That is the promise, and it usually works.
AGC holds level. It does not hold CNR. As optical input falls, carrier-to-noise at the photodiode falls with it while the meter still reads the same dBmV. A technician sent to a complaint finds a normal level, closes the ticket, and the subscriber calls back.
Level problems live in the RF chain and the AGC window. Noise problems live in the optical budget. A tight loop can mask the second one for months.
What AGC Actually Controls
A datasheet that gives only one of these three is not a specification:
- The AGC window — the optical input range over which output is held at reference, stated in dBm at the receiver's optical port. Outside it the loop has nothing left to give.
- The reference level — and at what channel count and OMI it is defined. "48 dBmV" means nothing without them.
- Slope and response time — how fast the loop tracks, and whether it overshoots on a fast transient such as a protected transmitter switchover.
Below the window, output falls roughly in step with optical input and CNR falls with it; the exact slope depends on the detector chain, so ask for the measured curve. Above the top of the window the failure inverts: excess optical power pushes the photodiode toward saturation and CSO/CTB degrade while the level still looks right.
Six Reasons the Level Drops With AGC Working
- Input below the window. Link loss grew: an extra splice, a contaminated APC connector, a UPC/APC mismatch at the panel, a splitter re-balanced for a new MDU branch, or a transmitter whose output has aged.
- MGC left enabled. Many units offer MGC (manual gain control). Set by hand, the loop is off and temperature drift reaches the subscriber directly.
- Reference set against the wrong load. Levels trimmed with an analog-era plan, or with DOCSIS 3.1 OFDM carriers unloaded, land wrong once the full spectrum is live — different loading, different OMI.
- Tilt, not level. On a 1.2 GHz HFC upgrade the equaliser often stays set for 870 MHz. The average holds while the top of the band droops.
- Thermal derating. An unconditioned riser or pole housing runs hotter than the bench. Ask for level stability across the rated range, not at 25 °C.
- A failing supply or RF stage. The loop commands gain the stage cannot deliver — sag under load, not a clean offset.
Measure It With the Conditions Attached
| Parameter | What to record | Why it decides the diagnosis |
|---|---|---|
| Optical input | dBm at the receiver port, at test | Places the unit inside or outside the window |
| Channel load | Count, analog vs digital, OFDM carriers | Sets the OMI the loop was trimmed against |
| RF output | Per channel, named test point | Without the test point the number is not comparable |
| CNR / CSO / CTB | Value plus channel count and OMI | Separates a level fault from a noise fault |
| Slope | Low to high band edge, dB | Catches tilt a mid-band reading hides |
| Cabinet temperature | °C at the unit, at test | Explains drift that vanishes on the bench |
Where the Loop Bites
1550 nm RF video overlay on GPON or XGS-PON. The receiver sits after a WDM coupler and an EDFA, so the input it sees includes WDM loss and splitter tolerance. Size the window to the worst-case span, not the design centre, and check the SBS margin upstream.
RFoG return. The RFoG upstream at 1610 nm carries burst traffic a downstream AGC loop was never designed for. Return level should be predictable, which usually means MGC on the return, AGC on the forward path.
MDU risers. One receiver feeds dozens of subscribers through a splitter tree. A reference 2 dB wrong is not 2 dB at the tap; it is multiplied by every stage behind it.
DOCSIS 4.0 planning. Higher-order modulation tolerates far less CNR droop. A receiver that held level through an analog era can still fail an OFDMA carrier — the loop never measured what now matters.
AGC or MGC: Decide From the Link
Choose AGC where the optical span moves — long feeders, wide temperature swings, splitter reconfigurations. Choose MGC where the system needs fixed, predictable gain: cascaded chains, return paths, headends that manage level centrally. Many networks run both, and that is a defensible default rather than a compromise. The mistake is treating the choice as a checkbox: both modes sit on the same board, and what differs is whether the window, reference and slope were specified for your link.
Warranty, RMA and the Question That Settles Them
A unit returned as "low output" is only arguable if it carries its own as-shipped numbers. Ask for a serial-numbered report stating optical input, channel load, OMI and the level held at those conditions — that turns a dispute into a comparison. Settle warranty term, return freight and turnaround before the PO: in an MDU riser or a live hub, turnaround is outage duration.
[待用户补充：HENSURE 质保期、RMA 条款（运费归属 / 换修时限）与备件供货清单。]
Price: You Are Buying the Window, Not the Word
Two receivers both advertise AGC. The difference is window width, reference accuracy, slope control and whether the conditions are documented. A narrow window on a long, hot feeder means truck rolls; a wide window with a measured curve survives its first summer.
Checklist Before You Specify
- [ ] Optical input range, worst case and design centre
- [ ] AGC window and reference, with channel count and OMI stated
- [ ] AGC or MGC per path — forward and return separately
- [ ] Slope target across the full 1.2 GHz band
- [ ] Level stability over the cabinet's real temperature range
- [ ] Serial-numbered report and measured level-vs-input curve
- [ ] Warranty term, RMA turnaround, spare-unit policy
Since 2015, HENSURE has built 1310/1550 nm transmitters, optical receivers, EDFA amplifiers and FTTH equipment for operators, integrators and distributors. Send your optical span, channel plan and cabinet conditions, and we will return a configuration with the AGC window drawn against your budget.