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N4391B Optical Modulation Analyzer

Data Sheets

Keysight N4391B

Optical Modulation Analyzer

General Information

The Keysight N4391B Optical Modulation Analyzer enables development of the coherent technologies required to achieve the next level of performance in modern communication systems such as between, and in, datacenters. It consists of a UXR real-time oscilloscope and a calibrated coherent optical receiver. The N4391B builds on the UXR oscilloscope series’ industry-leading noise performance and Keysight’s well-known, reliable, and flexible vector signal analysis (VSA) software. The most compact OMA solution of its class, the N4391B not only solves your coherent measurement problems but also makes your test equipment decision easier and protects your investment: it provides two paths of upgradability that can be accomplished in minutes through the installation of licenses. While complex modulated signals are commonly used in long-distance transmission across DWDM links, newly proposed technologies for intra-datacenter interconnect use the coherent approach in the O-band, addressing the higher data rate need with improved power efficiency. Keysight now provides OMA coherent receivers for the O-band in addition to the current coherent receivers that work in the C-band and L-band.


The ever-growing demand for higher transmission capability drives symbol rates on an ever-increasing trajectory. Yesterday’s systems at 64 GBaud are yielding to newly proposed ones at 124 GBaud and beyond to 200 GBaud. The Keysight N4391B helps you keep pace with this evolution that demands test equipment that can handle the symbol rate classes of transceivers for 800 Gb/s, 1.2 Tb/s to 1.6 Tb/s, from the first day in advanced research through the development phase. Not only symbol rates are challenging, but also modulation formats are getting more demanding due to the higher-order quadrature amplitude modulation which requires a step up in noise performance. The N4391B meets this challenge with the UXR’s Effective Number of Bits performance having an industry leading ENOB of 5.4. The N4391B, based on the latest industry-leading UXR Oscilloscope series, is the best-suited optical modulation analyzer to support these application requirements. The Keysight N4391B provides an operating bandwidth up to 110 GHz and a myriad of options for selecting modulation formats. Even user-defined modulation formats are supported through an additional option, and all at an outstanding high ENOB at highest bandwidths.

Product Details

The N4391B is offered with two versions of optical coherent receivers: one is calibrated to 70 GHz operation and equipped with 1.85 mm connectors. It includes C-band and O-band variants. A second one is factory calibrated to 110 GHz and equipped with 1.0 mm connectors. The bandwidth of operation is dependent on the oscilloscope bandwidth and the coherent receiver bandwidth chosen. In each C-band configuration, the optical coherent receiver is comprised of an internal power monitor, internal LO, LO output and external LO input. The O-band version requires an external LO tunable laser operating in the 1.3 µm band.

Bandwidth upgradability

Each of the N4391B’s configurations are upgradable to higher bandwidths: configurations based on the 1.85 mm connector can be upgraded from 40 GHz to 50 GHz, or 59 GHz, up to 70 GHz, while configurations based on the 1.0 mm connector can be upgraded from 40 GHz to 59 GHz, 70 GHz, 80 GHz, 100 GHz, up to 110 GHz. There are even configurations possible that feature different bandwidths for electrical and optical measurements. Upgrading the coherent receiver from option 007 to option 011 requires hardware rework at Keysight.


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