Analysis Of High-Speed Optical Signals Using Sampling Oscilloscope
n view of the dynamic growth in demand for information transfer, increasing requirements for the quality of transfer, security, and connections control, ITU-T (International Telecommunication Union) develops and improves standards for information transmission in optical systems. One of the main activities of ITU-T is the adoption of the concept of transport network building, published in the form of Recommendation G.805, and transport network modeling development based on fiber-optic and radio-relay transmission systems. At the same time fiber-optic systems, of course, have to play the main role.
Keysight DCA-X 86100D sampling oscilloscope allows to perform accurate characterization measurements of digital devices with a data rate of 50 Mbit/s to 80 Gbit/s or more.
Typical applications of this oscilloscope include:
characterization of optical signals when designing and manufacturing transceivers;
characterization of electrical signals when developing and testing ASICs and FPGAs;
The design of Keysight 86100D DCA-X oscilloscope (Fig.1) provides a successful combination of performances. An analogue wide bandwidth, a small value of jitter inserted in the signal and a low noise level allow for accurate measurement of optical and electrical characteristics of devices with a data rate of 50 Mbit/s to 80 Gbit/s. The mainframe provides a basis for an in-depth design analysis, including the compensation function for the cables and test fixtures effects that improves measuring accuracy and provides an opportunity to determine the actual characteristics of the developed devices.
The modular design allows increasing the capabilities of the system if needed. DCA-X oscilloscope supports a wide range of modules for testing optical and electrical devices. It is possible to select an appropriate module, which provides the desired bandwidth value, filtering and sensitivity. Any module of DCA family can be used with DCA-X mainframe, besides that 100% backward compatibility with the previous 86100C mainframe is provided.
The system has typical configurations (Fig. 2). For ease of use, Keysight DCA-X 86100D oscilloscope has two user interfaces. One is a classic interface of the digital communications analyzer (DCA) to ensure full backward compatibility with previously released mainframe models. Another one is the new FlexDCA interface, which provides additional measurement functions and advanced signal analysis.
Using various combinations of the mainframe and modules, developers of digital devices can get four instruments with industry-leading features:
a full-function oscilloscope with a bandwidth up to 80 GHz, ensuring accurate signal measurements;
a digital communications analyzer with automated eye diagram analysis for standard compliance testing;
the industry’s most accurate jitter and interference analyzer, providing single-button measurements of jitter subcomponents sources;
a full-function time domain reflectometer for high precision impedance and S-parameter analysis.
With the help of the toolbar and drop-down menu, you can perform different types of measurements, and specific types of measurements depend on the mode of DCA-X. The components of the demonstration measuring system for high-speed optical signal analysis are shown in Figure 3. In this measuring system, the digital-pattern generator of Keysight N4960A bit error rate tester (BERT) is used as a signal source. Signal parameters: frequency of the clock synthesizer is 5 GHz (thus, signal rate is 10 Gbit/s), signal amplitude is 0.5V. The type of signal sequence is pseudorandom (PRBS). Sequence length is 127 characters.
Keysight 81490A electro-optical converter (reference transmitter) located in the measuring optical system is set to a converted optical signal wavelength of 1305.3 nm. The sampling oscilloscope mainframe includes two modules: 83496B clock recovery module and 86105C measuring module with two channels, one optical and one electrical. It should be noted that the bandwidths of the optical and electrical channels are 9 and 20 GHz respectively.
In Scope Mode (when Scope key is highlighted on the front panel or Oscilloscope caption in a green oval is displayed in the upper left corner of the screen), the signal is displayed on the screen. Since the triggering system is not set up, initially you cannot observe a clear digital sequence on the oscilloscope screen (Fig.4).
Now, adjust the clock recovery system. For this, use 83496B clock recovery module, which is capable to restore a clock pulse sequence of electrical or optical signals. Further, this module sends a recovered signal to the triggering system, and a residual (70%) signal to the output channel of 86105C measuring module. In this case, the synchronization system will operate a clock-data-recovery (CDR Mode) (Fig. 5). When the clock recovery module is not used, then a trigger signal shall be separately sent to the front panel. Moreover, the value of its frequency shall be devisable by the frequency of the signal.
Now the conditions for the measurement and analysis of the optical signal are created. It should be remembered that the oscilloscope also offers several other modes of operation: jitter analysis, eye diagram. Let us analyze the eye diagram of the tested digital signal. To do this, go into Eye Mask Mode by pressing the button on the front panel or by clicking the color button in the upper left corner and selecting Eye/Mask Mode.
In the eye diagram mode (Fig. 6), the user is provided an opportunity to determine extinction ratio, average power of the signal, rise time and fall time, total jitter and other characteristics.
Let us now analyze jitter in the digital signal tested. To do this, go into Jitter Mode by pressing the button on the front panel or you can select Jitter/Noise with one mouse click on the colored button in the upper left corner. When activating the jitter measuring mode, the instrument automatically decompose jitter into components and displays the results in graphical and tabular forms (Fig.7).
In this mode, we can determine the values of different jitter components and, depending on which one is dominant, understand the problem of this digital stream. Thus, it is easy to determine TJ, RJ, DDJ, PJ, DCD and others (Fig.8).
Using BERT N4960A system, it is possible to insert various jitter components at different frequencies and with different amplitudes in the generated signal. Thus, "stress" tests of data transmitting-system components are conducted. The "stress" tests are required for a calibrated deterioration of the transmitted signal which, in turn, is used to test the device on the permissible level of external interferences. Three classes of external interferences ("stress") are considered:
Interference to the jitter signal (Jitter Stresses) is manifested in the form of temporary deviations from the ideal theoretical eye diagram (Fig. 9a). When we look at the eye diagram, we see signal deviations and the eye tending to close in the horizontal direction.
Interference, representing in this case signal data amplitude modulation, which is observed in the form of a vertically closed eye (Fig. 9b).
Cross-talk interferences from "aggressors" are a new class of "stress" signals, they cause periodic closing of the eyes (Fig.9c). These "stress" signals are often included in the testing of data transmission line components on their compliance with the standards. When testing, these independent control signals are sent to the end of the communication line and to the alternate line that is to the beginning.
Pay attention to the more blurred signal image in the lower right corner of Figure 10, it occurs due to increased jitter that is due to external interference to the resulting signal jitter.
Thus, the use of the system based on Keysight Infiniium DCA-X 86100D sampling oscilloscope provides us with the following key features:
generating the signal sent to the receiver at a rate up to 32 Gbit/s or more (using multiplexers of various designs;
inserting various jitter components in the signal and measuring the receiver’s response (eye closure, jitter, etc.);
flexibility in relation to different interfaces: electrical and optical ones;
the most accurate analysis of waveforms and eye diagrams;
automated testing to meet the requirements for jitter (Jitter Tolerance Test).
Thus, using the measuring system, it is possible to test receivers of various digital standards (e. g., IEEE 802.3ae Ethernet 10GbE, 8G and 10G Fiber Channel standards, etc.).