Jitter can be measured and analyzed by three instruments:
Bit error rate (BER) tester, jitter analyzer and oscilloscope (digital oscilloscope and sampling oscilloscope).
Which instrument to choose depends on the application, that is, electricity or light, data communication and bit rate. Because jitter is the main cause of error code, the first thing to measure is BER. If the BER of the network, network element, subsystem or IC exceeds the acceptable limit, the error source must be found.
Most engineers and technicians hope to track the jitter problem with instrument combination, first with BER tester, and then with jitter analyzer or oscilloscope to isolate the error source.
BER tester manufacturers need to measure the BER of their products to ensure that the products meet the telecommunications standards. BER testing is also important for testing high-speed serial data communication equipment when data communication components and systems need to be characterized.
BER tester sends a predefined data stream to the system or equipment under test, which is called pseudo-random bit sequence (PRBS). Then, every bit in the received data stream is sampled and the input bits are checked against the required PRBS pattern. Therefore, the BER tester can carry out strict BER measurement, which some jitter analyzers or oscilloscopes cannot do.
Although the BER tester can accurately measure BER, it will take several hours to test the network or equipment with the accuracy of10-12ber (1012 bit error). In order to shorten the test time from several hours to several minutes, the BER tester adopts "BERT sCAN" technology and uses statistical technology to predict BER.
The BER tester can be programmed to sample the input bits at any point in the bit time (called "unit interval" or "UI"). The bathtub curve shows that BER is a function of sampling position. If the BER tester detects the bit (0.5UI) in the center of the bit period, the probability of jitter causing error code is very small. If the BER tester detects bits located near the eye intersection, it will increase the possibility of obtaining bit errors caused by jitter.
The BER tester of jitter analyzer can't provide enough information about jitter persistence or jitter source. Jitter analyzer (usually called timing analyzer or signal integrity analyzer) can measure the jitter of any clock signal and provide information for the fault diagnosis of jitter. The jitter analyzer also uses jitter characteristics to predict BER, which takes much less time than the BER tester.
Jitter tester is very useful for testing devices used in high-speed data communication buses (such as optical fiber communication, SerialATA, Infiniband, Rapidio, and the data rate of each channel is as high as 3. 125Gbits/s). Because jitter analyzer can predict BER in a few seconds, it is very useful for production line testing. Many ATE manufacturers add jitter testers to the test system according to the requirements of users.
The jitter analyzer detects signal edges and measures the time between edges. After collecting time series data, the jitter analyzer executes algorithms to generate histograms, frequency curves and other intuitive data images. These images show clues to the interference signal. The jitter analyzer separates the whole jitter into random jitter and deterministic jitter by calculating histogram and frequency curve.
Such as deterministic jitter with special sources. The interference signal phase modulates the reference signal, thus generating jitter in the measurement signal. The jitter analyzer can calculate the frequency (phase 1-4) in jitter. Once the jitter frequency is known, the jitter source can be isolated and the jitter effect can be reduced. If the frequency of the interference signal corresponds to other clock frequencies, the signal source can be isolated by adding EMI shielding or other methods to solve the problem.
Hybrid instruments Recently, some test equipment manufacturers have developed hybrid instruments. The traditional error rate tester can only give the error rate. Now BER tester does some jitter analysis, and some even include sampling oscilloscopes. Now jitter analyzer also includes a sampling oscilloscope, such as SIA-3000. These sampling oscilloscopes can observe eye patterns, but they don't have the bandwidth of special sampling oscilloscopes. At present, the oscilloscope bandwidth of the hybrid instrument is as high as 6GHz. Real-time and equivalent time sampling oscilloscopes now provide software for measuring jitter and calculating BER.
Oscilloscopes Two types of oscilloscopes have proved to be useful for jitter testing and analysis. In order to test the equipment, cable, subsystem or system with communication speed of 3. 125Gbits/s (data transmission on copper wire, which may be the highest speed), a real-time sampling oscilloscope can be used. They are similar to jitter analyzers and can measure the jitter of any clock signal.
In order to measure optical signals, such as OC- 192 and 10 Gigabit Ethernet (9.952 GB its/s) or OC-768(39.808Gbits/s), a sampling skin indicator (such as Agilent digital communication analyzer or Tek communication signal analyzer) with a bandwidth of 50 GHz to 75 GHz is needed. These oscilloscopes can also be used for electronic data signals.
Broadband oscilloscope is useful for testing the jitter of the highest bit rate used today. Because of the low sampling rate (150ksamples/s or lower), they need to repeat signals (such as PRBS) to establish an eye diagram, from which a jitter histogram can be established.
Oscilloscope manufacturers provide jitter analysis software on their oscilloscopes.
Timing error diagram is the effective instantaneous phase diagram of data stream. This indicates that jitter contains periodic components. The fast Fourier transform (the third picture) of the timing error diagram is scaled to 1MHz/div, showing the frequency of jitter. This frequency may correspond to the clock frequency of the switching power supply or crosstalk from the system data cable.
The eye diagram intersection histogram shows that there are two peaks in the distribution. Double peaks represent deterministic jitter, which comes from external interference (such as switching power supply). Another kind of jitter-random jitter follows Gaussian analysis, and their sources cannot be determined.