9.2.2.1 Probing LVDS Transmission Lines

Always use high impedance (> 100 kΩ), low capacitance (< 2 pF) scope probes with a wide bandwidth (1 GHz) scope. Improper probing gives deceiving results.

9.2.2.2 Data Rate vs Cable Length Graph Test Procedure

A pseudo-random bit sequence (PRBS) of 2⁹−1 bits was programmed into a function generator (Tektronix HFS9009) and connected to the driver inputs through 50-Ω cables and SMB connectors. An oscilloscope (Tektronix 11801B) was used to probe the resulting eye pattern, measured differentially at the input to the receiver. A 100-Ω resistor was used to terminate the pair at the far end of the cable. The measurements were taken at the far end of the cable, at the input of the receiver, and used for the jitter analysis for this graph (Figure 16). The frequency of the input signal was increased until the measured jitter (t_tcs) equaled 20% with respect to the unit interval (t_tui) for the particular cable length under test. Twenty percent jitter is a reasonable place to start with many system designs. The data used was NRZ. Jitter was measured at the 0-V differential voltage of the differential eye pattern. The DS90LV047A and DS90LV048A can be evaluated using the new DS90LV047-048AEVM.

Figure 24 shows very good typical performance that can be used as a design guideline for data rate vs cable length. Increasing the jitter percentage increases the curve respectively, allowing the device to transmit faster over longer cable lengths. This relaxes the jitter tolerance of the system allowing more jitter into the system, which could reduce the reliability and efficiency of the system. Alternatively, decreasing the jitter percentage has the opposite effect on the system. The area under the curve is considered the safe operating area based on the above signal quality criteria. For more information on eye pattern testing, please see AN-808 Long Transmission Lines and Data Signal Quality (SNLA028).