High-Speed and Certification for Differential Signal Transmission Cable Test
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Certification:signal transmission cable test
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Differential signal transmission cable test
With the development of the information industry, the requirements for signal transmission rate are getting higher and higher, which requires more signal bytes to be packed into the limited bandwidth. The shrinking semiconductor units require the signal level to be lower and lower. The reduction of signal bandwidth and the reduction of signal level will inevitably lead to an increase in bit error rate. In order to reduce the bit error rate, differential transmission technology came into being. Now, differential signal transmission technology is widely used in high-speed and long-distance transmission scenarios, greatly improving the reliability of signal transmission.
Differential transmission is a signal transmission technology. Different from the traditional practice of one signal line and one ground line, differential transmission transmits signals on both lines. The two signals have the same amplitude and opposite phase. The signal transmitted on these two lines is a differential signal. The signal receiving end compares the difference between the two voltages to determine the logical state sent by the sending end.
Differential transmission transmits signals on both lines. The amplitudes of the two signals are the same, but the phases are opposite.
Technical advantages
Compared with ordinary single-ended signal routing, the most
obvious advantages of differential signals are reflected in the
following three aspects:
1. Strong anti-interference ability, because the coupling between
the two differential routings is very good. When there is noise
interference from the outside, it is almost coupled to the two
lines at the same time, and the receiving end only cares about the
difference between the two signals, so the common mode noise from
the outside can be offset to the greatest extent.
2. It can effectively suppress EMI. For the same reason, since the
polarities of the two signals are opposite, the electromagnetic
fields they radiate to the outside can offset each other. The
tighter the coupling, the less electromagnetic energy is released
to the outside.
3. Accurate timing positioning. Since the switching change of the
differential signal is located at the intersection of the two
signals, unlike the ordinary single-ended signal that relies on the
high and low threshold voltages for judgment, it is less affected
by the process and temperature, which can reduce the timing error,
and is also more suitable for circuits with low amplitude signals.
The currently popular LVDS refers to this small amplitude
differential signal technology.
Main test parameters of differential signal pairs
Although differential transmission has many benefits, its
disadvantages are also obvious. For example, when PCB wiring, the
two differential lines must be strictly equal in length and
equidistant, which places high demands on the design and process of
the PCB board. When using twisted pair to transmit differential
signals, the two wires in the twisted pair must be equal in length
and must be tightly twisted together. If the manufacturing process
does not meet the standards, it will not only fail to reduce the
bit error rate, but may even cause serious transmission problems.
At this time, the performance test of the differential transmission
cable can be used to verify whether the cable can meet the needs of
signal transmission.
Beice Group has high-bandwidth network analyzers, electronic calibrators, and dedicated differential cable test fixtures, as well as many experienced RF test engineers, which can accurately test the various parameters of differential transmission cables and effectively help customers improve the performance of RF products.
Test indicators
The common test indicators of differential transmission cables are
as follows:
★ Differential impedance: In addition to the characteristic
impedance of the two wires, the characteristic impedance of the
differential signal should also be added with the impedance
generated by coupling, which is generally slightly smaller than the
sum of the characteristic impedances of the two separate cables.
This indicator needs to be realized by using the TDR function of
the network analyzer. The differential characteristic impedance is
represented by TDD11 on the network.
★ Return loss: The ratio of the reflected power at the input end of
the differential line pair to the input power. The cause is the
discontinuity of the impedance. The return loss is represented by
SDD11 on the network.
★ Insertion loss: The ratio of the power received at the output end
of the differential line pair to the input power. The insertion
loss is represented by SDD21 on the network. Depending on the needs
of the customer, it is sometimes represented by SDD12.
★ Near-end crosstalk and far-end crosstalk: During the signal
transmission process, due to the existence of mutual inductance and
mutual capacitance, the signal generates different interference
signals to the input end and the far end in the adjacent wire
pairs. The near-end crosstalk is measured at the input end of the
interfered line pair, and the far-end crosstalk is obtained at the
far end of the interfered line.
★ Delay difference within the differential pair: The relative time
difference after the specific phase of the differential signal is
transmitted through equal-length wires. This indicator is very
important for the accuracy of differential signal transmission
information.
Note: The DD in SDD and TDD here represents the relationship
between the input (Stimulus) and feedback (Response) of the
differential mode.
Test application
Components of signal transmission equipment, such as PCB boards,
twisted pairs, RF cables, etc.
Test application of differential signal transmission cables
Related standards
GJB 9386-2018 Test method for data transmission performance of
electrical connectors
YD/T 838.1-2016 Twisted pair/star-twisted symmetrical cables for
digital communications
GB/T 5441-2016 Test method for communication cables
High-Speed and Certification for Differential Signal Transmission Cable Test
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