1、 Channel resistance

Channel resistance is the resistance added to the signal path due to the presence of cables, connectors, and switches. There are some basic knowledge about the role of resistance in the circuit. For example, resistance can reduce noise interference as a part of the filter network, or it can be used as an attenuator to introduce high voltage signals into the connecting equipment within an acceptable range. On the other hand, resistance also causes some side effects on the interconnection system, such as bad voltage drop, transmission signal attenuation and heating. The resistance in the signal routing system will cause partial loss of signal in the process of reaching the destination.

2、 Signal routing and AC measurement

Automatic Test Equipment (ATE) switch systems, especially those that include RF measurement, are usually built with smaller relays. Depending on the application, the type of cable you choose may be critical.

Two types of coaxial cables RG178 and RG316 are commonly used in the ATE system. Both cables have the same propagation delay time and the same capacitance per foot. Both types of cables need to be used in a 50 Ω environment. You can notice that the two are different in attenuation values. If we consider connecting a 12 foot coaxial cable between the signal source instrument outputting 140MHz square wave and the equipment under test in a test system, the 5th harmonic of the waveform is located at 700MHz. The attenuation of signal is a function of frequency. The loss of RG178 is 1.6 dB more than that of RG316 at 700 MHz. In the case of higher harmonics, the loss is greater. If the cable length increases, the loss at 700MHz will also increase accordingly. This will have a huge impact on the dynamic test. For example, higher harmonics will make the edge of the waveform very sharp, resulting in longer time and wider pulse. Conversely, any method that can shorten the overall signal routing path can allow more harmonics to exist in the signal, and the waveform is less prone to distortion.

A longer cable is conducive to a longer support service period, which is more conducive to maintenance. To shorten the signal routing path, we usually consider cost, system pin, flexibility and performance comprehensively. This is the most effective way.

Since ATE switch system is equivalent to the hub of input and output of all instruments, it usually occupies the largest number of pins on the user interface. For a switching system, it is not rare to have thousands of nodes, so the number of cables used in the whole system will be the largest. If the distance between the user end and the system can be shortened, the overall cable length of the system will be shortened, which is of great significance to the entire system.

If the switch card and other instrument cards are placed together in a PXI chassis, one way to shorten the distance can be considered is to directly place the chassis behind the connecting panel. The wires coming out of the switch are wrapped in a funnel shaped device with very short threads. Although the upfront cost of mass interconnects with funnel components is higher than discrete cables, it can be compensated by improving maintainability and performance.

Finally, it is smart to consider the switch module topology as a possible connection trunk to build a switch system to shorten the cable length.