I broke the cable between cal in and cal out port of pxie-5644R by accident. Should I send it back to factory for a new cable and calibration? Or just replace it with any other one cable which is in low cost?
It is highly recommended that you send VST for a repair, but you can also replace Cal In-Cal Out cable yourself with high quality one.
If you replace cable by yourself NI can not guarantee that all parameters will match to specification. But with some parameter degradation you still will be able to use device and Self calibration feature.