What might the reason, that i have difficulties to achieve trace noise specs (<0.001dB rms at lower freqs, S21) during ENA E5071B (2-port version) performance test. It's relatively simple test where have to connect short and good quality cable through port 1 to port 2 and follow the instructions. Result arounds somewhere 0.003 to 0.006dB rms. Can used optio IE5 (high stability timebase) cause some extra noise on trace? Performance test dosen't take any account, should the test perform with or without IE5. Analyzer own VBA "external_test" can't see any problem on trace noise and gives "Pass" mark for it.