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Why do I get a flat line using N5425A with U1818B and E5052B?

Question asked by Rob007 on Dec 23, 2020

I am evaluating phase noise on an eval board from SiLabs for a 100MHz oscillator. The eval board has SMA connectors on the oscillator outputs and when I connect through them to my U1818B probe to my E5052B SSA, I can almost perfectly replicate the phase noise profile offered in the SiLabs datasheet (see Orig_config_w-gnd.png attachment). However, when I try to probe using an N5425A probe tip through the U1818B probe into the E5052B SSA, I get a flat line profile at -130dB for the required range of 12kHz to 20MHz (see Ext pwr supply at 3.3V_R9_C7_C9-missing_w-gnd_Agilent probe w-gnd.png attachment). Some explanation: the missing C7 and C9 are the coupling caps at the oscillator outputs to the SMA connectors which I removed to eliminate the trace stub to the SMAs thereby probing the oscillator output. The missing R9 is a zero ohm res removed to be able to attach an external DC power supply at 3.3V to the oscillator thereby removing the onboard supply and any associated effects. We have three different N5425A probe tips and all three behave exactly the same way. I can't find any sort of setup on the SSA required for the N5425A probe tips and don't remember ever having to do any specific setup in the past when using these tips. It seems unlikely all three of my tips are "broken" but I don't rule that out. I don't question the U1818B probe itself because of how well it reports a profile so close to what I see in the SiLabs datasheet so the U1818B/E5052B pictures seems good. Any suggestions?