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I want to measure LVDS signals with a scope.

Will my solution work? (Shown at bottom.)

I know of other ways of doing this, but I am hoping this way is cleaner and easier.

Specs:

  • Frequency:~100MHz
  • Load:100Ω differential

Required:

  • Measure DC-coupled signal levels (in reference to GND.) This cannot be inferred, it must be measured with decent accuracy. This solution won't cut it: How to connect LVDS signals to oscilloscope?
  • Measure differential amplitude (Q-*Q)
  • Measure common-mode level (mean(Q+*Q))

Preferred:

  • Avoid differential probes
  • Use standard BNC or SMA terminating to 50Ω

I am using a ferrite bead to reference the SCOPE-GND to PCB-GND (for measuring DC levels). The bead blocks AC currents terminating to PCB-GND, insuring I(Q)=I(*Q)∙(-1).

Problem Overview

My solution

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  • \$\begingroup\$ I don't understand what the stackup is showing. If that means a PCB, then first why is there a dedicated PCB design with poor grounding for this (test fixture I guess), whatever it is? Where would the transmission-lines diagram be applied? You say by the "AVOID" you cannot afford loading the signal (I guess) so terminating pairs into the scope seems N/A. Your "preferred" list subtly says "avoid diff probes" (plural) but "use standard [coax]" (singular), but you cannot read both DM and CM simultaneously over a single connection. And what's wrong with the "IDEAL" example, common ground? \$\endgroup\$
    – Tim Williams
    Commented Sep 8, 2023 at 16:00
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    \$\begingroup\$ A pair of good 10x RF passive probes might be the way to go. It's not difficult to make this type of probe yourself--a 450 Ω resistor (not wirewound) in series with a length of coax (I suggest RG-316, double-shielded if you like) terminating in the scope's 50 Ω termination impedance is all you need. You can also buy them premade, LeCroy sells one as the PP066 for instance. \$\endgroup\$
    – Hearth
    Commented Sep 8, 2023 at 16:03

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