DSO Probe Effects at the DUT

[mawyatt]

While commenting on another thread about DSO probe effects at the DUT thought it might be useful to start a new thread dedicated to such.

https://www.eevblog.com/forum/testgear/siglent-sds800x-hd-12-bit-dsos-coming/msg5759401/#msg5759401

This is about the effects of the DSO probe as "Seen" by the DUT, rather than the usual probes effects as displayed on the DSO.

The setup is crude (not technically correct and calibrated) but good enough to show the effects, see thread above for details. An AWG signal generator (SDG6022X) is set to 0dBm and swept from 1MHz to 300MHz. The test DSO is the SDS824X HD (enhanced to 200MHz) in this case and thus the selected 1MHz to 300MHz Frequency Range. For the AWG 50Ω termination we utilized the input of a Spectrum Analyzer (SSA 3121X +) so we could "see" the DSO Probe effects on the SA since it's now the DUT for the DSO Probe. A BNC "T" was used to allow the DSO Probe BNC Probe Adapter to be inserted and the various test probes inserted into the adapter, see mentioned thread for setup images.

The SA was set to 1MHz to 300MHz range, peak detect, and +1dBm Reference level with 1/2dB/div. A reference was captured to show the results without the DSO probe and should show ~0dBm across the 300MHz span as the Yellow Trace. This shows about ~1/2dB variation due to the AWG, cables, adapters and such, and was not calibrated out. A more diligent test would calibrate this out and utilize better cables, adapters and such, however we felt this unnecessary for this case.

Anyway, the Yellow Trace is the reference without any DSO Probe attached. The Magenta trace is with the Siglent PP510 Probe @ 10X, the Cyan trace is the Siglent 10X SP2030A and the Green is the Rigol PVP3150 @ 10X. All probes were attached to a Siglent SDS824X HD, altho we found the SA traces didn't change when the Probe was attached or not on the DSO side. The PNG116 trace is from the Siglent SDS824X HD FFT with the PP510 Probe @ 10X, compare with the SA Magenta trace, note scale differences tho.

Note that the SA is showing a result which indicates the probe represents a more complex "load" to the DUT (SA in this case) than a simple shunt capacitance. This may cause the DUT to behave differently than expected and one should also realize that the result displayed on the DSO screen is not exactly what the DUT is experiencing due to the probe loading. In addition, the frequency response behavior can't be just related to a clock rate at the DUT for example, since the clock edges contain significant energy well beyond the clock rate and this must be considered

Happy Holidays,

Best

[tautech]

Yes, typically any measurement will affect the measurement with the measurement system effects on the DUT.

In a thread last year we explored this a little:
https://www.eevblog.com/forum/testgear/probe-into-probes-whats-up/msg4678768/#msg4678768

[mawyatt]

Yes, typically any measurement will affect the measurement with the measurement system effects on the DUT.

One could postulate that any measurement is fundamentally wrong (uncertainty principle), and the question becomes how wrong, and can the wrong be repeated with some certainty!!

WRT to this thread, this shows the DSO Display is somewhat distorted from what the DUT actually sees, and one can't simply infer that the DSO display represents what's actually taking place at the DUT, even at modest frequencies well within the DSO and Probe bandwidths. Also shows how much the DSO Probe can influence the DUT behavior without regard to the actual DSO connected, as we could completely disconnect the DSO end of the probe with almost no effect on the DUT result as shown by the SA (DUT in this case). Most are lead to believe the DSO Display represents what the DSO and Probe produce without much regard to what the Probe does to the DUT behavior!!

We would like to repeat this same test with same equipment and same (crappy) cables and adapters with an Active Probe and compare, if we had one. Have a pretty good idea what the results will show from our prior career, tho

Happy Holidays,

Best

[mawyatt]

The above technique also works utilizing the 50Ω Input of the SDS2000X+ to replace the SA. Here the DUT becomes the SDS2000X+ DSO Input set to 50Ω.

Everything else same as described above and with a PP510 DSO Probe. The SDS2000X+ FFT was setup same as SDS824X HD.

PNG1 is the SDS2000X+ FFT without the DSO Probe connected.

PNG2 shows the FFT with DSO Probe connected.

PNG3 attempts to show on the SDS2000X+ FFT the result with and without the Probe connected by changing the AWG sweeping rates.

PNG118 is FFT from SDS824 where the Probe is attached.