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New Siglent SP6150A - Passive probe 1.5 Ghz

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tautech:

--- Quote from: 44kgk1lkf6u on October 13, 2024, 09:18:21 pm ---The 1.8 pF capacitance seems a little large.  The P6056 and P6156 Z₀ probes from Tektronix have 1 pF input capacitance and are rated for 3.5 GHz.

--- End quote ---
Yeah well I wasn't splashing out on the 350 fempto Farad SAP5000D or SAP4000P Power Rail Probe just yet..... so the $250 SP6150A for tests against SAP1000 isn't outta the play budget.

nctnico:

--- Quote from: folays on October 13, 2024, 10:04:52 pm ---Please pardon my amateurism, but at 1 Ghz and 1 pF, things I may have learned recently (incorrectly ?) point me to the fact that the 1pF probe tip would have an « capacitive reactance » loading the circuit at 159 Ohm ?

What would change the 1k Ohm vs the 500 Ohm, if the probe would anyway be at ~159 Ohm ?

--- End quote ---
In most cases the capacitance will affect the edges but not the overall amplitude as the frequency content of the signal could be less than 1 GHz so there is less chance of bringing a signal below logic threshold levels (for example).

44kgk1lkf6u:

--- Quote from: KungFuJosh on October 13, 2024, 10:03:08 pm ---I don't generally do any HF probing, but I thought the consensus was that over 300MHz or so that active probes were preferred?

--- End quote ---

Z₀ probes are much cheaper than active probes with similar high frequency performance.  They are also easier to connect to things like spectrum analyzers and oscilloscopes of another brand.  Dr. Johnson lists some benefits here:

https://www.signalintegrity.com/Pubs/straight/probes.htm

https://www.signalintegrity.com/Pubs/news/5_4.htm


--- Quote from: folays on October 13, 2024, 10:04:52 pm ---What would change the 1k Ohm vs the 500 Ohm, if the probe would anyway be at ~159 Ohm ?

Unless I may have missunderstood and this Ohmic capacitive reactance would not be as impactful and should be somehow regarded as also having a 1:10 or 1:20 more Ohm loading impact ?

--- End quote ---

Some circuits are more sensitive to DC loading than AC.  For example, the receiver side of AC-coupled protocols such as PCIE.  I recently wanted to look at the waveform at the base of a transistor wired as a Colpitts oscillator.  A 500 Ω probe lowered the base to emitter voltage enough to stop the circuit from working.  I can't recall a situation when changing from 500 Ω to 1 kΩ made a difference though.  I ended up using an active probe on the oscillator.

Some of the input capacitance is there across the input resistor to help with the signal attenuated by the resistance in the cable.  Theoretically a higher attenuation requires less input capacitance.  But I don't know if this is actually the case.

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