New Siglent SP6150A - Passive probe 1.5 Ghz - Page 1
Forum section: electronics / testgear

author: folays
author_rank: Contributor
timestamp: October 11, 2024, 01:47:59 pm
content: https://siglentna.com/product/sp6150a-1-5-ghz-oscilloscope-probe/QuoteDatasheet :https://siglentna.com/wp-content/uploads/dlm_uploads/2024/09/SP6150A_Datasheet_EN01A.pdfQuoteAlso viewable in the "all probes list" athttps://siglentna.com/products/accessories/probes/passive-probes/

author: folays
author_rank: Contributor
timestamp: October 11, 2024, 02:29:17 pm
content: So I am an amateur, can you please confirm my (very possibly wrong )understanding ?The SP6150A cannotAT ALL"replace" a SP3150A probe on a SDS3xxx HDThey are both very different probes, very different tools, aimed at very different tasksProbes others than the SP5150A are generally intended to be used with the 1M Ohm inputthe 50 Ohm scope input is generally only useful if you want to match a coax of 50 Ohm impedance, to avoid reflections ?The scope 50 Ohm input can only dissipates 0.5 Watt, so that's why U = sqrt(P * R) == 5 V,the 50 Ohm input can only accept 5VEven transient spike above 5V can damage the 50 Ohm input, so if you were to use it, a 1:10 probe is a good ideaThe SP6150A is rated up to only 8.5V anyway, so if you were to be respecting this limit, it will present only up to 850mV on the 50 Ohm scope inputThe 500 ± 10 Ohm "input resistance (Probe + scope)" means that the probe has an internal 450 ± 10 Ohm resistance ?And now for theunknown(for me) :Why the probe is listed as having an 1.8pF "input capacitance" whereas scopes have around'ish 15 pF ?Can the SP6150A probe ever be used on a SDS3xxxx HD ?Why does it seems to be using the 50 Ohm input ? Just because the output of the probe should not exceed 850 mV, well below the 5V, and that the 50 Ohm input has a more direct path to the ADC ?Thanks & Best Regards,

author: bob1033
author_rank: Contributor
timestamp: October 11, 2024, 02:58:11 pm
content: Looks like what I would call a Z0 probe, you can look around the forum for various discussions about these. I think most of your understanding is correct.This probe is most likely just a low capacitance/inductance 450 ohm resistor connected to 50 ohm coax. The simple resistive tip keeps the tip capacitance (1.8pF) low which helps keep the frequency response flat til 1.5 GHzThis probe needs to be used with 50 ohm termination only and to get the rated response the termination must occur inside the scope (AKA 50 ohm mode on the scope)Since you use 50 ohm termination with this probe, yes, you must limit the input signal since the scope is only rated 5V max on 50 ohmSince the probe is a resistor and a transmission line, we don't need to worry about the scope's input capacitance (15pF) since we are providing a good impedance match using the scope's 50 ohm terminationYou should be able to use this probe no problem on the SDS3xxx HD. I use similar homemade probes on my SDS2000X+. Just be sure to use the 50 ohm termination mode in the channel setting and do not exceed the maximum voltage rating.

author: noisyee
author_rank: Contributor
timestamp: October 12, 2024, 09:12:42 am
content: Quote from: folays on October 11, 2024, 02:29:17 pmYes you are right, this is a very different probe than normal passive probe and aims at very different use conditions. They are a kind of trade off between bandwidth and probe load (more bandwidth than normal passive probe and less load than direct feed signal into 50Ω plus much much cheaper than active probes).Transient spike with not too much over voltage would not cause damage to most oscilloscopes. But how should you know how much is too much for a particular oscilloscope. Better to follow the warring sticker.Quote from: folays on October 11, 2024, 02:29:17 pmOscilloscope input capacitance specified at 1MΩ input, no meaning for 50Ω.Unlike passive high impedance probes, transmission line probes can be used on almost any instrument with 50Ω input, even a Spectrum Analyzer.They are designed to be used with matched 50Ω instruments. They have small low value resistor attenuator and matching network on the probe tip, which limited their input voltage.

author: tautech
author_rank: Super Contributor
timestamp: October 12, 2024, 12:26:13 pm
content: Critical info for SP6150A is on P8 of the datasheet captured below.Put simply, these are a low voltage autosense 1.5 GHz Z0 10:1 probe for 50 Ohm input use.SP6150A is a much cheaper option than an active probe when low probe tip loading is required.I'll have some in a few weeks for tests.

author: 44kgk1lkf6u
author_rank: Regular Contributor
timestamp: October 13, 2024, 09:18:21 pm
content: 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.

author: nctnico
author_rank: Super Contributor
timestamp: October 13, 2024, 09:53:48 pm
content: Quote from: 44kgk1lkf6u on October 13, 2024, 09:18:21 pmI agree. And in my experience having a 500 Ohm input resistance is too low to maintain the amplitude (as in not attenuation/loading too much) of the signal you are measuring. Better use a 1:20 probe with a 1k Ohm input resistance. I love the P6156 probes I have.

author: KungFuJosh
author_rank: Super Contributor
timestamp: October 13, 2024, 10:03:08 pm
content: I don't generally do any HF probing, but I thought the consensus was that over 300MHz or so that active probes were preferred?

author: folays
author_rank: Contributor
timestamp: October 13, 2024, 10:04:52 pm
content: 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 ?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 ?

author: nctnico
author_rank: Super Contributor
timestamp: October 13, 2024, 10:24:05 pm
content: Quote from: KungFuJosh on October 13, 2024, 10:03:08 pmNot really. If the signal has a fairly low source impedance (which most high speed signals have), you can load it with 1 K Ohm without much effect. The low-Z probes are passive probes which divide the signal. The upside is that these probes are relatively simpel and deliver high bandwidths. The downside is attenuation. An active probe typically doesn't have the attenuation and generally has a much higher input resistance but at high frequencies the input capacitance is dominating the input impedance anyway.

author: tautech
author_rank: Super Contributor
timestamp: October 13, 2024, 10:26:00 pm
content: Quote from: 44kgk1lkf6u on October 13, 2024, 09:18:21 pmYeah 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.

author: nctnico
author_rank: Super Contributor
timestamp: October 13, 2024, 10:26:26 pm
content: Quote from: folays on October 13, 2024, 10:04:52 pmIn 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).

author: 44kgk1lkf6u
author_rank: Regular Contributor
timestamp: October 13, 2024, 10:43:17 pm
content: Quote from: KungFuJosh on October 13, 2024, 10:03:08 pmZ₀ 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.htmhttps://www.signalintegrity.com/Pubs/news/5_4.htmQuote from: folays on October 13, 2024, 10:04:52 pmSome 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.