Products > Test Equipment
> 1 GHz DIY differential probes
nctnico:
--- Quote from: 0xdeadbeef on October 01, 2020, 07:31:50 pm ---A typical active probe has like 0.5pF capacitance, so anything you attempt to attach it somewhere will actually add a capacitance in the same magnitude or higher. I don't see how a purely resistive probe would help there. Plus, its low resistance makes it problematic for digital outputs which might as well create a burst clock only now and then.
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Well, you should try the low-Z passive probes. These have very low capacitances and high bandwidth. I have a bunch of Tektronix P6156 probes ( http://w140.com/tekwiki/wiki/P6156 ) and these work very well (3.5GHz bandwidth with less than 1pf of capacitive loading!) but you'll need to use the 1:20 attenuator (or higher) to get an input resistance of at least 1k Ohm otherwise digital signals like LVDS will be loaded too much.
tggzzz:
--- Quote from: 0xdeadbeef on October 01, 2020, 07:31:50 pm ---
--- Quote from: tggzzz on October 01, 2020, 04:04:59 pm ---I expect you mean a 40Mb/s LVDS signal. The maximum frequency will be much higher, dependent solely on the transition time.
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Well, one of the LVDFS channels is a 40MHz clock. So since when is frequency only a valid unit for sine waves? Of course the sharp edges have much higher frequency components. No need to mention that in this board, do we?
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Yes, it is frequently (ho ho) necessary to mention it on this board - including in this case.
I mentioned the abbreviated version. For a longer version see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/
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--- Quote from: tggzzz on October 01, 2020, 04:04:59 pm ---It is unsurprising you see a distorted waveform, given that a *10 "10Mohm" impedance probe has a (capacitive) input impedance of <100ohms at the frequencies you are likely to see on an LVDS signal.
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For a 11pF probe, a 400MHz component will "see" a resistance of ~36Ohm, while the DC components will "see" 10MOhm. How is it surprising that this distorts the waveform?
Anyway, have you ever actually tried to measure a 40MHz square wave with a passive probe?
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Er, I noted distortion is unsurprising. It appears that you had not considered the way in which it will affect the waveform.
I have frequently (ho ho) measured such waveforms with passive probes over the decades.
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--- Quote from: tggzzz on October 01, 2020, 04:04:59 pm ---A pair of resistive divider Z0 probes would be a better bet, since they have a much lower capacitance and a well defined input resistance of 500ohms (*10) or 1000ohms (*20). They are easy to make at home.
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A typical active probe has like 0.5pF capacitance, so anything you attempt to attach it somewhere will actually add a capacitance in the same magnitude or higher. I don't see how a purely resistive probe would help there. Plus, its low resistance makes it problematic for digital outputs which might as well create a burst clock only now and then.
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My resistive divider probes are "<0.7pF", so yes it does help considerably compared with the typical *10 passive probe you used[1].
This is nothing new; I have an "operating note" dated 1972 for my hp10020a resistive divider probes.
[1] Here's the context you chose to snip...
--- Quote from: 0xdeadbeef on October 01, 2020, 02:03:02 pm ---...Compared to the default 500MHz passive LeCroy probes, the difference is like night and day. I.e. with the passive probes, the signal looks like a distorted sine and with the active probes, it's a nearly perfectly rectangular signal....
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0xdeadbeef:
Yeah, I really don't know if this brought up anything worthwhile. I can't day it did for me. Hope you feel better at least.
Mechatrommer:
--- Quote from: tggzzz on October 01, 2020, 11:56:29 pm ---My resistive divider probes are "<0.7pF", so yes it does help considerably compared with the typical *10 passive probe you used[1].
This is nothing new; I have an "operating note" dated 1972 for my hp10020a resistive divider probes.
[1] Here's the context you chose to snip...
--- Quote from: 0xdeadbeef on October 01, 2020, 02:03:02 pm ---...Compared to the default 500MHz passive LeCroy probes, the difference is like night and day. I.e. with the passive probes, the signal looks like a distorted sine and with the active probes, it's a nearly perfectly rectangular signal....
--- End quote ---
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while Z0 probe is the cheapest way, it doesnt help with high impedance nodes such as 1Kohm lines (regardless of frequencies low or high)... most logics will simply follow the distorted clock fine, but some other circuit will be screwed, maybe like comparator or serial comm/timing that needs strict synching... ymmv.
since at it, here is a 50ohm 40MHz clock line loading effect i probed with my diy 1/10X passive probe vs Rigol bundled probe (DS1054Z grade BW) last night just out of curiosity. 1st and 2nd picture is for reference only (unloaded signal on 1GSa/s 1 channel activated only vs 500MSa/s both channel activated) granted most of high frequency content of my 6GHz function (clock) Gen already absorbed by DSO capacitance and phase distortion (delay or synch) is not so obvious from this snapshots. anyway, for 1Kohm nodes such as usually found in mcu output, Z0 probe will not be suitable however slow the clock is, this is where active probe is more of a general purpose probe ie we dont need to keep track of using different probes...
tggzzz:
--- Quote from: 0xdeadbeef on October 02, 2020, 12:27:55 am ---Yeah, I really don't know if this brought up anything worthwhile. I can't day it did for me. Hope you feel better at least.
--- End quote ---
There's an English proverb: "you can lead a horse to water, but you can't make it drink".
Perhaps it will avoid other readers being mislead.
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