Products > Test Equipment
Probing at high speeds. Will the 10x probe overload the circuit?
joeqsmith:
--- Quote from: tggzzz on May 09, 2024, 07:50:50 am ---
--- Quote from: joeqsmith on May 09, 2024, 02:50:29 am ---
--- Quote from: tggzzz on May 08, 2024, 07:21:38 pm ---
--- Quote from: joeqsmith on May 08, 2024, 07:08:33 pm ---
--- Quote from: tggzzz on May 08, 2024, 07:00:14 pm ---What do you think "most certainly matters"?
--- End quote ---
Anything that effects the SI.
--- End quote ---
Precisely. SI is an analogue phenomenon not digital, and the same issues also occur in analogue circuits.
--- End quote ---
As I stated, it just provides context which I thought would have been obvious. Sure we can become pendant but it doesn't add anything to the discussion.
--- End quote ---
The key distinction between digital signals (0,1,etc) and analogue signals (volts, amps, frequency, etc) is not pedantry.
False distinctions between analogue waveforms and RF/microwave etc is not pedantry.
Failing to grok those is the source of much bafflement, many incorrect statements (especially w.r.t. sampling rate), and many problems seen all too often in circuits (especially SI).
Hence it is important, not mere pedantry.
--- End quote ---
When you consider details, digital is analog. I use the term digital to provide context about what we are probing. Again not 0's and 1's but loading, drive strengths, transition levels, overshooot.... As you previously posted:
--- Quote ---Strictly speaking it doesn't matter. If you are looking at a waveform with a scope you are looking at an analogue waveform that something will interpret as a digital signal.
--- End quote ---
Much like my own post so far, I suspect these comments are too high level and obvious. They add little to no value to the discussion.
tggzzz:
--- Quote from: joeqsmith on May 09, 2024, 01:01:41 pm ---
--- Quote from: tggzzz on May 09, 2024, 07:50:50 am ---
--- Quote from: joeqsmith on May 09, 2024, 02:50:29 am ---
--- Quote from: tggzzz on May 08, 2024, 07:21:38 pm ---
--- Quote from: joeqsmith on May 08, 2024, 07:08:33 pm ---
--- Quote from: tggzzz on May 08, 2024, 07:00:14 pm ---What do you think "most certainly matters"?
--- End quote ---
Anything that effects the SI.
--- End quote ---
Precisely. SI is an analogue phenomenon not digital, and the same issues also occur in analogue circuits.
--- End quote ---
As I stated, it just provides context which I thought would have been obvious. Sure we can become pendant but it doesn't add anything to the discussion.
--- End quote ---
The key distinction between digital signals (0,1,etc) and analogue signals (volts, amps, frequency, etc) is not pedantry.
False distinctions between analogue waveforms and RF/microwave etc is not pedantry.
Failing to grok those is the source of much bafflement, many incorrect statements (especially w.r.t. sampling rate), and many problems seen all too often in circuits (especially SI).
Hence it is important, not mere pedantry.
--- End quote ---
When you consider details, digital is analog. I use the term digital to provide context about what we are probing. Again not 0's and 1's but loading, drive strengths, transition levels, overshooot.... As you previously posted:
--- Quote ---Strictly speaking it doesn't matter. If you are looking at a waveform with a scope you are looking at an analogue waveform that something will interpret as a digital signal.
--- End quote ---
Much like my own post so far, I suspect these comments are too high level and obvious. They add little to no value to the discussion.
--- End quote ---
Lewis Carroll referred humourously to similar issues 170 years ago, via his Humpty Dumpty character :)
The drive strength at the far (rx) end of a 50/100/etc ohm transmission line is the same whatever is at the near (tx) end.
Digits are captured in digital tools such as logic analysers, or protocol analysers, or printf() statements. Scopes capture, ahem, analogues.
A key principal is to use an (analogue) scope to observe (analogue) signal integrity, then flip to using a digital tool to observe digital signals. But you know that.
If beginners were taught that and understood it, fewer questions would need to arise - because they sorted out many problems on their own. Their remaining questions would be more interesting and less repetitive :) That's why I think it is important.
jonpaul:
Suggest you read the Tektronix Circuits Concepts books:
Oscilloscope Probe Circuits
062-1146-00.pdf
Oscilloscope Probe measurements
https://w140.com/tekwiki/images/1/19/062-1120-00.pdf
also:
https://download.tek.com/document/51W_30013_0_MR_Letter.pdf
We use TEK P6136 Zo, P6137 10X and DIY Zo just a 50 Ohm coax and series small axial R 450 or 4950 Ohm at the tip.
Enjoy
Jon
nctnico:
--- Quote from: ballsystemlord on May 08, 2024, 05:05:38 am ---Hello,
So when I first learned about probing, I learned that you should use the 10x setting of the probe whenever possible so that you don't load down your circuit very much.
Recently, I came upon a video by Mr. Ganssle.
https://youtu.be/aJsJibDNg9M
He shows that a 10pF capacitor goes from over 300ohms of capacitive reactance to just 159ohms of capacitive reactance.
What happened to the 9Mohm resistor in the probe?
Like, it should still be a 10 to 1 input impedance, right?
Formula: sqrt((1/(PI×2×100 000 000×0.000 000 000 01))^2+(9 000 000^2))
He goes on to discuss using just a piece of coax and 1K resistor.
Wouldn't that need some sort of capacitive compensation?
--- End quote ---
Typically it does need a bit of compensation but it greatly depends on what kind of accuracy you are after. Most high speed signals you encounter nowadays are driven from a source impedance between 40 to 60 Ohm. Loading that with a resistor of 1k Ohm and some capacitance can easely lead up to a difference in amplitude of 10% between the original (no probe attachted) signal and with a low-Z probe attached. But still, a low-Z probe is very suitable for use on high-speed signals. I'm using these quite a lot myself.
This page has some info on how high speed low-Z probes (basically a piece of coax with a resistor attached to it) are constructed including a DIY example:
https://emcesd.com/1ghzprob.htm
joeqsmith:
When ballsystemlord titled Probing at high speeds, they never defined what they were attempting to probe. I provided what I considered the topic to be about based on their mention of resistive probes, thinking the OP would provide more details. I really have no idea what that were actually after and assumed after reading some of the posts, lost interest. A shame really.
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