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Old 25th Jul 2017, 10:22 am   #21
ionburn
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Default Re: Home-brew 100:1 'scope probe.

I am researching VTVM probes at the moment, and see good overlap as the functions end to be very similar.
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Old 25th Jul 2017, 11:35 am   #22
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Default Re: Home-brew 100:1 'scope probe.

Yes, the cable looks like 50 ohms. It's a transmission line. I visualise a transmission line as lots of little sections, each of which looks like 50 ohms. At the end of the line will be a 50 ohm termination, but to the signal travelling down the line, the terminator looks just like another short section of line.

My usual reference: https://www.youtube.com/watch?v=I9m2w4DgeVk

At the probe end, he has R2 set to 50 ohms to establish a 50 ohm source impedance for the sending end of the transmission line. But when sizing R1 to get his desired 40:1 ratio, we have to remember that the lower portion of the potential divider is indeed 25 ohms.

Or to put it another way, both ends of the cable are correctly terminated in 50 ohms, so in combination that's 25 ohms.

Last edited by mhennessy; 25th Jul 2017 at 11:45 am.
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Old 25th Jul 2017, 11:45 am   #23
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Default Re: Home-brew 100:1 'scope probe.

Can't find any mention of a 50 ohm termination at the 'scope end?

Lawrence.

Last edited by ms660; 25th Jul 2017 at 11:50 am.
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Old 25th Jul 2017, 12:09 pm   #24
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Default Re: Home-brew 100:1 'scope probe.

Would you expect put a DC to 1GHz signal down a cable with no termination at the end?
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Old 25th Jul 2017, 12:28 pm   #25
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Default Re: Home-brew 100:1 'scope probe.

The author didn't mention it (termination at the 'scope) and I was wondering whether that fact might have led to some confusion regarding Al's query.

Lawrence.
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Old 25th Jul 2017, 3:53 pm   #26
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Question Re: Home-brew 100:1 'scope probe.

Thanks for your comments, gentlemen.

Upon a re-read, it seems that I may have mis-interpreted what the author has written: I'm certainly confused. In his figure 1, he shows R2. This is his 50Ω termination (four 200Ω resistors in parallel). But the "25Ω" load resistor is not similarly shown. So - where is this "25Ω load" resistor?

Al.
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Old 25th Jul 2017, 4:09 pm   #27
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Default Re: Home-brew 100:1 'scope probe.

R2 in parallel with the impedance presented by the coax cable (not shown) is equal to 25 ohms is how I'm reading it.

Lawrence.
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Old 25th Jul 2017, 4:56 pm   #28
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Default Re: Home-brew 100:1 'scope probe.

Doesn't post #22 explain it fully?

It's 25 ohms because there is 50 ohms (the 4 200R resistors) in the probe, and 50 ohms at the end of the cable. The transmission line is happy because it sees Zo at each end, but for the purposes of potential division, 25 ohms is the net impedance.

As Lawrence says, the author didn't explicitly mention the 50 ohms at the end of the cable, but as I said in post #24, you'd surely automatically assume a correct cable termination in a system that promises DC to 1GHz coverage

He does mention using a spectrum analyser with a tracking generator to check the response of the probe. These are always terminated internally (usually 50 ohms, but I've seen 75 ohms in the past).

Hope that helps...
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Old 25th Jul 2017, 5:15 pm   #29
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Default Re: Home-brew 100:1 'scope probe.

O.K., please let me try again: it seems that everyone else here understands what he means, but I'm still struggling. Your patience is much appreciated.

I quote from his Theory of Operation para.:
. . . and a 25Ω load - the 50Ω termination in parallel with the 50Ω characteristic impedance of the coaxial cable.

Now it may just be me, but the way I read & comprehend that, he is saying that the coaxial cable, whose Zo is 50Ω, will automatically present - all by itself - a 50Ω load at the end of that cable, by virtue of the fact that the cable has a Zo of 50Ω!
Now we all know that that is nonsense - don't we? Hence my confusion.

But perhaps I blinked at the wrong moment: nowhere did I read any reference to the probe + cable being specifically connected to an 'external' 50Ω load - such as a spectrum analyzer - in order to produce that net "25Ω". Yes, he does refer to using a spectrum analyzer to check the performance of the finished probe, but that is not the point.

Al.
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Old 25th Jul 2017, 5:27 pm   #30
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Default Re: Home-brew 100:1 'scope probe.

He's right to say that the cable has a 50 ohm impedance, irrespective of the termination. It's root(L/C) - each little "slice" of the cable has some inductance and some capacitance, which together give 50 ohms.

The termination, which looks exactly like the previous "slices" to the signal - is purely there to stop the signal reflecting back to the start, which will constructively or destructively interfere with the source signal, depending on wavelength.

Obviously, if you consider it at purely DC, then the termination is essential. But as I say, you surely wouldn't omit it...
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Old 25th Jul 2017, 6:20 pm   #31
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Arrow Re: Home-brew 100:1 'scope probe.

Post #30: all of that I am well aware of.
However, I've just re-read your earlier post:

Quote:
Originally Posted by mhennessy View Post
The author didn't explicitly mention the 50 ohms at the end of the cable, but as I said in post #24, you'd surely automatically assume a correct cable termination in a system that promises DC to 1GHz coverage
"Automatically assume": well, maybe - and that would indeed explain the 25Ω. But I'm very sorry, but to my way of thinking, any article - especially one like this that goes in for such detailed construction, theory of operation, etc. - that makes such an assumption results in a very poor article indeed.

Al.

Last edited by Skywave; 25th Jul 2017 at 6:30 pm.
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Old 25th Jul 2017, 6:26 pm   #32
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Default Re: Home-brew 100:1 'scope probe.

http://audiosystemsgroup.com/TransLines-LowFreq.pdf

Lawrence.
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Old 25th Jul 2017, 6:59 pm   #33
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Default Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by Skywave View Post
any article - especially one like this that goes in for such detailed construction, theory of operation, etc. - that makes such an assumption results in a very poor article indeed.
I posted the link to the Z0 probe, and I'm pretty amazed that it has sparked such a heated debate. The Z0 probe is a whole lot easier to get your head around as compared with the potential divider probe, because the latter needs a very complicated compensation system to maximise the bandwidth.

The article I linked is not a recipe in the sense that it covers all aspects of connecting the probe to the scope, such as you would read in an electronics magazine. It assumes that the reader knows that a transmission line has to be terminated at each end by its characteristic impedance (or at the very least at one end),that the construction of the probe head is 50 ohms and you need 50 ohms at the other end of the cable. And it assumes that if you look at the DC resistance it is, and has to be, 25 ohms.

I also posted a link to the Tektronix Concepts book on probes; there is a whole chapter of that about Z0 probes, of which Tek made both x10 and x100 varieties.

Craig
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Old 25th Jul 2017, 7:05 pm   #34
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Default Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by Skywave View Post
But I'm very sorry, but to my way of thinking, any article - especially one like this that goes in for such detailed construction, theory of operation, etc. - that makes such an assumption results in a very poor article indeed.
As I said earlier, this is a system that's covering DC to 1GHz, so correct termination of the 50 ohm transmission line is essential. For that reason and more, I believe it's a fair assumption.

Sure, he could have worded things better, but technical writing is very difficult, and speaking as someone who does a lot of it, I'm prepared to give him the benefit of the doubt - especially as this project is aimed at folk who will be aware of transmission line effects.
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Old 25th Jul 2017, 7:51 pm   #35
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Arrow Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by mhennessy View Post
Sure, he could have worded things better, but technical writing is very difficult, and speaking as someone who does a lot of it, I'm prepared to give him the benefit of the doubt - especially as this project is aimed at folk who will be aware of transmission line effects.
We'll have to agree to differ, I'm afraid. I do not think that is reasonable to assume that anyone who makes and uses this probe will "be aware of transmission line effects." I built my first 'scope when I was about 14 years of age - and had no concept at all of "transmission line effects". Besides, how much effort would it have been for the author to be a teeny-weeny bit more explicit? He seems to put a lot of effort in what he has written.

And there is also the topic of what this probe will be used with. There is the embedded assumption that that will be a 50Ω input resistance spectrum analyzer (or something like that) - which will provide the necessary 50Ω of course. But if someone builds this and uses it with a 1MΩ / 30 pF input Z 'scope . . . . .

As for technical writing, quality of, I also did a lot of that in my last three employments before I retired. Then - and now when I do a write-up for myself about something I have designed and constructed, I always make sure that
(a) it is comprehensive and
(b) that there are silly errors in it.
And then when I've produced any piece of technical documentation, I leave it for a day or so, go back to it and then read it again: I usually find that I have made a few assumptions in that write-up, etc.

Which, of course, brings me back neatly to where I came in.

Al.
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Old 25th Jul 2017, 8:44 pm   #36
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Default Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by Skywave View Post
We'll have to agree to differ, I'm afraid.
Fair enough!


Quote:
Originally Posted by Skywave View Post
There is the embedded assumption that that will be a 50Ω input resistance spectrum analyzer (or something like that) - which will provide the necessary 50Ω of course. But if someone builds this and uses it with a 1MΩ / 30 pF input Z 'scope . . . . .
Conversely, what if someone tries to use a 10X or 100X 'scope probe with something other than 1M? For example, your Tek 2465 has a 50 ohm option. Spectrum analysers are usually 50 ohms, and frequency counters are usually 1M but often offer a 50 ohm option...

We make assumptions all the time. Just as well, as otherwise we'd be explaining Ohm's law in every post. Luckily, there is always context.


Quote:
Originally Posted by Skywave View Post
(b) that there are silly errors in it.
One thing I've learnt over the years is that it's impossible to proof-read my own work

All the best,

Mark
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Old 25th Jul 2017, 9:13 pm   #37
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Default Re: Home-brew 100:1 'scope probe.

Quote:
whose Zo is 50Ω, will automatically present - all by itself - a 50Ω load at the end of that cable
It presents a source with an impedance of 50 ohms (assuming the other end is also fed by 50 ohms) and needs a 50 ohm load to absorb all the energy. A 50 ohm cable isn't a 50 ohm resistor, it is merrily describes the best feed and load for it.
 
Old 25th Jul 2017, 11:19 pm   #38
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Arrow Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by Craig Sawyers View Post
I posted the link to the Z0 probe and I'm pretty amazed that it has sparked such a heated debate.
I'm glad that you did make that post: thank you. Why am I glad? Because when you have an understanding about something as fundamental to electronics as has featured here, but you find that your understanding - which you have held for so long and used it, successfully, in many applications - appears to be quite wrong, you are then faced with a dilemma. You can shrug your shoulders and walk away - convinced that you are right. Or you can challenge what has been said and ask others' opinions and help - which is what I have done here. For myself, I'd rather be proven wrong and thus enlightened rather than carry on in ignorance and with a mistaken belief. In the long term, with the former, I benefit; with the latter I don't. And that's why it appears to have become "heated": my persistence to clear my confusion with clear answers to my questions.

So my thanks go to all who have borne with me in that persistence: it is much appreciated.

Al.
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Old 25th Jul 2017, 11:30 pm   #39
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Arrow Re: Home-brew 100:1 'scope probe.

Quote:
Originally Posted by merlinmaxwell View Post
It presents a source with an impedance of 50 ohms (assuming the other end is also fed by 50 ohms) and needs a 50 ohm load to absorb all the energy. A 50 ohm cable isn't a 50 ohm resistor, it is merrily describes the best feed and load for it.
I'm not too sure if I understand all that, and I did not say that the cable presented a 50Ω load all by itself. What I did say was that the author's theory of operation implied that. However, do I know that you can have a source whose internal resistance is not the same as the Zo of a coaxial cable and that provided that cable is terminated in a resistance equal to Zo, no reflections will take place at the load. In a past employment, working with high power VHF / F.M. transmission equipment, that principal was successfully used extensively.

Al.
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Old 25th Jul 2017, 11:49 pm   #40
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Default Re: Home-brew 100:1 'scope probe.

Actually a 50 Ohm transmission line doespresent a 50 Ohm resistive impedance al on its own. But just only for a while.

Transmit a signal into the line from a matched signal source, and at the far end dump the signal into a 50 Ohm load. God is in his heaven and you have a fully-mtched system. The maximum power transfer theorem is in-play as well.

But wait a moment. There's this pesky matter of the speed of lght. It took time for the signal to reach the load, and for any reflection from the load to get back to the source. So what did the cable look like to the source in the meantime? It had to look like a 50 Ohm resistor to make sense. If the 50 Ohm line is terminated in a good 50 Ohms, the reflected wave amplitude is nil. There is no reflected wave to modify the apparent impedance presented to the source at all. So if it doesn't change, then the initial value must be the same as the final value : 50 Ohms.

We can play this game both ways. Feed the signal into the cable in a burst, and make the burst quicker than the time to traverse the line/ The source has stopped making the signal before the first of it reaches the end, so all that reaches the end was that stored in the movement along the cable. Out pops a voltage wave and a current wave in the ratio of 50 volts per amp... That's what went in! that's what comes out. THe load thinks it got fed from a 50 Ohm source impedance.

If the load is a mismatch and the signal is transmitted for a long while, at irst the cable looks like 50 Ohms to the source then late, after the reflection has made it back, the impedance shifts.

David
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