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25th Jul 2017, 10:22 am | #21 |
Heptode
Join Date: Dec 2014
Location: Lincoln, Lincolnshire, UK.
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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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25th Jul 2017, 11:35 am | #22 |
Dekatron
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
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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. |
25th Jul 2017, 11:45 am | #23 |
Dekatron
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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. |
25th Jul 2017, 12:09 pm | #24 |
Dekatron
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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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25th Jul 2017, 12:28 pm | #25 |
Dekatron
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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. |
25th Jul 2017, 3:53 pm | #26 |
Rest in Peace
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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. |
25th Jul 2017, 4:09 pm | #27 |
Dekatron
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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. |
25th Jul 2017, 4:56 pm | #28 |
Dekatron
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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... |
25th Jul 2017, 5:15 pm | #29 |
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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. |
25th Jul 2017, 5:27 pm | #30 |
Dekatron
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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... |
25th Jul 2017, 6:20 pm | #31 | |
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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:
Al. Last edited by Skywave; 25th Jul 2017 at 6:30 pm. |
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25th Jul 2017, 6:26 pm | #32 |
Dekatron
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Re: Home-brew 100:1 'scope probe.
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25th Jul 2017, 6:59 pm | #33 | |
Dekatron
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Re: Home-brew 100:1 'scope probe.
Quote:
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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25th Jul 2017, 7:05 pm | #34 | |
Dekatron
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Re: Home-brew 100:1 'scope probe.
Quote:
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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25th Jul 2017, 7:51 pm | #35 | |
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Re: Home-brew 100:1 'scope probe.
Quote:
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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25th Jul 2017, 8:44 pm | #36 | |
Dekatron
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Re: Home-brew 100:1 'scope probe.
Fair enough!
Quote:
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. 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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25th Jul 2017, 9:13 pm | #37 | |
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Re: Home-brew 100:1 'scope probe.
Quote:
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25th Jul 2017, 11:19 pm | #38 | |
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Re: Home-brew 100:1 'scope probe.
Quote:
So my thanks go to all who have borne with me in that persistence: it is much appreciated. Al. |
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25th Jul 2017, 11:30 pm | #39 | |
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Re: Home-brew 100:1 'scope probe.
Quote:
Al. |
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25th Jul 2017, 11:49 pm | #40 |
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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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