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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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25th Jul 2017, 11:51 pm | #41 | |
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Re: Home-brew 100:1 'scope probe.
Quote:
Phew! In the end, you and I have a different 'take' on this. As I said, I think we'll simply have to agree to disagree. The validity - or otherwise - of the assumption the author has made is a matter of opinion, not scientific, provable fact. Al. |
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26th Jul 2017, 12:01 am | #42 |
Dekatron
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Location: Cheltenham, Gloucestershire, UK.
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Re: Home-brew 100:1 'scope probe.
Just to give my 2p.... I've read quite a few articles by Douglas Smith over the years and they usually contain useful info. I found some of his old articles about E and H field probes to be very informative.
However, his 'free' articles are often a bit sketchy in places and test results tend to be described verbally with minimal pictures or plots. I think he earns income via seminars and books so there will be a limit to how much free info you can expect to get on probes etc in the free online stuff. In the case of the 40:1 probe article there isn't a proper circuit or system diagram or a typical response curve (showing under/over compensated curves as well?). Even the probe looks rushed and fragile. I'd expect the 1206 tip resistor to snap fairly often and the 200R resistors may crack and become erratic over time. Even though I know it is supposed to be used with 50R test gear I do think this should be made clear in the introduction or at least in a decent system drawing including the (50R) scope or analyser. The introduction should ideally have given the ratio of the probe and given some indication of the impedance it presents. But I guess the people likely to build it will often know this stuff anyway so no big deal. But the article looks incomplete to me without these simple and basic bits of info at the beginning. You have to dig down to the construction steps to find out the ratio of the probe for example. This should ideally be stated in the introduction. I'm not sure I'd recommend using it to measure 50V rms because the 1206 tip resistor will get quite toasty even in a 100:1 format. But I guess it depends on the application and how long you connect the probe for...
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Regards, Jeremy G0HZU Last edited by G0HZU_JMR; 26th Jul 2017 at 12:09 am. |
26th Jul 2017, 12:13 am | #43 |
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Re: Home-brew 100:1 'scope probe.
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26th Jul 2017, 1:45 am | #44 |
Heptode
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Location: Heysham, Lancashire, UK.
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Re: Home-brew 100:1 'scope probe.
Making high impedance probes with good bandwidth and flat frequency response - necessary for accurate capture of fast digital signals, is very difficult. Hence the high price for good probes from the likes of Tektronix. Getting the signal into the probe with good signal integrity is also difficult - you can't just use the 6 inch earth lead that comes with cheap probes - good probes can be fitted with different length earth leads, an earth clip, or even be plugged into test sockets, depending of what it's being used for.
Low impedance probes can be made with good bandwidth much more easily, and are more tolerant of less than perfect earth connections. You don't need the 50 ohm termination at the tip end of the cable, but you do need one at the scope (it wasn't made clear in the article discussed here). For example, the Tektronix P6035 and P6034 probes are 100x and 10x probes with 5k and 500 ohm input impedance respectively, so don't seem to have the 50 ohm at the tip end of the cable. These probes are designed for high bandwidth and good signal fidelity, and sacrifice input impedance and the need to crank up the gain in the scope. Without the 50 ohm at the tip end, low impedance probes are quite easy to make, all you really need is a resistor at the end of some 50 ohm coax - and a terminator at the scope end. How high the resistor can go depends on the amplitude of the signal you're trying to look at, the loading the circuit can tolerate, and how sensitive your scope is. Stuart |
26th Jul 2017, 2:12 am | #45 |
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Re: Home-brew 100:1 'scope probe.
Here at home and at work I've made several passive low impedance probes and these are mainly for use with a spectrum analyser. I just use a series resistor and a tiny compensating cap for the higher ratio probes and I use a coaxial DC block at the analyser to strip away any DC. The other gotcha with the 40:1 probe in the article is that it presents a 1000 ohm DC path to ground so could easily mess up the biasing of active circuits when they are probed. Again, the article doesn't warn the reader about this limitation.
The Pease active probe is quite elegant but I think it will generate a negative resistance up towards VHF so could give false readings depending on the impedance of the circuit under test. It could add energy and make VHF readings look bigger than they really are for example. It could probably also go completely unstable and oscillate up at VHF if the right passive load was presented to it. Also, how is it meant to be interfaced to any test gear and what is the input impedance of that test gear? It would also not be able to cope with 50V rms at the input but a passive divider could be fitted to the input I suppose.
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26th Jul 2017, 9:10 am | #46 | |
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Re: Home-brew 100:1 'scope probe.
Quote:
Craig Last edited by Craig Sawyers; 26th Jul 2017 at 9:17 am. |
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26th Jul 2017, 9:44 am | #47 | |
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Re: Home-brew 100:1 'scope probe.
Quote:
I'm going to be honest though and I think poking around high frequency stuff is nasty with a scope. Too many side effects. Would rather approach the problem in the frequency domain if possible and sneak a known amount of power out somewhere. |
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26th Jul 2017, 10:02 am | #48 | |
Dekatron
Join Date: Nov 2010
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Re: Home-brew 100:1 'scope probe.
Quote:
The P6026 had: 7 attenuator heads 5x, 10x 20x 50x, 100x, 200x, 500x 2 loads to plug each of those into, both 50 ohms, one DC coupled and one AC coupled 1 adaptor from the load to a GR874 connector 1 10ns GR 874 cable 1 low inductance ground clip. It was an eye-popping $140 in 1963 - about $3000 now. The basic configuration of a series resistor feeding into a 50 ohm load, then a 50 ohm cable terminated in a 50 ohm oscilloscope load is exactly the same configuration as http://emcesd.com/1ghzprob.htm It will probably come as no surprise that I have one of these pieces of Tek history. Craig |
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26th Jul 2017, 11:18 am | #49 |
Dekatron
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Location: Croydon, Surrey, UK.
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Re: Home-brew 100:1 'scope probe.
We use Hantek T3100 100:1 probes in our lab. These are available on eBay for not too much money (or our lab manager wouldn't have bought them....) and free postage. We use 100:1 probes for certain discharge measurements where minimal influence of the scope probe is required.
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26th Jul 2017, 10:56 pm | #50 |
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Re: Home-brew 100:1 'scope probe.
I see that his finished probe - which is very short in length - has a BNC female attached. Presumably, a coax cable will be required to connect that probe to a spectrum analyzer. So what would be a suitable Zo for that cable? 50Ω ? And what about its length-dependent capacitance?
Al. |
26th Jul 2017, 11:48 pm | #51 |
Heptode
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
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Re: Home-brew 100:1 'scope probe.
Low impedance probes designed to drive a 50 ohm load at the scope or spectrum analyser end can have any length of 50 ohm coax between the probe and the scope, it's just a terminated transmission line. Of course, any coax is lossy, pasticularly at higher frequencies, but a couple of metres of good quality coax should not be a problem.
Stuart |
27th Jul 2017, 11:03 am | #52 |
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Re: Home-brew 100:1 'scope probe.
Thanks Stuart: your remarks confirm my thinking about this. Just wanted to check with other members.
Al. |