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What use for a 640 Ohm 1x Probe?
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I recently obtained a batch of old and diverse probes. Many are junk but a few are rather interesting. Out of a number of old 1x Tek probes I should be able to put together 2 nice ones (as soon as a get a hex wrench small enough).
Another I have studied is an old Fairchild that is in good condition that has a resistance of 640 ohms end to end on the center conductor. To what use could one put to such a probe? |
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Alex
640 ohms ought to be enough for anyone.
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--- In TekScopes@..., "Philip" <ndpmcintosh@...> wrote:
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Peter Gottlieb
Be careful with that!
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Danger, 640 Ohms! On 2/18/2013 5:50 PM, Alex wrote:
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Don Black
开云体育I think you are measuring the inner conductor resistance. This is made of resistance wire (often wound in a spiral) to damp reflections back the cable. The cable isn't terminated (the input impedance of the scope is 1 Meg) and like any transmission line energy is reflected where it isn't absorbed by matching termination. This is a compromise to damp such reflections, useful at low frequencies.Tektronix explains their techniques in their concept book series, the one you want is called 'Oscilloscope Probe Circuits". It's available to download on the Internet and is an excellent guide to probe design. Silicon Chip also has a good article on probes a while back that I think explained it. Don Black. ? |
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Yeah. It doesn't have any markings on it other than the name "Fairchild" on the cable, and I am glad I checked it before I actually tried to test it.
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In looking at an old but classic oscilloscope book it mentions a "direct" probe that has essentially 0 resistance used for checking low impedance, low frequency circuits. And, it also describes the "isolation" probe which has an R of 4.7 to 10kOhms. It might be something intended to serve as one of these. It looks like it is from the early 60's and perhaps was used on the Fairchild 766H. I doubt there would be much current use for this probe.
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I agree with what Don posted. The 640 ohms is just the resistance of
the inner conductor of the coaxial cable for a x1 oscilloscope probe. I measured about 250 ohms on a x1 probe I happen to have in reach. On Tue, 19 Feb 2013 04:55:41 -0000, "Philip" <ndpmcintosh@...> wrote: Yeah. It doesn't have any markings on it other than the name "Fairchild" on the cable, and I am glad I checked it before I actually tried to test it. |
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Mark Wendt (Contractor)
So spake Mr Alex Gates... ;-)
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Mark On 2/18/2013 5:50 PM, Alex wrote:
640 ohms ought to be enough for anyone. |
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On Wed, 20 Feb 2013 00:24:09 -0000, "Philip" <ndpmcintosh@...>
wrote: The publication on scope probes mentioned earlier is good and I am working my way through it. I already had it in my document collection and it was on my reading list.I get almost exactly 9.00 MOhms on each of several different x10 probes within reach. |
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Don Black
开云体育It should be 9 Meg ohms. Then 90% of the signal is dropped across the probes 9 Meg and 10% across the scope's 1 Meg input impedance, giving 10:1 ratio.The compensating capacitors across them are adjusted for the same division at high frequencies to maintain the flat response, that's hat you're setting when you adjust for flat square wave with the trimmer. Don Black. On 20-Feb-13 12:18 PM, David wrote: ? |
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开云体育So, I've had the idea of building a 50
ohm fixed 10x attenuator to use inline with a 50 ohm cable. What
kind of impedance matching should I use for the 1meg ohm on the
scope?
It should be 9 Meg ohms. Then 90% of the signal is dropped across the probes 9 Meg and 10% across the scope's 1 Meg input impedance, giving 10:1 ratio. |
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Bob Albert
--- On Tue, 2/19/13, Cliff White wrote:
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Don Black
开云体育The usual way to build such a probe is simple and excellent for measuring very fast sub-nanosecond pulses. It consists of a length of 50 ohm coax that's terminated in 50 ohms at the scope, either with a 50 ohm input on the scope (usual on high bandwidth scopes) or with a separate? termination. At the input end a small, low inductance resistor is soldered to the center conductor and the shield twisted into a small tail for the ground connection. The lead length is kept as small as possible and the leads often soldered directly to the test points. With a 450 ohm input resistor the probe divides by ten, a 4950 ohm resistor divides by 100. The input resistance is 500 and 5000 ohm respectively. This seems very small, however at high frequencies any capacitive loading is severe, the reactance of 1 pf at 1 GHz is approx. only 160 ohms. These probes are quick and cheap and the short leads are essential for reliable signal coupling; an inch of lead has significant inductance at high frequencies. Even the most professional engineers with access to the latest equipment make them up since they are so handy. Incidentally, the multi thousand dollar many GHz probes are often soldered directly into the test points to minimize lead inductance. I've attached the instructions for making a probe, they are often simpler if a commercial termination is used at the scope end and doesn't have to be built into the cable. The resistor can be tinier too. It's a bit of art to make them with a really flat response, suitable test equipment to check response is valuable, but if made carefully they are capable of useful performance as is. There was a link to a you-tube discussion forum a few weeks back on probes that was very good, if anyone has the link to it, it's worth watching. Tektronix sold some coax probes like this, I don't know if they still do so. I have a 100:1 probe that has a switchable 50 ohm termination at the scope end. For really high frequencies the simple minimalist home made ones are probably better since the lead length is small. The ground lead in particular is important and is often a short spike to touch a ground point.On the other hand, if you want to look at mid frequencies, use a standard high impedance 10:1 probe. The low impedance of a coax cable will load the circuit. An unterminated coax with clip leads is OK for audio frequencies but has too much capacitance for much higher than that. Don Black. Don Black. On 20-Feb-13 3:05 PM, Cliff White wrote: ? |
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I should have been more specific. Actually I get about 9.3 MOhms on the 10x probe I checked. I am learning a lot though by thinking about this probe, reading everyone's comments and reading the various probe manuals available. Thanks for everyone's input.
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I also ohmed out an old 1x Tek probe and got 300 Ohms. The center conductor on the cable is a tiny single strand of wire. Phil... --- In TekScopes@..., David <davidwhess@...> wrote:
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This probe (it says Fairchild Scientific Instruments 106 on the cable) is a good 1x probe. I bit the bullet and tested it on a scope compensation signal and it has a slight overshoot but a very flat top on the trace. I'll use it for checking ripple and the like. It appears to be very well made and has cushy handle so I rather like it.
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Phil... * --- In TekScopes@..., "Philip" <ndpmcintosh@...> wrote:
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David Hess
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