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| making an 20:1 coax probe |
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| tggzzz:
--- Quote from: imo on July 11, 2022, 10:35:42 am ---The internal wire in the oscope probe's coax isn't made of copper, but it is a higher resistance wire usually, something like 200ohm, afaik.. --- End quote --- In a standard "high" impedance *10 probe, yes. Not in the HP10020, which is a Z0 probe. There the coax's conductor is <1ohm. |
| G0HZU:
The Tek 485 is an interesting example because I think this scope switches in a completely different signal path when the internal 50 ohm termination is selected. There is a dedicated signal path that is optimised for 50 ohms and so this will show a resistive input without the shunt capacitance. I think this partly explains the big difference in the two scope plots by tggzzz. When the external termination is fitted in 1M mode the scope will have 20pF input capacitance and this explains why the blip is so prominent with this arrangement. I tried a similar test on my old Tek 465 scope last night using an external 50 ohm BNC termination and this showed the blip in the expected place. Sadly, the old 465 scope doesn't have an internal 50 ohm termination switch. |
| G0HZU:
--- Quote from: David Hess on July 11, 2022, 03:33:11 am --- --- Quote from: G0HZU on July 10, 2022, 08:59:40 pm --- --- Quote ---My fastest oscilloscope with an internal switched termination is only 300 MHz and I have never seen any extra aberration from using a 50 ohm source or probe interacting with the input capacitance. There is something there, but a fast high impedance probe shows the same thing. --- End quote --- A typical shunt 15-20pF capacitance from a scope input will cause quite a discontinuity up at VHF. At the tip end of the cable is a 950 ohm resistor so the cable is therefore mismatched at both ends up at VHF. This means the initial positive edge of the pulse hits a shunt capacitance at the scope. This is a lower impedance than 50R so there will be an inverted reflection that lasts only a brief amount of time as the capacitance charges up. This inverted blip will travel back up the cable where it hits the 950R resistor. This huge mismatch causes nearly all of the blip to reflect back but this time it will stay inverted as 950R resistance is much higher than 50R so there is no waveform inversion with this reflection. If the cable has a delay of 5ns then after 10ns from the rising edge of the pulse the blip will arrive back at the scope input where it will put a little dent in the top edge of the waveform about 10ns after the rising edge seen on the scope just as in the spice simulations. This all assumes the pulse has a fairly fast rise-time. I'd expect a rise time of 2-3ns to cause visible artefacts on a typical 200MHz scope. The Pico scope only has 13pF input capacitance so maybe it won't be as affected as some other scopes that can have 15-20pF input capacitance. --- End quote --- Has anybody actually observed that with a real low-z probe when a feedthrough termination is used? I thought Tektronix might have made a separate set of low-z probes for their early or late oscilloscopes that used feedthrough termination that include the t-coil termination to prevent the problem, but I have not been able to find them. The only low-z probes in that category are the ones I mentioned which work with a 1 megohm input and have the termination built in. I know those show no such glitch. From what I remember, the t-coil termination sort of transforms the capacitive load into a resistive load. --- End quote --- That's interesting stuff, thanks! I'll have a play this evening with that t-coil circuit. |
| tggzzz:
--- Quote from: G0HZU on July 11, 2022, 11:12:42 am ---The Tek 485 is an interesting example because I think this scope switches in a completely different signal path when the internal 50 ohm termination is selected. There is a dedicated signal path that is optimised for 50 ohms and so this will show a resistive input without the shunt capacitance. --- End quote --- That's why I chose it to display the difference :) There are indeed two signal paths with two attenutors, one 50ohm and one 1Mohm, selected by an RF relay at the front end that pops out if you overload the 50ohm path. (A greatly underrated feature!) N.B. it isn't optimised for 50ohm, it is 50ohm :) --- Quote ---I think this partly explains the big difference in the two scope plots by tggzzz. When the external termination is fitted in 1M mode the scope will have 20pF input capacitance and this explains why the blip is so prominent with this arrangement. I tried a similar test on my old Tek 465 scope last night using an external 50 ohm BNC termination and this showed the blip in the expected place. Sadly, the old 465 scope doesn't have an internal 50 ohm termination switch. --- End quote --- With most scopes, including that majority with an internal 50ohm resistor banged in parallel with the 1Mohm input, the best that can be hoped for is to use an attenuator to reduce the effect of the 15pF+1m cable. Lose gain, gain fidelity; that's always the story at RF. But hey, gain is cheap nowadays :) |
| bson:
--- Quote from: sicco on July 10, 2022, 02:58:44 pm ---If your scope does not have a 50 ohms input option (selected) then you do need to add something like this on the scope BNC connector: https://www.distrelec.nl/en/feed-through-termination-50-ohm-2w-rohde-schwarz-hz22/p/11085041?ext_cid=shgooaqnlnl-p-shopping-fallback&gclsrc=aw.ds&?pi=11085041&gclid=EAIaIQobChMIiK7X08Tu-AIVy513Ch2y8wCQEAQYASABEgJO2_D_BwE A 50 ohms coax cable connected to just a 1 Mohm scope input is a pure reflection point for sharp fast pulses. The coax cable is a transmission line, a waveguide. Waveguides shall be terminated, on both ends, with a resistor that is as many ohms as the waveguide’s characteristic impedance. 50 ohms that is for the coax you’re considering to use. For a Z0 probe, one can omit the 50 ohms resistor in the probe because there is no need to cancel echoes from what might travel as pulse/edge in the cable back from scope to probe. But then we must first guarantee that nothing could have bounced at the scope BNC input. The 50 ohms scope input resistor does give that guarantee. If it is there… The spice models posted just above confirm it all… --- End quote --- But the thing is to increase its impedance as a pure 50Ω Z0 probe is practically useless; there's almost nothing you can observe with it. Put it on an XO µC pin and the oscillator stops. On a 100Ω LVDS signal, and the transceiver faults. On a device D+/D- USB pin, and the host wants to reset and reenumerate, and likely completely quarantines unless you're very quick to remove the probe. On an ethernet RMII interface and it goes belly-up. There are such extremely limited uses for it that its existence is mostly an academical novelty. |
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