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| making an 20:1 coax probe |
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| G0HZU:
--- Quote ---The above may not apply to an oscilloscope at all because of how they are calibrated. The calibration source for transient response at the input connector is a fast reference edge. When the transient response calibration is done, the ripple should be removed by the opposite ripple in the response of the oscilloscope, but that does not account for reflections in the cable and who knows how long that is? At higher frequencies no cable is used, and at lower frequencies it should not matter if the pulse source has a reverse termination to absorb the reflection, and the oscilloscope may not be fast enough to see it anyway. At higher frequencies the probe itself does part of the transient response calibration for the oscilloscope, which is why Tektronix releases otherwise identical probe revisions for different oscilloscope models, or at least they used to. --- End quote --- I was really just describing how the frequency response of the scope can be affected by any mismatch at the scope end of the cable. There are equations for mismatch uncertainty that can show how much extra uncertainty (ripple) will be introduced if the cable is fairly long. If it helps anyone see the point I'm making I did a quick video showing my old 500MHz Infinium scope looking at the swept output of my VNA on ports 1 and 2. The video starts with the green trace and this is a 50R to 50R connection to the scope. This shows a small amount of droop up at 500MHz. The scope response isn't flat to 500MHz as expected but the response is smooth. The yellow trace shows a probe made with a series 1k resistor feeding about a 1m long RG174 cable that has a crimped BNC at the far end and I've selected the 1M input of the scope and fitted a commercial (unbranded) BNC through terminator at the scope input. There is a lot of ripple in the yellow trace and it's up to the individual to decide if this matters or not. Not everyone will be limited to a 200MHz scope so I'm just showing everyone what can happen with a 500MHz scope. This scope only has 8pF input capacitance and a 200MHz scope might have >15pF input capacitance. So the input VSWR of the 200MHz scope will be worse at 200MHz compared to the VSWR of the 500MHz scope at 200MHz. This is a really old scope so sorry for the quality of the display. |
| tggzzz:
--- Quote from: G0HZU on July 09, 2022, 11:58:54 am ---If it helps anyone see the point I'm making I did a quick video showing my old 500MHz Infinium scope looking at the swept output of my VNA on ports 1 and 2. The video starts with the green trace and this is a 50R to 50R connection to the scope. This shows a small amount of droop up at 500MHz. The scope response isn't flat to 500MHz as expected but the response is smooth. The yellow trace shows a probe made with a series 1k resistor feeding about a 1m long RG174 cable that has a crimped BNC at the far end and I've selected the 1M input of the scope and fitted a commercial (unbranded) BNC through terminator at the scope input. There is a lot of ripple in the yellow trace and it's up to the individual to decide if this matters or not. --- End quote --- If you had "Z0 cable"" -> 3dB pad -> 50ohm terminator -> 1M//xpF scope input, would the "ripple" be reduced as predicted by theory/simulation? |
| Bud:
Why do you want to crimp? Jyst buy a premade cable of a length you heed with connectors and cut one connector off. |
| G0HZU:
--- Quote from: tggzzz on July 09, 2022, 02:10:18 pm --- If you had "Z0 cable"" -> 3dB pad -> 50ohm terminator -> 1M//xpF scope input, would the "ripple" be reduced as predicted by theory/simulation? --- End quote --- It should be reduced a bit in theory. The OP is looking at fast edges so I'd expect the discontinuity at the scope input to cause visible reflections if the mismatch/discontinuity was significant. If the cable had (say) 4ns delay then the impact of the discontinuity would appear at about 8ns from the start of the waveform edge as it has to make two trips along the cable to arrive back at the scope. If a fast clock was being examined rather than a single edge then I'd expect to see each discontinuity spoil the look of the waveform on the scope but it depends on the individual how significant this is to them. They might not care. The input VSWR of my Infinium scope is very low when the internal 50R mode is selected but it does degrade up towards V/UHF. I would recommend placing an attenuator ahead of the scope input for critical measurements. This does demand a scope with a low noise input. Maybe try 3dB, 6dB or 10dB to see if it helps. I found that using an SMA connector on the end of the cable, then an SMA attenuator followed by a decent SMA to BNC adaptor and then selecting the internal 50R termination works the best for me. |
| Zeyneb:
I didn't thought I would get so many responses on this topic. Ok, my 200MHz scope input is 1Mohm and 13pF. I do have two channels. Still, some parts of the discussions go over my head. Maybe G0HZU is right and if RG-316 is easier to solder I might go for that. And I also agree with what Bud said and just buy a coax that has the BNC connectors. I found Amphenol RF part 115101-01-M3.00. This might be the coax most suitable for me. Cut it in 2 equal lengths for my two channel scope. This has 7 stranded wires. And as I have learned an 1k SMT resistor has less parasitic capacitance than an THT resistor. Would it be doable to solder an 0603 or 0805 chip resistor to this coax 7 strand conductor? Also the other end of the resistor is not a suitable probe tip. How to do all this in practical terms? I know I do like to use these E-Z hook clips: But they add about 50mm stub length before I can have the 1k resistor. Would that be ok upto 200 MHz? |
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