EEVblog #1367 - 5 Types of Oscilloscope Passive Probes COMPARED

[tggzzz]

Just do the simulation and you'll see I'm right. You will get a step in the signal if the beginning of the coax cable isn't terminated.

I'm 100% sure the Lecroy 7300A has proper 50 Ohm inputs. Proof of that is that the 'dip' doesn't show with a direct cable connection. A 3GHz oscilloscope would be utterly useless without proper 50 Ohm inputs. People would have come to Lecroy's headquarters with pitchforks, tar and feathers if they screwed it up.

Do the measurements, and you'll see I'm right!
Or, how do you explain those measurements?

[tggzzz]

Just do the simulation and you'll see I'm right. You will get a step in the signal if the beginning of the coax cable isn't terminated.

I'm 100% sure the Lecroy 7300A has proper 50 Ohm inputs. Proof of that is that the 'dip' doesn't show with a direct cable connection. A 3GHz oscilloscope would be utterly useless without proper 50 Ohm inputs. People would have come to Lecroy's headquarters with pitchforks, tar and feathers if they screwed it up.

Here's the simulation, and you are wrong!

No 50ohm load at the source.
With a pure 50ohm you get a perfect trace.
With 50ohm//20pF you get the dips you see in your trace.

Your scope isn't pure 50ohms

[nctnico]

That isn't a lossy transmission line model! Model it with a lossy transmission line using real coax cable parameters and you'll see I'm right and your simulation is wrong. The 'regular' transmission line model is an over simplification. In the pspice I use I don't even get the 'dip' if I put 22pf in parallel with the 50 Ohm resistor.

[tggzzz]

That isn't a lossy transmission line model! Model it with a lossy transmission line using real coax cable parameters and you'll see I'm right and your simulation is wrong. The 'regular' transmission line model is an over simplification. In the pspice I use I don't even get the 'dip' if I put 22pf in parallel with the 50 Ohm resistor.

Of course it isn't a lossy transmission line model; it is only 2m of RG174 or similar!

Resistive divider Z0 probes use standard 50ohm coax, not the lossy lines used in 10Mohm probes.

If you try to make a Z0 probe with lossy cable, you are misunderstanding the theory and will get suboptimal results!

[Tom45]

No 50ohm load at the source.
With a pure 50ohm you get a perfect trace.
With 50ohm//20pF you get the dips you see in your trace.

Could you elaborate on your two simulations? The circuits shown seem to be the same for both. So what is different between them?

[tggzzz]

No 50ohm load at the source.
With a pure 50ohm you get a perfect trace.
With 50ohm//20pF you get the dips you see in your trace.

Could you elaborate on your two simulations? The circuits shown seem to be the same for both. So what is different between them?

The value of Cscope; factor of 10⁶ difference.

[nctnico]

That isn't a lossy transmission line model! Model it with a lossy transmission line using real coax cable parameters and you'll see I'm right and your simulation is wrong. The 'regular' transmission line model is an over simplification. In the pspice I use I don't even get the 'dip' if I put 22pf in parallel with the 50 Ohm resistor.

Of course it isn't a lossy transmission line model; it is only 2m of RG174 or similar!

Resistive divider Z0 probes use standard 50ohm coax, not the lossy lines used in 10Mohm probes.

If you try to make a Z0 probe with lossy cable, you are misunderstanding the theory and will get suboptimal results!

You are misunderstanding. A lossy transmission line model has nothing to do with lossy coax! A lossy transmission line is a Spice simulation model with a better approximation of real coax so you will also get more realistic results. And you'll also need to model the probe head itself more accurately (including parasitic capacitances). You'll see that the latter will introduce a dip.

[tggzzz]

That isn't a lossy transmission line model! Model it with a lossy transmission line using real coax cable parameters and you'll see I'm right and your simulation is wrong. The 'regular' transmission line model is an over simplification. In the pspice I use I don't even get the 'dip' if I put 22pf in parallel with the 50 Ohm resistor.

Of course it isn't a lossy transmission line model; it is only 2m of RG174 or similar!

Resistive divider Z0 probes use standard 50ohm coax, not the lossy lines used in 10Mohm probes.

If you try to make a Z0 probe with lossy cable, you are misunderstanding the theory and will get suboptimal results!

You are misunderstanding. A lossy transmission line model has nothing to do with lossy coax! A lossy transmission line is a Spice simulation model with a better approximation of real coax so you will also get more realistic results. And you'll also need to model the probe head itself more accurately (including parasitic capacitances). You'll see that the latter will introduce a dip.

Your second sentence directly contradicts your third sentence.

Explain the measurements I have shown.

Prove your contention by showing us your simulation that

• matches your description and measurements
• does not match my description and measurements

I'm not holding my breath.

[nctnico]

That isn't a lossy transmission line model! Model it with a lossy transmission line using real coax cable parameters and you'll see I'm right and your simulation is wrong. The 'regular' transmission line model is an over simplification. In the pspice I use I don't even get the 'dip' if I put 22pf in parallel with the 50 Ohm resistor.

Of course it isn't a lossy transmission line model; it is only 2m of RG174 or similar!

Resistive divider Z0 probes use standard 50ohm coax, not the lossy lines used in 10Mohm probes.

If you try to make a Z0 probe with lossy cable, you are misunderstanding the theory and will get suboptimal results!

You are misunderstanding. A lossy transmission line model has nothing to do with lossy coax! A lossy transmission line is a Spice simulation model with a better approximation of real coax so you will also get more realistic results. And you'll also need to model the probe head itself more accurately (including parasitic capacitances). You'll see that the latter will introduce a dip.

Your second sentence directly contradicts your third sentence.

No it doesn't; your statement only shows a lack of understanding Spice models for transmission lines. You seem to get stuck at the word 'lossy' but it is used in an entirely different context which has nothing to do with coax. The lossy transmission line model can be used to model any kind of transmission line which does not have to be coax at all.

Explain the measurements I have shown.

All your measurements show is that your probe shows seemingly correct behaviour. However, your probe is a 1:10 model with a 450 Ohm series resistor instead of a 1:20 model with a 950 Ohm series resistor. Crafting the attenuator cartridges for low-Z probes is an art in itself. Maybe HP is better at that than Tektronix but a simple 1k resistor in series with a piece of coax is not going to cut it. Try that first!

[tggzzz]

That isn't a lossy transmission line model! Model it with a lossy transmission line using real coax cable parameters and you'll see I'm right and your simulation is wrong. The 'regular' transmission line model is an over simplification. In the pspice I use I don't even get the 'dip' if I put 22pf in parallel with the 50 Ohm resistor.

Of course it isn't a lossy transmission line model; it is only 2m of RG174 or similar!

Resistive divider Z0 probes use standard 50ohm coax, not the lossy lines used in 10Mohm probes.

If you try to make a Z0 probe with lossy cable, you are misunderstanding the theory and will get suboptimal results!

You are misunderstanding. A lossy transmission line model has nothing to do with lossy coax! A lossy transmission line is a Spice simulation model with a better approximation of real coax so you will also get more realistic results. And you'll also need to model the probe head itself more accurately (including parasitic capacitances). You'll see that the latter will introduce a dip.

Your second sentence directly contradicts your third sentence.

No it doesn't; your statement only shows a lack of understanding Spice models for transmission lines. You seem to get stuck at the word 'lossy' but it is used in an entirely different context which has nothing to do with coax. The lossy transmission line model can be used to model any kind of transmission line which does not have to be coax at all.

Explain the measurements I have shown.

All your measurements show is that your probe shows seemingly correct behaviour. However, your probe is a 1:10 model with a 450 Ohm series resistor instead of a 1:20 model with a 950 Ohm series resistor. Crafting the attenuator cartridges for low-Z probes is an art in itself. Maybe HP is better at that than Tektronix but a simple 1k resistor in series with a piece of coax is not going to cut it. Try that first!

If you really think that the issue is related to 1:10 vs 1:20 then there is little point in continuing this conversation.

Please explain why Z0 probes use standard coax, not the very resistive coax used in 10Mohm probes.

Please provide your simulations that justify your conjectures. If you choose not to, please starts why not!

Come on; it took me 15minutes.I sure it won't take you much longer than that!

[radar_macgyver]

Could someone with a VNA look at the S11 of their scope input in 50 ohm mode? I don't have one at home.

[nctnico]

That isn't a lossy transmission line model! Model it with a lossy transmission line using real coax cable parameters and you'll see I'm right and your simulation is wrong. The 'regular' transmission line model is an over simplification. In the pspice I use I don't even get the 'dip' if I put 22pf in parallel with the 50 Ohm resistor.

Of course it isn't a lossy transmission line model; it is only 2m of RG174 or similar!

Resistive divider Z0 probes use standard 50ohm coax, not the lossy lines used in 10Mohm probes.

If you try to make a Z0 probe with lossy cable, you are misunderstanding the theory and will get suboptimal results!

You are misunderstanding. A lossy transmission line model has nothing to do with lossy coax! A lossy transmission line is a Spice simulation model with a better approximation of real coax so you will also get more realistic results. And you'll also need to model the probe head itself more accurately (including parasitic capacitances). You'll see that the latter will introduce a dip.

Your second sentence directly contradicts your third sentence.

No it doesn't; your statement only shows a lack of understanding Spice models for transmission lines. You seem to get stuck at the word 'lossy' but it is used in an entirely different context which has nothing to do with coax. The lossy transmission line model can be used to model any kind of transmission line which does not have to be coax at all.

Explain the measurements I have shown.

All your measurements show is that your probe shows seemingly correct behaviour. However, your probe is a 1:10 model with a 450 Ohm series resistor instead of a 1:20 model with a 950 Ohm series resistor. Crafting the attenuator cartridges for low-Z probes is an art in itself. Maybe HP is better at that than Tektronix but a simple 1k resistor in series with a piece of coax is not going to cut it. Try that first!

If you really think that the issue is related to 1:10 vs 1:20 then there is little point in continuing this conversation.

Not the issue but a much larger impedance mismatch. Please try using a 1k resistor attached to a piece of coax.

[tggzzz]

Could someone with a VNA look at the S11 of their scope input in 50 ohm mode? I don't have one at home.

A sensible suggestion. I doubt nctnico will make such a measurement on his LeCroy.

The Tek 485's input attenuators (plural) can be seen in the schematic.

[tggzzz]

That isn't a lossy transmission line model! Model it with a lossy transmission line using real coax cable parameters and you'll see I'm right and your simulation is wrong. The 'regular' transmission line model is an over simplification. In the pspice I use I don't even get the 'dip' if I put 22pf in parallel with the 50 Ohm resistor.

Of course it isn't a lossy transmission line model; it is only 2m of RG174 or similar!

Resistive divider Z0 probes use standard 50ohm coax, not the lossy lines used in 10Mohm probes.

If you try to make a Z0 probe with lossy cable, you are misunderstanding the theory and will get suboptimal results!

You are misunderstanding. A lossy transmission line model has nothing to do with lossy coax! A lossy transmission line is a Spice simulation model with a better approximation of real coax so you will also get more realistic results. And you'll also need to model the probe head itself more accurately (including parasitic capacitances). You'll see that the latter will introduce a dip.

Your second sentence directly contradicts your third sentence.

No it doesn't; your statement only shows a lack of understanding Spice models for transmission lines. You seem to get stuck at the word 'lossy' but it is used in an entirely different context which has nothing to do with coax. The lossy transmission line model can be used to model any kind of transmission line which does not have to be coax at all.

Explain the measurements I have shown.

All your measurements show is that your probe shows seemingly correct behaviour. However, your probe is a 1:10 model with a 450 Ohm series resistor instead of a 1:20 model with a 950 Ohm series resistor. Crafting the attenuator cartridges for low-Z probes is an art in itself. Maybe HP is better at that than Tektronix but a simple 1k resistor in series with a piece of coax is not going to cut it. Try that first!

If you really think that the issue is related to 1:10 vs 1:20 then there is little point in continuing this conversation.

Not the issue but a much larger impedance mismatch. Please try using a 1k resistor attached to a piece of coax.

Nonsense.

Stop avoiding the issues and provide measurements or simulations!

Please explain why Z0 probes use standard coax, not the very resistive coax used in 10Mohm probes.

Please provide your simulations that justify your conjectures. If you choose not to, please starts why not!

Come on; it took me 15minutes.I sure it won't take you much longer than that!

[nctnico]

See the earlier posting. I added a simple simulation schematic and results. But keep in mind that this simulation does not model the probe head and parasitic capacitances / inductances. However the effects of the improper termination of the coax are clearly visible!

[Tom45]

However the effects of the improper termination of the coax are clearly visible!

Maybe I missed something, but which termination are you talking about when comparing the two traces? At the scope end of the coax, or at the probe end?

[nctnico]

However the effects of the improper termination of the coax are clearly visible!

Maybe I missed something, but which termination are you talking about when comparing the two traces? At the scope end of the coax, or at the probe end?

At the beginning of the coax there is resistor R4 which is either 52 Ohms or (as shown in the schematic) has such a high value that it is basically left out. The resulting signal is measured at the end of the coax across the 50 Ohm resistor which resembles the oscilloscope's input signal.

[tggzzz]

However the effects of the improper termination of the coax are clearly visible!

Maybe I missed something, but which termination are you talking about when comparing the two traces? At the scope end of the coax, or at the probe end?

No, you haven't missed something; nctnico's statements are ambiguous and unclear.

[nctnico]

However the effects of the improper termination of the coax are clearly visible!

Maybe I missed something, but which termination are you talking about when comparing the two traces? At the scope end of the coax, or at the probe end?

No, you haven't missed something; nctnico's statements are ambiguous and unclear.

Come on. All this time we have been talking about where the termination should be and now it is unclear?  Can't you read a simple diagram? I'd almost pull the troll card.

For your own sake please don't post until you have tried a 1k resistor at the end of a piece of coax.

[tggzzz]

However the effects of the improper termination of the coax are clearly visible!

Maybe I missed something, but which termination are you talking about when comparing the two traces? At the scope end of the coax, or at the probe end?

At the beginning of the coax (at the point where the 1k input resistor connects to the transmission line). The resulting signal is measured at the end of the coax across the 50 Ohm resistor which resembles the oscilloscope's input signal.

I can't see that, and have no inclination to search for how you have "revised history".

Post the simulations here, where we can see them in the current context.

Address the issue of why Z0 probes use standard coax, not highly resistive coax.

[nctnico]

  Nobody is talking about highly resistive coax. Why do you keep bringing that up? 

Scroll a few posts up and you see the diagram and simulation results.

[tggzzz]

However the effects of the improper termination of the coax are clearly visible!

Maybe I missed something, but which termination are you talking about when comparing the two traces? At the scope end of the coax, or at the probe end?

No, you haven't missed something; nctnico's statements are ambiguous and unclear.

Come on. All this time we have been talking about where the termination should be and now it is unclear?  Can't you read a simple diagram? I'd almost pull the troll card..

Where, exactly, is your simulation?

Two people (at least) are confused by your postings, so you are being unclear.

[tggzzz]

Nobody is talking about highly resistive coax. Why do you keep bringing that up?  .

You introduced them.

[Tom45]

At the beginning of the coax there is resistor R4 which is either 52 Ohms or (as shown in the schematic) has such a high value that it is basically left out. The resulting signal is measured at the end of the coax across the 50 Ohm resistor which resembles the oscilloscope's input signal.

Thanks. You had two simulations but didn't say which resistor was being changed. I didn't know whether you meant R2, R4, or R3.

[tggzzz]

Nctnico's simulation and measurements are very different: note the difference between steps and notches.



My simulation and measurements of a scope with a 50ohms//20pF input show the same principal features.

 

Conclusion: my model matches reality better than nctnico's.