Z0 probe resistor location

[electrolust]

So all the commercial Z0 probes (4 I looked at, and a Tek P6150 I bought), all the DIY probes, and all the literature describes putting the resistor right at the tip.  Why not use the bare center wire from the SMA cable, or a formed pin, and use a barrel attenuator on the cable?

When I look at the FFT of a noise signal on my 200MHz scope, I see the same response whether bare cable (SMA that comes with the P6150) or cable + 20dB attenuator.  Which I would expect, since my Pasternack-branded attenuator is rated to 2GHz and my SMA-BNC adapter is rated to 8GHz.  I say "the same" and not "flat" because there's a peak at 156.25MHz I don't understand, but anyway it's the same for both configs.

When I probe a specific test pulse with the P6150 with 10x probe, I get a 6.4ns rise time.  When I switch to the 1x probe but use the 20dB attenuator, I get a 10.4ns rise time.  Given that the frequency response to a noise signal was the same, I didn't expect the difference in rise time.

I guess the attenuation is probably a PI network and not a single resistor, but I am not clear on why this affects the rise time so much.  If that is the issue, I did find someone that makes a ready built PCB sandwiched between 2 connectors.  It comes in different configs (Pi, etc) including just straight through, so I could just put in a 500r resistor.  And Pomona sells something similar, with some fixed resistor values (not 450 or 500 unfortunately), but no one seems to stock these.

AIUI the 20dB attenuator is going to be on the order of 500r instead of 450r, which would affect the voltage scale but the 50r difference shouldn't itself be a concern for my purposes (rise time).

The reason I am thinking to use an attenuator is that the P6150 probe tips are rather fragile.  The two 10x probes I received in my P6150 kit are both somewhat damaged in different ways, and both of them are rather difficult to probe with.  I figured if I can probe with a bare wire that'd solve the tip wear problem and I could come up with probe attachment however I like, including solder to board.  The 1x probe in my kit is rather in good condition and easier to use, but 1x probe isn't useful ... unless I could attenuate it!!

One clue I found is that in the HP 54006A documentation, which uses a caddock axial lead resistor as opposed to the rather more exotic construction of the Tek probes, it says that the resistor lead (which is the probe tip, just like DIY designs) is trimmed to a precise length to establish .25pF capacitance, and that cutting it shorter will reduce capacitance further.  Wow.  Axial lead length changes the capacitance?  OK, I guess I'll just accept that, but what does that imply about barrel attenuators?

[w2aew]

So all the commercial Z0 probes (4 I looked at, and a Tek P6150 I bought), all the DIY probes, and all the literature describes putting the resistor right at the tip.  Why not use the bare center wire from the SMA cable, or a formed pin, and use a barrel attenuator on the cable?

When I look at the FFT of a noise signal on my 200MHz scope, I see the same response whether bare cable (SMA that comes with the P6150) or cable + 20dB attenuator.  Which I would expect, since my Pasternack-branded attenuator is rated to 2GHz and my SMA-BNC adapter is rated to 8GHz.  I say "the same" and not "flat" because there's a peak at 156.25MHz I don't understand, but anyway it's the same for both configs.

When I probe a specific test pulse with the P6150 with 10x probe, I get a 6.4ns rise time.  When I switch to the 1x probe but use the 20dB attenuator, I get a 10.4ns rise time.  Given that the frequency response to a noise signal was the same, I didn't expect the difference in rise time.

I guess the attenuation is probably a PI network and not a single resistor, but I am not clear on why this affects the rise time so much.  If that is the issue, I did find someone that makes a ready built PCB sandwiched between 2 connectors.  It comes in different configs (Pi, etc) including just straight through, so I could just put in a 500r resistor.  And Pomona sells something similar, with some fixed resistor values (not 450 or 500 unfortunately), but no one seems to stock these.

AIUI the 20dB attenuator is going to be on the order of 500r instead of 450r, which would affect the voltage scale but the 50r difference shouldn't itself be a concern for my purposes (rise time).

The reason I am thinking to use an attenuator is that the P6150 probe tips are rather fragile.  The two 10x probes I received in my P6150 kit are both somewhat damaged in different ways, and both of them are rather difficult to probe with.  I figured if I can probe with a bare wire that'd solve the tip wear problem and I could come up with probe attachment however I like, including solder to board.  The 1x probe in my kit is rather in good condition and easier to use, but 1x probe isn't useful ... unless I could attenuate it!!

One clue I found is that in the HP 54006A documentation, which uses a caddock axial lead resistor as opposed to the rather more exotic construction of the Tek probes, it says that the resistor lead (which is the probe tip, just like DIY designs) is trimmed to a precise length to establish .25pF capacitance, and that cutting it shorter will reduce capacitance further.  Wow.  Axial lead length changes the capacitance?  OK, I guess I'll just accept that, but what does that imply about barrel attenuators?

The resistor at the tip of the Z0 probe is designed mainly to increase the input impedance of the probe (at the expense of reducing the signal amplitude). In the case of the P6150, it increases it to 500ohms.  This is useful when the circuit that you are probing cannot drive a 50ohm load. The design of the probe tip resistor/mounting is done carefully to get very low capacitance and thus give you high bandwidth.

If you simply use an inline attenuator with a 50 ohm line, you have only reduced the signal amplitude.  You have *not* increased the input impedance, it is still 50 ohms.  The input impedance of the coaxial attenuators is 50 ohms, so sticking one in series with a 50 ohms line does not make a Z0 probe.

The change in risetime that you observed was likely not a function of the probe bandwidth, but rather a change in your circuit response when driving 50 ohms vs. a 500 ohm load.

[electrolust]

If you simply use an inline attenuator with a 50 ohm line, you have only reduced the signal amplitude.  You have *not* increased the input impedance, it is still 50 ohms.  The input impedance of the coaxial attenuators is 50 ohms, so sticking one in series with a 50 ohms line does not make a Z0 probe.

I guess I don't quite understand resistance vs. impedance (I'll do my own homework though), although at a trivial level I should have understood that some XXdB attenuator still has a 50ohm impedance.

So instead of a pi-pad type attenuator, what about a straight through resistor, like a Pomona 4391 http://www.pomonaelectronics.com/pdf/D4391_100.pdf ? The main difference to a typical Z0 probe would be that the resistor is at the other end of the cable.

[electrolust]

The change in risetime that you observed was likely not a function of the probe bandwidth, but rather a change in your circuit response when driving 50 ohms vs. a 500 ohm load.

yup.  Obvious to me now.  Had I looked closer at the trace I'd have seen that the output/final voltage was only 2.2V compared to 3.3V.  I already knew that my circuit can't drive 50r.  Had I looked more closely at the trace (at 1V/div) I would have noticed that the output voltage was a full division smaller.

[NorthGuy]

I figured if I can probe with a bare wire that'd solve the tip wear problem ...

This may help to solve tip wear problem:

https://www.digikey.com/product-detail/en/cal-test-electronics/CT2711A-0/BKCT2711A-0-ND/5765523

[T3sl4co1l]

Attenuator has three resistors so it looks like 50 ohms from both sides.  Hi-Z probe does not.

It would be alright if you had a length of 500 ohm transmission line... but coax in that value is hard to come by.

Tim

[Gyro]

I figured if I can probe with a bare wire that'd solve the tip wear problem ...

This may help to solve tip wear problem:

https://www.digikey.com/product-detail/en/cal-test-electronics/CT2711A-0/BKCT2711A-0-ND/5765523

Or socket the resistor for easy replacement (and minimum capacitance) ...

www.eevblog.com/forum/projects/lo-z-probe/msg801772/#msg801772


P.S. You probably want to read through that entire thread.

[electrolust]

Attenuator has three resistors so it looks like 50 ohms from both sides.  Hi-Z probe does not.

Ahhh!!!  I get it now.

It would be alright if you had a length of 500 ohm transmission line... but coax in that value is hard to come by.

You joke, but could one build a homemade 500 ohm transmission line out of twisted pair, or something else?

[tggzzz]

You joke, but could one build a homemade 500 ohm transmission line out of twisted pair, or something else?

The impedance of free space is 377ohms. Where e_r greater than 1, it will have a lower impedance.
https://en.wikipedia.org/wiki/Impedance_of_free_space

[electrolust]

ahh ... funny joke is funny now 

[T3sl4co1l]

Maximum is 2*Z_0, because there's a contribution from each conductor; but this only works, I think, for twin lead (which goes up to 600 ohm, 300 ohm being the most common).

The coaxial transmission line formula simply goes as ln(D/d), which implies you can get any impedance you wish, given a ridiculous enough ratio of diameters (shield ID to core OD).  Not that log(x) grows very fast: 5 ~= ln(148), 10 ~= ln(22026) and so on.

For a practical ratio that large, you'll have a center conductor that's so thin as to be very resistive on its own (= high attenuation, especially at high frequency), and an outer shield that's so large that you get waveguide modes in the passband (sub GHz).  It won't be much longer than its width anyway, nor will the connections make much sense (how do you probe a component pin, when your probe is ~feet across?).

The consequence of Z being limited to around Zo, is that you get radiation within your transmission line, which is to say, waveguide modes and stuff.  Twin lead will radiate quite effectively (to space) at frequencies, and in directions, where the waves don't cancel out (the trick is to physically flip or twist it around, to minimize radiation).  Coax is self-contained though, which means it reflects the radiation back into itself.  Which means you can get whatever impedance you want (including >Zo), but you only get a low-dispersion TEM mode up to whatever frequency corresponds to the OD.

Tim

[electrolust]

OK I've acquired some parts and been able to complete an experiment.  I suppose the Tek and HP parts aren't in high demand and that's why I was able to acquire them in short order.

Tek P6150 x10 tip
HP 54006A x20 tip
PRL DIY 953r (x20)

If you recall from above, I have a Tek P6150 kit with poor quality x10 tips.  The x1 tip is quite good (looks new AFAIK) presumably because it never got used.

I acquired an 54006A x20 probe body and resistor (not a whole kit).  I used it with the SMA cable from the Tek kit, I suppose a high quality cable with good connectors and good construction.

The PRL trace is for a Pulse Research Labs "PiNet" universal attenuator kit.  It's a PCB between 2 BNC connectors.  Would be easy to make the whole thing yourself, but the cost would be close and you wouldn't have the nice enclosure.  I used an 0805 0.1% thin film 953r resistor -- note that this is actually closer to the nominal 950r than the 54006a which is only within 1% (I only have 1 resistor, not a full kit, and measured it as 956r).  Not that that matters for this experiment.  I wired it just as a straight through resistor, not as a Pi pad.

So, you can see that the P6150 and 54006A traces are extremely clean, with the 54006A having a 6% better rise time.  I guess that implies something about the capacitive load of the probe, although the P6150 is supposed to have only 0.15pF vs the 0.25pF of the 54006A.  However as I said my P6150 x10 tips are is sorry shape, smushed into the barrel a bit so this is damage which possibly affects the response.  "New" tips are over $125 on ebay so that's not very attractive, esp. since it would be impossible to evaluate the actual condition without having it in front of you to inspect, and clearly they do get damaged so even if you got a new one it won't last.

The PRL trace is a bit slower than the others.  I guess that represents higher capacitance?  For the cost, and the 3% difference from the P6150 I think that's great.  It is 10% off of the 54006A though.  Still, it's ok.  The bigger problem with the PRL trace is the overshoot.  Where does that likely come from?

The PRL trace looks exactly the same whether I use an SMA connector with 6" RG316 pigtail coax ($6 preassembled from amazon), or the good x1 probe and gold plated ground tip from my P6150 kit.

Which is very good news.  It means my theory to put the resistor at the other end of the cable might be valid.  It means the overshoot and the extra capacitance comes from the construction of the "attenuator", and not so much from the bare wire "probe".  If I can address that with better PCB design, better connectors, or other fix, then the probe end can just be a consumable wire pigtail, with numerous DUT attachment possibilities.

I've also attached a pic of the PRL board.  Note that it's 2 sided, the other side is in parallel to the first side.  I suppose the layout is not ideal for series resistance with lowest possible capacitance, but would I be able to do significantly better with a purpose built PCB, without the extra ground "stubs" etc?  And specifically would I likely be able to address the overshoot?  Full pdf of the kit is here.

When I get a pulser going, I'll redo this so as to get a better appreciation of the performance.  I only have a 200MHz scope, but that should be good for 1.75ns rise time so that is still significantly better than my current test circuit.

[T3sl4co1l]

Wow, that pulse response (PRL) sucks...

You've got a mild resonance or reflection/mismatch somewhere.

Tim