Why does Tektronix need 250 kΩ oscilloscope inputs?

[44kgk1lkf6u]

The TekVPI oscilloscope probe connector, the BNC connector with a 2 by 3 grid of pogo pins, has a 250 kΩ mode in addition to 50 Ω and 1 MΩ.  Does anyone know why?

It seems to be a hidden feature in oscilloscopes with the TekVPI interface.  The feature can only be activated with certain TekVPI probes.  The only place where I found a reference to it is in the specifications for the 4 Series MSO https://www.tek.com/en/datasheet/4-series-mso.  The spec for DC gain accuracy says, "1 MΩ and 250 kΩ: ±1.0%".

Probes that make use of said feature include some TekVPI probes with with 10 MΩ // 4 pF input impedance, for instance the TPP0500 and TPP1000.  The manufacturer does not seem to say they are 10X.  TekWiki says they are 10X and classifies them as 1 MΩ probes, but there is no citation.  By measuring them, I found that they have 40X attenuation, and that the oscilloscope would switch its input resistance to 250 kΩ when one is attached.  When not connected, they measure 9.75 MΩ end to end, and open from the tip to ground.  When connected, there is 10 MΩ from the tip to ground.  When the replaceable tip hybrid is removed and a voltage is fed to the socket, 40 times the voltage shows up on the scope.

It makes sense that they want a 40 times attenuation for less input capacitance.  It also makes sense that they may want to make input hybrids with a similar resistance to what they already know how to make instead of figuring out how to make 39 MΩ ones.  But when the TekVPI connector has plenty of space for a 330 kΩ resistor, the only reason I can think of for putting it in the scope is maybe they did not think figuring out how to switch to 1 MΩ without compromising the 50 Ω path was enough to show off their competence.

[calibrationfixture]

Hi,

On Tekscopes I found this:

Re: scope Probe <--> Scope Compatibility

Clark Foley
11/20/22   #196825   
There is a fundamental difference between traditional input configuration and that intended for use with TPP1000.  Apparently the TPP1000 does not have a 9.0M ohm series resistor that would terminate to a 1M ohm resistor to form a 10:1 voltage divider. According to a FAQ on the Tek website, the TPP1000 has a series R=9.75M ohm and it terminates with a series 250k ohm resistor (10M total) that connects to a transimpedance front end.  I suggest and old Russian saying, later used by Pres. Regan, “Trust but verify!”
In other words check the series resistance of the probe to determine if it is indeed 9.75M ohm.  It was news to me.  Visit the link below for more clues.

https://www.tek.com/en/support/faqs/why-isnt-tpp1000-(tektronix-passive-probe-1ghz)-compatible-my-scope

The link isn't active any more. Maybe renamed.

Calibrationfixture

[David Hess]

It seems like you answered your own question.  The lower input resistance produces more attenuation so the capacitance at the probe tip may be even lower, increasing passive probe bandwidth.  In the extreme case, the input resistance is the same as the impedance of the probe cable and the capacitance at the probe tip can be zero ohms.

The disadvantage is that more attenuation means greater noise when referred to the probe tip, but most oscilloscope applications are not limited by noise.

[44kgk1lkf6u]

It seems like you answered your own question.  The lower input resistance produces more attenuation so the capacitance at the probe tip may be even lower, increasing passive probe bandwidth.  In the extreme case, the input resistance is the same as the impedance of the probe cable and the capacitance at the probe tip can be zero ohms.

What I don't understand is why they had to put the 250 kΩ resistor in the oscilloscope and deal with switching it on and compatibility problems.  It is easier to put a 330 kΩ resistor in the probe.

[David Hess]

It seems like you answered your own question.  The lower input resistance produces more attenuation so the capacitance at the probe tip may be even lower, increasing passive probe bandwidth.  In the extreme case, the input resistance is the same as the impedance of the probe cable and the capacitance at the probe tip can be zero ohms.

What I don't understand is why they had to put the 250 kΩ resistor in the oscilloscope and deal with switching it on and compatibility problems.  It is easier to put a 330 kΩ resistor in the probe.

Long ago Tektronix made probes that worked exactly like that.  They can be identified as a x10 probe, but they had an input resistance lower than 10 megohms.

[Someone]

It seems like you answered your own question.  The lower input resistance produces more attenuation so the capacitance at the probe tip may be even lower, increasing passive probe bandwidth.  In the extreme case, the input resistance is the same as the impedance of the probe cable and the capacitance at the probe tip can be zero ohms.

What I don't understand is why they had to put the 250 kΩ resistor in the oscilloscope and deal with switching it on and compatibility problems.  It is easier to put a 330 kΩ resistor in the probe.

They aren't voltage mode probes, adding a 250k termination resistor does not make it compatible with an infinite (1M/10M) input impedance.

https://www.eevblog.com/forum/testgear/can-tektronix-tpp0500-probe-be-used-with-non-tek-oscilloscopes/