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High voltage oscilloscope probes, make or buy?
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Rupunzell:
Lossy coax alone does not cure the probe problem as it still has reflections/ringing and requires damping in the probe termination and peaking to compensate for the loss in HF response.

This is the probe termination box of a Tektronix P6105 which is spec'd at 100Mhz. It is a pretty basic Kobbe-Polits scope probe using a lead-lag network. Not the best scope probe as it does produce reflections at the probe tip and has less waveform fidelity compared to Tek and other higher quality probes.

Tek P6105, Term box side:



Tek P6105, Term box bottom:



This is Tektronix P6106 probe (200Mhz)  termination box, it has two inductors and a LOT more Trimmer and Cap adjustments. Why is this?


Tektronix P6106, termination box's other side:



Scope probes of this design work well, have low internal reflections with good pulse and frequency response.
Answer to this can be found starting at page 15 in this Tektronix probe book from 1969.
 http://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf

A bit of probe development history:
http://www.vintagetek.org/oscilliscope-patents/

Low quality, poorly designed and produced probes are a MAJOR source of measurement error. This coupled with inadequate probe application and insensitivity to how the circuit behaves when probed ADDs to the measurement errors. Passive probes are the most error prone due to their loading at HF, probe reflections and grounding problems. Active probes can make a BIG difference when properly applied, Zo and numerous other probe variations can make the different between low circuit reaction to being probed with good waveform-waveshape fidelity and extreme measurement error. Analog folks can be extremely demanding on probe performance. Every lump, bump, ring and all manner of wave shape aberration can have meaning directly related to circuit behavior. 


This is the probe termination box from the P6015 HV probe, it is not a simple network and is LOTs of fun to set up. These are often found with the termination box completely out of whack which results in lumpy, bumpy pulse & frequency response. Getting one of these properly adjusted is a good network learning experience.

 

Bernice



--- Quote from: DanielS on April 13, 2015, 01:01:13 pm ---
--- Quote from: Rupunzell on April 12, 2015, 04:46:34 pm ---Beyond this, this design does not account for probe cable reflections or impedance mis-matches.
--- End quote ---
When was the last time probe cable reflections and mis-matches have ever been an issue? Passive probes use lossy coax to prevent any significant signal reflections from appearing despite the gross mismatch between the unknown source impedance, the probe, the probe's cable and the scope's input.

--- End quote ---
Mark:
Excellent info Bernice!  Thank you, I will bear that in mind if I attempt to adjust anything, I'm certainly glad I decided to buy and not make/bodge now. 
HighVoltage:
If you bought one of the older P6015 probes that require to be filled with fluorocarbon, you can fill them instead with a high voltage stable silicone gel to have them permanently filled. This is the same as in the P6015A. Just make sure the probe works perfectly, before you fill it with silicone gel.
Mechatrommer:

--- Quote from: DanielS on April 13, 2015, 01:01:13 pm ---
--- Quote from: Rupunzell on April 12, 2015, 04:46:34 pm ---Beyond this, this design does not account for probe cable reflections or impedance mis-matches.
--- End quote ---
When was the last time probe cable reflections and mis-matches have ever been an issue?
--- End quote ---
when you try to diy it yourself. all this while we took this for granted, with off the shelf probe.


--- Quote from: DanielS on April 13, 2015, 01:01:13 pm ---Passive probes use lossy coax to prevent any significant signal reflections from appearing..

--- End quote ---
i will be very glad if you can give me ebay link where to get lossy 50ohm coax. afaik its none, so reflection problem solving is you need to put a resistor at the very tip of your probe side (with some degree of "success"), not at the active circuit or dso side. otoh, yes building a voltage divider and its compensation is easy, the hard part is specifying it with real world signal measurement, in this case, you need HV generator with acceptable output impedance. if its easy to be get, then its easy to be done.
korlatos:

--- Quote from: HighVoltage on April 14, 2015, 08:45:03 am ---If you bought one of the older P6015 probes that require to be filled with fluorocarbon, you can fill them instead with a high voltage stable silicone gel to have them permanently filled. This is the same as in the P6015A. Just make sure the probe works perfectly, before you fill it with silicone gel.

--- End quote ---

Does anybody have any recommendations for the type (and source of it in the USA) of a silicone gel suitable for filling P6015?
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