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Tek p5205 HV differential probe teardown. BTW, what are the red and brown wires?

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MarkL:
You could go in the other direction and make it x100 and x1000 by doing a simple divide by 2 (with compensation) on the coax side to the scope.

If your plan was to modify the input side of the probe, this alternative preserves the CMRR and other input characteristics.

Mechatrommer:
agreed, making divider on the output is much less risk. looking at your picture, 50X is unterminated, simply adding proper 50 ohm termination at dso end you'll get 1/100X output (@ 50X setting) and 1/1000X (@500X setting), set your dso to suit, no need mental math ;)

MarkL:
Yes, this works to a first approximation and a simple 50 ohm pass-through terminator can be used.  However, be aware of this from the probe specs:


--- Quote ---Output Type: Single-ended. Source Impedance of 50 ohms drives 1 M ohm oscilloscope input. Load impedance must be greater than 50 k ohm for stated accuracy.

--- End quote ---

So, a compensated divide by two attenuator with an impedance greater than 50k is needed if you want to stay accurate.  You could use a couple of 25k resistors as a divider, or you might even be able to get away with a single 1M and rely on the 1M inside the scope.  In either case you'll need an adjustable cap for compensation across the resistor, around 5 to 25pF.  If you put the arrangement in the probe box instead of at the scope you'll need even more pF to compensate for the cable.

There's no information I can find that says exactly *which* specs are invalidated if you go with 50 ohms (I suspect accuracy with max signal input), so in reality the above may be overkill for your purposes.  I would say try the 50 ohm termination with various signals that you typically measure to see if it works well enough.

(Edit: Ohms symbol didn't make through cut/paste from specs.  Changed  to "ohms".)

David Hess:

--- Quote from: MarkL on October 08, 2014, 02:40:25 am ---You could go in the other direction and make it x100 and x1000 by doing a simple divide by 2 (with compensation) on the coax side to the scope.
--- End quote ---


--- Quote from: MarkL on October 08, 2014, 01:34:30 pm ---So, a compensated divide by two attenuator with an impedance greater than 50k is needed if you want to stay accurate.  You could use a couple of 25k resistors as a divider, or you might even be able to get away with a single 1M and rely on the 1M inside the scope.  In either case you'll need an adjustable cap for compensation across the resistor, around 5 to 25pF.  If you put the arrangement in the probe box instead of at the scope you'll need even more pF to compensate for the cable.
--- End quote ---

What I find really odd about this discussion is that such a device used to exist!

Oscilloscope input "normalizers" are used to adjust the input capacitance of the vertical inputs of an oscilloscope as well as the compensation of the input attenuators but these normalizers also function as compensated precision divide by 2 attenuators.  I have a couple of them in my collection of calibration instruments.


--- Quote from: MarkL on October 08, 2014, 01:34:30 pm ---There's no information I can find that says exactly *which* specs are invalidated if you go with 50 ohms (I suspect accuracy with max signal input), so in reality the above may be overkill for your purposes.  I would say try the 50 ohm termination with various signals that you typically measure to see if it works well enough.
--- End quote ---

The accuracy of the 50 ohm termination will degrade the calibration.  The high voltage differential probes I am familiar with specify two different attenuation factors depending on if their output is terminated into 50 ohms or not.

MarkL:

--- Quote from: David Hess on October 08, 2014, 07:42:26 pm ---What I find really odd about this discussion is that such a device used to exist!

Oscilloscope input "normalizers" are used to adjust the input capacitance of the vertical inputs of an oscilloscope as well as the compensation of the input attenuators but these normalizers also function as compensated precision divide by 2 attenuators.  I have a couple of them in my collection of calibration instruments.

--- End quote ---
I figured there had to be; this didn't seem like such an unusual situation.   But I couldn't find any to say, "Here, buy this!"  Thanks for the pointer.  Is there a compensation adjustment on a "normalizer"?


--- Quote ---
--- Quote from: MarkL on October 08, 2014, 01:34:30 pm ---There's no information I can find that says exactly *which* specs are invalidated if you go with 50 ohms (I suspect accuracy with max signal input), so in reality the above may be overkill for your purposes.  I would say try the 50 ohm termination with various signals that you typically measure to see if it works well enough.
--- End quote ---

The accuracy of the 50 ohm termination will degrade the calibration.  The high voltage differential probes I am familiar with specify two different attenuation factors depending on if their output is terminated into 50 ohms or not.

--- End quote ---
Right.  I'm expecting the P5205 won't be able to swing to the peaks of its specified output with 50 ohms and the waveform will get squashed.

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