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

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David Hess:

--- Quote from: MarkL on October 08, 2014, 07:59:53 pm ---
--- 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"?
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Yes, it is not shown in the photo because it is on the bottom but there is an access hole for the trimmer capacitor.

In practice they were compensated against one channel and then used to adjust the compensation of the other channels and all of the input attenuator sections.  They could also be used to compensate multiple oscilloscopes against each other so probes could be moved between them easily.  A x10 probe may be used to do the same thing but it will have 5 times more attenuation of the test signal which may or may not be a problem.


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--- 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.
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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.

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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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The waveform will certainly be halved but if they designed it to drive a 50 ohm transmission line to its 100 MHz bandwidth, I doubt it will have problems besides that.

MarkL:

--- Quote from: MarkL on October 08, 2014, 07:59:53 pm ---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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Did a quick test and this is correct.  Instead of the output swinging 6.2V Pk-Pk (measured; 5.2V is the spec), I only get 2.8V Pk-Pk with 50 ohm termination.

A side effect of 50 ohm termination, which I didn't expect, is that the overrange LED does not light.  It's obviously looking at its own output to determine overrange and the level never gets high enough.  So, 50 ohm termination with this probe is not a good thing.

Edit: Since the source impedance is 50 ohms, 2.8V is actually not too far off expectations.

Mechatrommer:
make active 1/2X divider or 5X amplifier with an opamp then as suggested by the OP earlier.

MarkL:
Not sure I saw that suggestion, but why would you want to start adding active components?  Plus added complications of DC to >100MHz?

The question I was trying to answer was if a simple, widely available, 50 ohm terminator could be used.  A: It can, but there's a tiny amount of non-linearity on max signals, and you lose the overrange light.  Or, to stay in compliance with the probe output specs, build something passive with 1 or 2 resistors and a trimmer cap.

onlooker:
Right, I did not mean to add my own active components. The talk about x100/x1000 (/2) or x10/x100 (5x) was to say that, after all, mentally dividing by 2 or multiplying by 5 may not be that bad, though from the beginning, I did contemplate about what might be possible by some simple unofficial adjustments of the current PCB to reach x10/x100.

If at all possible, x10/x100 (5x) is more preferable than x100/x1000 (/2) since it reduces the attenuation that the device currently has and smaller signals will likely be less noisy (yes, small signal after P2505 appeared to be a lot more noisy).

As you guys had mentioned,  x10/x100 can be done by rearranging the voltage divider at the HV side, then, at the expense of de-rating the HV performance (but, how much de-rating, 5 times worse? I do not have a quick answer). Or, is there any room in the x50/x500 calibration pots on the PCB?

On the other hand, as I mentioned earlier, for now, I am just trying to make it usable with minimum effort. My eBay adventure just failed. The RG174 I got was junk. The shield has probably less than 60% coverage, that resulted to significantly high signal loss. The new hunt for lower loss RG174 is in progress and the price is likely more than double. 

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