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

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onlooker:
Thanks for all the suggestions and comments. I think I will do the easy part first, that is, to  do minimum rewiring  to make it usable again.

The first thing is the coax. I have just received some eBay version of RG174. Its small diameter is the reason for the choice. I need to have both the coax and the power wires go through the existing outlet.

From an online coax loss calculator, for a run of 5ft of generic RG174, the loss is about 0.1dB@10Mhz and 0.4dB@100MHz (comparing to RG58 with <0.1dB@10Mhz and 0.2dB@100MHz). 

Is a generic RG174 good enough or I need to have a low loss coax? Additionally, is this 0.1dB vs 0.4dB difference enough of a concern?

The other main part is the power. I did more tests. For a 50Mhz signal, the current consumption of the device is already more than 200mA, which is significantly more than that for a low frequency signal that I checked earlier.

Lastly,  two pictures are attached for the entertainment. The pictures are just quick checks of 10MHz aquare and triangle waveformss before (yellow) and after (red) P5205. They are not with proper termnation or best possible cabling; the square one looks particularly ugly.



Mechatrommer:

--- Quote from: onlooker on October 07, 2014, 04:19:24 am ---From an online coax loss calculator, for a run of 5ft of generic RG174, the loss is about 0.1dB@10Mhz and 0.4dB@100MHz (comparing to RG58 with <0.1dB@10Mhz and 0.2dB@100MHz). 

--- End quote ---
no. by the right mind, you should concern more on the impracticality of the bigger RG58, which you cant do pretty much anything about. the major artistic side of it is "to make it work" by hook or by crook, 58 or 174. i think you are fine with 174, just recompensate if the result is not good, thats the biggest part anyway.... knowing your stuffs ;)

edit: btw thanks for the pcb picture of the probe. if you can post the back side of it i will appreciate it. from the signal capture... triangle wave got roll off at peak and bottom, thats normal due to BW limitation, at 10MHz, triangle still got so many harmonics which your probe cant show. second picture... too much ringing on both red and yellow signal. i'm suspecting reflection (or black magic) on yellow stuff. not sure how you test it.

wait a minute, the yellow is original triangle signal and red is probe output? you have overcompesated probe ;) redo again with proper termination.

onlooker:
Ok, a picture of the bottom side is just added to the 1st post. The PCB has visible middle layer(s).

Yes, I will try RG174 1st, though the little leftover of the original cable shows it was more close to RG58 in size.

And yes, the P5205 output is closer to the true waveform shape; the yellow traces were taken before P5205 with an x10 probe having its tip touching one of the input terminals of P5205.

And yes, I found the problem, I did not use the probe GND connection since I  mistakenly thought the GND connection was already there.  In fact, it was indeed there, but it was in the form of a big ground loop going from the FG GND to the scope GND. This loop dumped the yellow traces and caused the delay of the yellows. 

After connecting the probe GND to another one of the input of P5205, the views are much better. Again the yellows are at the input of P2505; and the reds are at the output of P2505. Now the yellows are ahead of the reds.





Anyway, I think, if nothing else, these pictures showed that 1). the waveforms are centered well enough without needing any additional offset compensation (though for small signals, the offset is more pronounced); and 2). the amplitudes are roughly right.

BTW, I did not like the forced mental math with the x50/x500 attenuation settings. This is also a good motivation to change it to x10/x100 later. 

MarkL:

--- Quote from: onlooker on October 08, 2014, 12:34:21 am ---BTW, I did not like the forced mental math with the x50/x500 attenuation settings. This is also a good motivation to change it to x10/x100 later.

--- End quote ---
Does your scope not support manually entered attenuation factors?  Surely it must have x1 and x10, but not x50 and x500?  (Sorry, I don't recognize the make/model from your screen shots.)

onlooker:
Right, it has only x1, x10, x100 and x1000. It is the now "infamous" Owon SDS7102. Ok, I can do  divide by 2 or multiply by 5.

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