Electronics > Repair

Tektronix TCP202 current probe repair - Schematic and suggestions needed

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tautech:
 :-//
Why is the trigger level outside the waveform ?

MarkL:

--- Quote from: Weston on July 03, 2020, 11:34:56 pm ---I don't think I ever explicitly check the square wave response on my TCP202 when I got it, so I am unsure if this is a new issue or an old issue, but I have realized the square current step response is quite bad on my probe. It almost seems like a HF/LF compensation issue, but there is no way to adjust that. Here is a waveform where I compare against at CT-2 current probe.
...

--- End quote ---
I have some non-flatness with my TCP202, similar to yours but not as extreme.

Do you have 50R termination on in the scope?  I never used an R&S scope, but I don't see an indication of the termination.  Or maybe using an external terminator?

What do you get with different input frequencies?  Does it eventually flatten out with lower frequency?

If it helps for comparison, below are some screen shots measuring the current from a signal gen output into my scope's Ch1 with internal 50R termination set.  The TCP202 is attached to Ch3 with a Tekprobe II adapter which sets 50R automatically on the scope.  I'd like to tweak the probe to be a little better, but as you point out there are no adjustments, except for the offset and gain. The TCP202 specs provide a very wide berth for "System Aberrations 10% p-p", but I think your probe should be doing better than you show.

The AM503 service manual has some good background on how all these DC current probes work in relation to combining the signals from the Hall sensor and AC transformer.  It may help you understand where to dig.  Perhaps there are some factory selected components in the TCP202 that could be tweaked.

Weston:
I am using an external 50 ohm load, I have already checked everything in relation to the tekprobe power supply, scope, and load.

I saw the "System Aberrations 10% p-p", so  guess this is technically in spec? I guess I am just surprised its so bad.  Talking about this issue with a friend got them to check their AM502 + A6302 and they were seeing a similar response, although not as bad. Based on that + your measurements I guess its just bad compensation and not any damage to any of the active parts. I wonder if something drifted over time.

It just seems to be some mid-band weirdness, its better at LF or HF. I measured a bode plot today using a 50 ohm shunt.


I will have to read through the AM503 manual and see if I can figure out any ways to tweak the compensation networks, thanks for the advice!

Tantratron:

--- Quote from: Weston on July 04, 2020, 09:30:53 pm ---It just seems to be some mid-band weirdness, its better at LF or HF. I measured a bode plot today using a 50 ohm shunt.
(Attachment Link)

--- End quote ---
One issue or explanation, this is a hybrid technology probe where they mix or fuse the hall sensor with the current sensor outputs. Normally tektronix design should compensate actively and passively the effect to get it much less that 10% otherwise what is the point to pay so much.

Similar topic happens with Rogowski coil where you cascade or hybrid three types of integrator (two passive and one active)...

So far I've never owned any hall-Transformer probe plus as mentioned before I got unlucky with a purchase on eBay-Austria. I've been refunded thanks to PayPal protection but do you guys confirm acquiring TCP202 is still good investment for laboratory use now we find this non-flat bump response ?

For years I've been using P6021 and P6022 but these are only AC probes with incredible performance... the DC-AC philosopher stone is not easy to design or find !

Weston:
I spent what is probably too much time this weekend fiddling with the TCP202. I reverse engineered a good portion of the schematic and took a lot of measurements (as a side comment, this board is really weirdly routed, did they use an autorouter or something??). The pulse aberrations are due to the nulling amplifier, nothing in the passive output network of the TCP202 seems to have relevant time constants.

As previous comments have said, the hall effect amplifier is amplified by a OP27 and then further amplified and buffered by a MC1458 + PNP/NPN stage. The schematics for the AM503 and the AM503A have a similar configuration. One thing I noticed was that those units had adjustable feedback on the nulling amplifier output stage and had a R + C feedback network on the hall effect amplifier stage. The TCP202 actually has two sets of unpopulated pads for what I assume is a R + C feedback stage.

I took some interesting waveforms from the TCP202, it is clear that the nulling amplifier is under damped. Here is a capture of the input current (green) probe output (blue) output of the nulling amplifier (orange) and output from the hall effect sensor ( yellow) (orange and yellow  should be inverted).




I guess if I took bode plots of a lot of things I could calculate more optimal feedback network values, but I already sunk more time into this than I wanted to and was getting tired of it. So instead I just iterated through possible tweaks of the two feedback networks. I tweaked the feedback resistor on the output stage, lowering it from 10k to 4.3k and that seemed to make the biggest improvement. Installing a 140p + 83k network on the unpopulated pads seemed to contribute a small additional improvement, lots of changes there just made the waveform worse, including RC corner frequencies used in the AM503 and AM503B.

I changed the current level, so the scales do not exactly align, but you can see the waveform is now greatly improved!



I assume manufacturing tolerances on the hall effect sensor lead to a range of gain values, so it seems to make sense that by changing the loop gain elsewhere I could improve the step response. The hall effect resistance in my probe seems to be lower than typical, I after the 220 resistors that drop the voltage from 5V to the hall effect sensor I was only reading ~+-1.8V instead of the +-3V others were mentioning. I think this may be related to why changing the loop gain was beneficial in my case.

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