Inside a Tek CT-1/2 - Curious about high frequency current transformers - Page 1
Forum section: electronics / testgear

author: ezalys
author_rank: Frequent Contributor
timestamp: March 26, 2018, 07:35:35 pm
content: Has anyone disassembled a tektronix CT-1 or CT-2? I'm curious what the actual current transformer looks like and how they got the bandwidth of the probe to be so wide. I'm curious if it's air-cored or uses some magnetic material in there. Curious about any insights as far as how you'd make a 1 GHz current probe even if you haven't seen inside these specific probes.

author: tautech
author_rank: Super Contributor
timestamp: March 26, 2018, 08:03:14 pm
content: Check out this great old book about Tek probe design:https://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdfP54 and on.

author: ezalys
author_rank: Frequent Contributor
timestamp: March 27, 2018, 12:31:22 am
content: Ooh this is an excellent reference. Of course, they don't seem to give away what the ferrite is made of

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 27, 2018, 12:12:06 pm
content: High frequency is determined by geometry; low frequency is usually determined by (drumroll please!).... nanocrystalline cores.Tim

author: ch_scr
author_rank: Frequent Contributor
timestamp: October 11, 2021, 02:29:06 pm
content: Reading the indicated part of the "TekProbeCircuits" and the datasheets carefully, matching information, one can learn the following:The P6040/CT1 is described as:"5 turn secondary with inductance of 125 uH"and"drives a terminated 50cable which is shunted by a 50"while the P6041/CT2 described as:"25 turn coil wound on a ferrite core shunted by 50resistor"and"inductance of the secondary 3.125 mH"which in both cases gives an core Al value of 5e-6 - indicating they might have used the same ferrite core for both probes.The attached datasheet of the ct1 (courtesy of tekwiki) indicates the use of a compound structure for the "50termination",composed of: a 430resistor, a 59resistor, a "peaking coil" and also additional tiny "cores" and "toroids".The "cores" might be ferrite beads, and the 430resistor looks different from the 59one - maybe the 430is a carbon comp used to de-Q an inductor?The exact circuit is not decipherable but it looks like some kind of balun, transformer or both.A DIY probe with relaxed bandwith requirements might do with just a 47termination and maybe a common mode choke around the coaxial cable?

author: Hydron
author_rank: Super Contributor
timestamp: October 12, 2021, 09:57:45 am
content: Quote from: T3sl4co1l on March 27, 2018, 12:12:06 pmMy split-core Pearson CTs (which have similarly large upper vs lower -3dB BW ratios, at in least the less-compromised non-split versions, though they don't go as high as 1GHz) are made from VERY thin laminations, but I wonder if they pre-date the availability of nano-crystalline stuff? Also, as thin as the laminations are (they need some magnification for their presence in the otherwise polished face to be obvious) I'm not sure they're as thin as nano-crystalline gets.The split core Tek CTs are ferrite cored, but you don't get an amazing low-freq response unless you have either the DC version (with hall sensor and nulling current) or use the active amplifier option with the AC probe.

author: tautech
author_rank: Super Contributor
timestamp: October 12, 2021, 11:35:09 am
content: Quote from: ch_scr on October 11, 2021, 02:29:06 pmBest I could tell from your diagram was from the year 1986.CT-1, 2 and 5 specs attached.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2021, 02:26:56 pm
content: Quote from: Hydron on October 12, 2021, 09:57:45 amPossible they were made with permalloy, but that's pretty fine foil to be rolling; not that it can't be done, just I'm not aware it's normally available. Amorphous strip is made in about that thickness (around 0.001" as I recall, maybe a bit less), and yup, it looks just like any spiral wound alloy, except it's very brittle.I suppose very high magnification on a polished face would be telling. Any other alloy, you should find crystals at 100x+. After etching that is. X-ray diffraction would be diagnostic but I'm not sure it can be done without damaging the device.Tim