Help on understand wideband current probe like P6021 P6022 - Page 1
Forum section: electronics / projects

author: NWerner
author_rank: Contributor
timestamp: March 02, 2025, 10:49:09 am
content: Hello,Recently I got interested in clamp-on current probes like classic P6021 or P6022.Manuals include schematic for probe and termination - but little information is given about winding, core construction etc.No further information is given regarding trimmer functions.I'm a bit lost and a quick literature-research only added to the confusionCan someone share some insights or papers relating to the physics and workings of such probes.Thanks

author: KrudyZ
author_rank: Frequent Contributor
timestamp: March 02, 2025, 06:37:38 pm
content: This should be a good starting point:https://w140.com/tekwiki/images/6/62/062-1146-00.pdf

author: voltsandjolts
author_rank: Unknown
timestamp: March 02, 2025, 07:00:08 pm
content: Quote from: NWerner on March 02, 2025, 10:49:09 amI'm no expert, but AFAIK the main 'trick' for achieving wide bandwidth, linearity and dynamic range of an oscilloscope current clamp is:keep the flux in the clamp core near zeroThis is achieved with flux sensor and control electronics which drive a coil to oppose and cancel flux generated by the current being measured. Thus the probe characteristics are determined more by the probe electronics than the magnetics.Tektronix patents may have some info.

author: Weston
author_rank: Regular Contributor
timestamp: March 02, 2025, 07:11:47 pm
content: Never seen that tektronix document, it has some cool peaking circuits I have not seen before1Like any transformer, the current transformer is limited by magnetizing inductance at the low frequency range and limited by resonances and leakage inductances at the high end of the frequency range.For the low frequency range the trick is to use an extremely high permeability core. The P6021 and P6022 use an amorphous (~metglass) core with permeability at least an order of magnitude higher than standard ferrite. With this high a permeability even small scratches or dirt particles between the mating halves of the core can significantly shift the LF -3dB point upwards. DC bias will also lower the permeability and shift up the LF -3dB pint,At high frequency the limiting factor is often not discrete leakage inductance, but resonances within the current transformer structure itself. Most relevant is the "slow wave" helical transmission line resonance, which is related to the pitch of the windings and stray capacitance. This resonance can be damped by segmenting the current transformer winding and having a distributed termination resistance. Looking at that tektronix document it looks like the P6021 is split into 3 segments. Pearson current transformers also use this technique and the patent may be more insightful than the Tektronix app notehttps://patents.google.com/patent/US3146417A/enAnd then of course, they use various tuned peaking circuits like the tektronix app note explains. With modern oscilloscopes I wonder if it would just be easier to compensate for that part of things digitally....

author: NWerner
author_rank: Contributor
timestamp: March 03, 2025, 09:06:23 pm
content: As far as I can tell those pearson probe use ferrites and are usually optimized for higher-current wideband transformers.The physics of those are nicely decribed by mentioned patents.In contrast I have no idea how eg trimming of 1K pots in P6021 works. Manual is not very helpfull in this regard. It looks like quite some signal is lost due to signal divider formed from those pots and 62R termination. I clearly miss somethingAlso manual of P6021 mentions "U-shaped ferrite core" - but it doesn't look like a ferrite to me. Very shiny and not high-ohmic.

author: edavid
author_rank: Super Contributor
timestamp: March 04, 2025, 12:42:15 am
content: Quote from: voltsandjolts on March 02, 2025, 07:00:08 pmThis was only done in the DC current probes (P6042, A6302), and only at low frequencies, so it's not relevant to the AC current probes OP is asking about (or the high frequency response of the DC capable probes).

author: Terry Bites
author_rank: Super Contributor
timestamp: March 06, 2025, 07:40:45 pm
content: This was a great idea, its not clear who had it first. It works all the way down to DC and your error amp output gives a linearly scaled V/A.I've seen them used in monster cable kiloamp circuits, I think they used used flux gate sensors.

author: edavid
author_rank: Super Contributor
timestamp: March 06, 2025, 08:28:37 pm
content: Quote from: Terry Bites on March 06, 2025, 07:40:45 pmNo, Tektronix used and uses only Hall sensors for DC.