Tektronix A6302 Current Probe Analysis and Modification - Page 1
Forum section: electronics / testgear

author: Hexley
author_rank: Regular Contributor
timestamp: May 20, 2021, 10:56:12 pm
content: A defective Tektronix A6302 current probe found its way to my lab bench recently. It **almost** worked, as can be seen in Figure 1. Driven by a 100 Hz, 100 mA p-p signal it showed only 84% of the proper amplitude, and had pronounced spikes on the waveform edges.While I can measure high-frequency current with a different probe and amp, I wanted the DC and low-frequency measurement capability of the A6302. So the question was whether its performance could be improved at all?A thorough exam of the core showed that the ferrite surfaces were smooth and mated perfectly, and that no obvious cracks could be seen. It seemed mechanically intact.So I decided to characterize various electrical aspects of the probe while searching for the root cause of the problem. Fortunately, a good A6302 was available for comparison.First, a quick review of the probe’s architecture: it has a ferrite core that surrounds the current-carrying test lead by means of a sliding cap. The core encompasses a probe coil and a Hall device. The coil senses high-frequency signals and also provides a means to generate a nulling flux. The Hall device measures net core flux.The turns count, N, of the probe coil was measured by passing a 100 mA square wave through the probe and measuring the coil’s output current into a short circuit. It was 2 mA p-p, indicating N=50. The coil in the bad probe also measured N=50, as shown in Figure 2.So the problems were not due to a shorted probe coil – the coil of the bad probe was intact.The next test was to look at the AC response of the Hall device. A Bode plot was generated using a 50 mA drive current from 10 Hz – 100 kHz. Output was measured after the Hall device preamp in an AM503 current probe amplifier, which was operating in an open loop condition. Figure 3 shows the results for a good probe. (Please ignore the phase wrap-around at low frequency; the artifacts reflect shortcomings in my Python script.)The bad probe has a worse response, as shown in Figure 4 – the low frequency response is reduced, and there is mild peaking before the roll off begins.Massaging the data in Excel produced a graph of the amplitude difference between good and bad probe, as shown in Figure 5.This suggested that a “bad probe compensator” amplifier with about 11 dB gain and an 11 kHz roll off could correct the bad probe to better match the good one. Such a circuit was built and installed in the AM503. The new result for the bad probe + compensator was as shown in Figure 6. The low frequency response was improved, though the slight peaking remained.Unfortunately, the closed-loop response of the A6302/AM503 barely changed. The probe was still reading too low. It seemed that the overall loop gain of the probe and amplifier was sufficient to swamp the effects of even a 10 dB change in Hall device sensitivity.So the defect was not due to a reduction in absolute sensitivity of the Hall device.[End of part 1]

author: Hexley
author_rank: Regular Contributor
timestamp: May 20, 2021, 11:03:38 pm
content: [Part 2]Having ruled out a shorted coil or a weak Hall device, it appeared that something subtle was going on to cause the probe to read low.A quick review of how the probe and amplifier work to measure DC current is in order:An applied test current induces a magnetic field in the probe core.The Hall device produces an output voltage that is proportional to the field.The DC feedback loop through the AM503 amplifies the Hall device output, applies it across the probe coil, and terminates it with a fixed load resistance.The current through the probe coil sets up an opposing magnetic field in the probe core.The feedback loop works to null the net magnetic field. At null, the current through the probe coil will be exactly 1/N times the applied test current.The nulling current flowing into the fixed load resistance therefore makes a voltage that is proportional to the applied test current.The probe was sensing a nulled field with **lower** current through the probe coil than expected. No physical model jumped to mind. But this was something that could be tested.An experiment was set up drive a test current through the probe, to apply various amounts of nulling current through the probe coil, and measure the output of the Hall device over these changing conditions. See Figure 7.The results for the good probe in Figure 8 are as expected. The good probe shows (a) a good null is obtained when the probe coil current is 100% of normal [i.e., is exactly 1/50th of the input current], (b) there is good symmetry around y=0 as the probe coil current ratio is varied on either side of the ideal value, and (c) the response is symmetric around x=0 as the center conductor current changes from negative to positive.The bad probe, on the other hand, shows (a) the best null happens when the probe coil current is 90% of normal, (b) there is skewed symmetry around y=0 as the probe coil current ratio is varied on either side of the ideal value – that is, the preamp output has greater magnitude for 125% coil current than for 75% coil current-- and (c) the response is symmetric around x=0 as the center conductor current changes from negative to positive.The behavior of (a) explains why the AM503 produced low readings at DC – null was achieved at lower coil current than expected. Note also that the peak magnitude of the output is about half that of the good probe. See Figure 9.Having found the root cause, the next question is what to do about it?The probe and amplifier were simulated in LT Spice, and it was found that bypassing the probe coil with a suitable resistor would reduce its sensitivity. Adding an inductor in series with the bypass resistor improved the low-frequency transient response.The modified bad probe, with 500 uH and 7.2 ohms across the probe coil, shows in Figure 10 how the modification changes the response. The results show (a) the best null happens when the probe coil current is just under 120% of normal, (b) there is even more skewed symmetry around y=0 as the probe coil current ratio is varied on either side of the ideal value, and (c) the response is still symmetric around x=0 as the center conductor current changes from negative to positive.The closed-loop response with the final version of this modification to the A6302 works as expected, as shown in Figure 11. The indicated levels are correct now. The transient response is not perfect but is adequate for low frequency use.As a practical matter, however, only the resistor fit inside the A6302 probe housing. The toroidal inductor (56 turns on an FT37-43 core) was simply too large to squeeze in. (SMD inductors were considered, but their internal resistance tended to be much too high for this application, where only 5 ohms of total series resistance is allowable.) So the final modification to the probe was simply the 4.9 ohm resistor across the coil.The resistor was attached directly to the rear of the potted ferrite plug-in module as shown in Figure 12.Removing the toroidal inductor degraded the transient response a bit, but the probe is still useful from DC up to a few hundred Hertz. So the original goal of getting DC current measurement capability was achieved, and the A6302 probe was saved from the scrap heap.It is unlikely that other defective probes will exhibit exactly the same behavior as mine did. But I hope that some of the measurements above will be helpful to people who want to investigate their own defective probes.# # #

author: alm
author_rank: Super Contributor
timestamp: May 23, 2021, 09:53:12 pm
content: Thanks for documenting and sharing your experience!Have you considered that the probe head might have been magnetized by a high current in the past? Did degaussing make any difference? I guess it could be that the built-in degaussing operation is not sufficient?Hall effect sensors are also great strain gauges, so otherwise it is plausible that the probe experienced a shock, as a result of plastic deformation due to that shock, the sensor is experiencing a permanent strain that causes the offset. I indeed don't see a way other than external compensation to fix this.

author: Hexley
author_rank: Regular Contributor
timestamp: May 24, 2021, 02:02:05 am
content: Quote from: alm on May 23, 2021, 09:53:12 pmThis seems the most likely explanation for the "bias" that is shown in the Hall response curve set. Perhaps a really large degaussing field would help. Maybe I should wind a coil just for that purpose, and drive it from a high-current filament transformer with a variac on the front end...

author: Calvin
author_rank: Regular Contributor
timestamp: May 24, 2021, 05:36:18 am
content: Hi,I had issues with the degaussing circuit in my P6042a.The only(!) cap broken in the whole circuit was the one that powers the degaussing oscillator.Still though after replacing it with a cap of same value the oscillation period remained smaller than the manual stated.Sims of the circuit also suggested a ~doubling of the cap to achieve the correct period.regardsCalvin

author: Amphion
author_rank: Contributor
timestamp: June 02, 2022, 05:25:30 pm
content: See my post here, in case this is relevant.https://www.eevblog.com/forum/repair/tektronix-tcp202-current-probe-repair-schematic-and-suggestions-needed/msg4211593/#msg4211593