Fluke 515A calibrator noise on resistance settings - Page 1
Forum section: electronics / general

author: guenthert
author_rank: Frequent Contributor
timestamp: February 04, 2024, 01:50:35 pm
content: I just acquired a (refurbished) Fluke 515A which seems to be in good nick. There is however quite some noise on the output when selecting high resistances. On the 10MOhm setting, it's in excess of 300mVpp @400Hz. Most DMMs seem to cope more or less (might yield a less stable measurement though), but my -hp- 3457A can't and just displays OVLD instead.The 515A came without batteries and w/o the battery charging PCB. Afaik, the batteries were meant to allow truly portable operation away from power lines and save on warm-up time. The specifications however don't mandate off-line operation. The +18V and -18V supplies, as well as the 14V and 10V references are pretty clean (else the other functions wouldn't perform so well methinks). Looking a bit more closely at the schematics, it appears the oscillator is never turned off (as long as the calibrator is on (*)), it just changes frequency and when no AC function is selected, it oscillates at 400Hz. Attaching a short wire to an oscilloscope's probe reveals that it radiates quite strongly with the case open (and shield removed). It seems the engineers at Fluke were aware of that, as there is a two-conductor cable with shield on the main PCB, but crucially, the cable connecting the high output with the resistor-selecting switch is not (and also happens to be fairly long).So, my question to the metrology community here is, what to do?- ignore the issue -- the attached meter ought to be immune to noise (you get what you pay for)- connect a capacitor with known, low leakage current in parallel with the calibrator when measuring resistance- modify the calibrator (solder above mentioned capacitor to the resistance selecting switch' wiper) (why didn't Fluke do that?)-*) Doh, just realized that theobvioussolution is to turn the calibrator off for resistance measurements. Not ideal, when one would like to measure multiple DMMs, but ought to work.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 04, 2024, 04:25:03 pm
content: Having mauch interference with the resistors suggest some problem with shielding, supply ripple of similar. If the problem is also with low restance there could be an isssue with a ground loop.It may be worth checking the supplies and capacitors, though this should normally be the first to check when doing a refurbishment. It may be worth checking solder joints on replaces capacitors in the supply.

author: guenthert
author_rank: Frequent Contributor
timestamp: February 05, 2024, 10:01:10 am
content: Quote from: Kleinstein on February 04, 2024, 04:25:03 pmForgot to mention, that the output noise is subdued reciprocal to the selected resistance.Ground loop was on my mind as well. I even tested (before opening the calibrator), whether it was mis-wired internally, assuming one of the power line connectors would be earthed, by turning around the (now) Schuko connector, but no, there's no difference and the wiring is in order. Further, all my instruments hang of the same wall outlet in order to minimize earth loops (and the noise is by far dominated by the 400Hz signal, 50Hz isn't even visible on the oscilloscope).Quote from: Kleinstein on February 04, 2024, 04:25:03 pmHidden above in the wall of text, I mentioned that the supplies seem clean. The device is shielded (double-walled, outer connected to earth, inner to guard, which itself is optionally linked to earth). The problem rather is, thatwithinthe device there is radiation, which the high resistance path is all too eager to pick up. There is a somewhat extensive area on the PCB which is on ground (verified), but clearly that's not enough internal shielding. The manual doesn't say anything about this, so I can't tell, whether it is within specifications (the -hp- 3457A wasn't around when the Fluke 515A was released, perhaps earlier instruments were also less fuzzy about noise over resistors -- after all, the performance verification procedure calls for a ESI 242D bridge; it's null detector isn't going to show the 400Hz).I wished the 515A would turn off the oscillator (but leave the references and peak-detector powered up) when no AC output is selected. Oh well, Fluke, next time!For now, I drop the idea of adding a capacitor, as it might unduly change the value of the high resistors and require a (meter-dependent) delay in the measurement til the cap is charged. I rather turn the calibrator off for the time I measure the resistors.

author: andrewtaylor
author_rank: Contributor
timestamp: February 05, 2024, 10:09:05 am
content: As Kleinstein already meintioned, shielding and good cabling is important, especially with HP3457A/3458 DMM.Try to measure caps as he proposed,and do not resolder joints if not really necessary.