Audio Precision ApX 525

[DavidTheElectrician]

I just recently completed my education as an Electrician and now suddenly find myself in an IEC 17025 laboratory, so please bear with me for being a complete newb.

I have recently calibrated a bunch of Audio Precision ApX 525's, but I stumple upon the same anomaly on every single one of these particular instruments.

The instrument specification gives an uncertainty of+/- 0,008 dB at 10 Hz relative to 1 kHz (voltage not specified but I have been using everything from 1 V p-p to 10 V p-p). However, every time I approach 10 Hz the measurements begins to flucuate wildly between approximately - 0,004 dB and -0,034 dB (relative to 1 kHz). 11 Hz is just fine. 10,8 Hz fine as well. At 10,6 Hz the readings, however, starts to become erratic.

I don't think the issue is related to the signal generation: I have tried using a Fluke 5522A, a Keysight 33250A as well as the ApX 525's own generator output. However, the issue remains the same.

I have also tried running the self diagnostics tests (generator to analyzer) and it came out as passed.

Also, every single one of the other 100's of calibrations points I put the ApX 525 through is within specs as well.

I have been in contact with Audio Precision (sharing my project file with them to rule out any setup errors in the software controlling the instrument) and they simply can't duplicate the issue (even using my project file).

So far Audio Precision has suggested that the instruments needs an adjustment, however, isn't it rather unusual that every single ApX 525 that passes my lab can't pass the aforementioned test?

This leads me to the conclusion that I surely must be doing something wrong... I just don't know WHAT that might be.

I am still new to electronics, and hence I have yet to understand what goes on behind the hood of the instruments.

So, what I am asking is this: Any ideas as to what might be going on?

Thank you.

[DavidTheElectrician]

Here is a video showing what is going on:

Also, it doesn't seem to matter whether I use XLR or BNC cable from the balanced I/O or BNC from the unbalanced I/O.

[blackdog]

Hi DavidTheElectrician, 

There are two things I think of in the effect you show.
It could be some kind of aliassing or that you are on the edge of a lock or control loop.
Audio precision dont share schematics, for my Audio Precision Portable One, I have never been able to find schematics or a service manual.

You may have found a little problem that AP may not want to admit, I've had the same with one other major measurement tool brand.
Let us know especially how this turns out.

Kind regards,
Bram

[MiDi]

Could also be an intermodulation e.g. of 50Hz mains and 60Hz (ADC aquisition time, LCD refresh rate).
AP is situated in the states where mains has 60Hz, that may be a point here.
If source is external it should have lower effect at 10V input in comparison to 1V, if not, source is likely internal.
What about the frequencies below 10Hz and multiple of 10Hz (20Hz, 30Hz, 40Hz, 50Hz)?

[DavidTheElectrician]

MiDi and Blackdog:

Thank you very much for your input.
I did some research on your suggestions, however, it eventually turned out to be a software issue (we haven't upgraded the Audio Precision Software as newer versions don't come free).

I will copy/paste the solution provided by the the Audio Precision technical service here (awesome support, by the way):

Hi David,

I agree that would be very unusual unless it has been some time since all these analyzers were calibrated.

I just performed the same steps in your video in v7.1, albeit with the internal "loopback" option instead of an external cable, and the values acquired at 10 Hz were within spec.

I then went back to v4.4 (since you are running 4.1.1 and this is the oldest version that can run on 64 bit Windows) and was able to reproduce the behavior you have described using the same steps and hardware. This made me suspect that there is some kind of software issue that has since been resolved in newer software.

Looking into it some more, I found that the readings at 10 Hz are within spec when I make an RMS Level reading in "Level and Gain" instead of Signal Path Setup.

Then I remembered something I should have told you earlier: in Sequence Mode, if you want to take a reading, it is better to run the measurement in a sequence then to take the live reading off the panel, as live reading will be unsettled.

Please try checking the Signal Path Setup RMS Level result in the Sequencer, running a sequence, then viewing the acquired value in the displayed report. If that is still not within spec, or even just for the sake of comparison, you might try running the same measurement in Level and Gain as well. I expect you will find the deviation from 0 dBrA to be much less with this approach.

I do not know why the Signal Path Setup produces a worse reading that Level & Gain, however this software version is very old, and this behavior has since been resolved in newer software.

If you still see readings outside the spec at this point, then I would ask two things:
1. Try again using the internal loopback setting instead of an external cable.
2. Please check the calibration dates of these analyzers. If it is not within one year, then I would suggest sending them in for service.