Viavi JDSU JD785A Calibration/Harware Issue on Startup - Page 1
Forum section: electronics / repair

author: Teknishun
author_rank: Newbie
timestamp: February 24, 2024, 03:58:10 pm
content: Hello good people of the EEVblog Forum.I've seen that there are some members here who know this equipment quite well here. I'm hopeful someone will know the answer to my question and I won't need to do too much guesswork and sleuthing. Either way, thanks for these great forums and thanks for taking a moment to read my post.My description of the issue would be as follows:On startup, but not always (see below), the calibration or perhaps some other issue with the hardware and/or software causes the CAA (Cable and Antenna Analyzer) to come up with a bad return loss plot (this would affect all other measurement modes of course, VSWR, DTF, Cable Loss, etc).The first photo shows the bad behavior and the second photos shows the correct behavior. Both plots are taken after start up with no other keys pressed. I do have it set to load the user settings, but that doesn't change the behavior. Default settings would produce the same issue. Note that the bad behavior shows that it did load my manual calibration and says calibration status "off", but this also it not always true as it will say Calibration Status "on" sometimes and still have a crappy plot.I feel like this issue started after updating to the most recent firmware, 3.120.043, but I cannot be 100% sure as I wasn't using often enough before the update. Also I have the impression that it could be temperature related as it never seems to start up correctly until after several restarts. i.e. it's always in the bad state on initial and several subsequent restarts. It wouldn't surprise me if one day it came up correctly on the initial startup however. So far it always takes many restarts to get into the correct "good" state.Thanks and all the best!Teknishun

author: nctnico
author_rank: Super Contributor
timestamp: February 24, 2024, 04:11:26 pm
content: I assume both measurements are taken with the same cable from input to output? And which input are you using? Vector or scalar mode?It could be a cracked soldering joint somewhere. But to know where to start looking, you need to test both vector and scalar modes with the error present.

author: Teknishun
author_rank: Newbie
timestamp: February 24, 2024, 04:23:53 pm
content: Thanks, since it's the return loss port, S11 basically, you're seeing full reflection with no cable. So yes the hardware is exactly the same each time, the analyzer only with no cables. I kind of doubt that there's a crack or break somewhere as once it's in the good state i can carry it around, bump it and handle it with no issues. Of course we've been fooled before by such things...

author: zrq
author_rank: Frequent Contributor
timestamp: February 24, 2024, 10:50:45 pm
content: I'm not sure but is it possible that when the reflection is not calibrated, the reading should be expected to be off? Although -18 dB at low frequency might be a bit too much.When started my JD785A in CAA mode without calibration, I get a return loss like this.

author: Teknishun
author_rank: Newbie
timestamp: February 27, 2024, 02:48:45 am
content: Thanks but unfortunately no, not that simple. When it's in this condition it will not calibrate correctly either. Recalling or performing a new calibration ends with poor results.