Logic Analyzer Displaying False Data - Page 1
Forum section: electronics / repair

author: ReedTriesToRepair
author_rank: Contributor
timestamp: March 27, 2023, 07:09:11 pm
content: Disclaimer: I have next to no experience with logic analyzers, but I have quite a bit of experience with oscilloscopes.I recently inherited an Agilent 16804A LA and had to do some work to get it to boot up and work (a story for another time perhaps). In trying to test it, when I turn on a bunch of channels on the various pods and ask it to run, the analyzer populates it with data. But the analyzer isn't connected to anything, so I'm not sure where this data comes from. It seems to do it regardless of what the threshold is set to (TTL 1.5v or CMOS 2.5v). The data being collected seems to change a little bit in each run, but not significantly. It happens whether or not I have the pod breakout cables plugged into the slots.Any advice? Is it broken, or am I just clueless about LAs?

author: wraper
author_rank: Unknown
timestamp: March 27, 2023, 07:14:02 pm
content: As it's not connected to anything it just picks up noise.

author: ReedTriesToRepair
author_rank: Contributor
timestamp: March 27, 2023, 07:23:34 pm
content: Thanks for your reply. Is there an easy way to connect it to something measurable? I have access to a basic function generator. I suppose I could just feed some of the channels a 3v square wave to measure?

author: Andy Watson
author_rank: Super Contributor
timestamp: March 27, 2023, 07:56:38 pm
content: Quote from: ReedTriesToRepair on March 27, 2023, 07:23:34 pmGround would be my first reference signal.

author: ReedTriesToRepair
author_rank: Contributor
timestamp: March 30, 2023, 07:01:44 pm
content: I connected all of Pod 1 to ground via some jumper wires (I wrapped them in foil for the measurement), and got the attached measurement photo. It looks like after M2, all channels read "0" for about 40ns or so, then the noise continues. Is this something to be concerned about?

author: tggzzz
author_rank: Super Contributor
timestamp: March 30, 2023, 07:50:35 pm
content: Your picture shows the inputs floating.The foil will be irrelevant. If you used the foil to connect the inputs together and to 0V, that won't work.Show a picture of the self-test results.Have you read the user manual and tried one of the simple examples in that?Give pictures showing how you have setup the inputs for the pod you are testing.Connect a single input directly to ground, and set the display to show only that input.

author: ReedTriesToRepair
author_rank: Contributor
timestamp: March 30, 2023, 08:30:35 pm
content: Thanks for your input.As I said, I wrapped foil around the pins of the leads. I grounded the foil for my test.The self-test is perhaps revealing, good suggestion. It fails 7 tests:Memory Data Bus Test (Failed in slots AB)Memory Signals Test (Failed in slots AB)Memory Unload Modes Test (Failed in slots AB)Dma Test (Failed in slots AB)Hardware Accelerated Search Test (Failed in slot A)Analyzer Chip Memory Bus Test (Failed in slots AB)System Clocks (J/K/L/M/Psync) Test (Failed in slots AB)Lots of things fail, I increased self test verbosity to the max and uploaded the results of the test. Not sure exactly how to interpret the results.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 30, 2023, 09:10:07 pm
content: Checking the power rails would be a good start.

author: tggzzz
author_rank: Super Contributor
timestamp: March 30, 2023, 10:15:47 pm
content: Check power rails for voltage and noise.Look carefully at each and every aluminium electrolytic capacitor, especially in the PSU, checking for bulging or vomited electrolyte.Reseat all the connectors. They may have worked loose and/or have some oxidation.Throw that foil out. Unlike scope probes, digital inputs are relatively low impedance. Aluminium foil contacts will be too high impedance.