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Data buffers on the 8757

Question asked by johnrf2 on Aug 30, 2019

I am currently working with an hp 8757e (https://d3fdwrtpsinh7j.cloudfront.net/Docs/datasheet/hp_8757.pdf ) along with a prologix GPIB-USB bus connector. Someone I work with has written some python to get the GPIB to successfully send messages to the 8757 and get replies. However, when we pull data off of the NA, the first array of data is only the beginning of the signal, when we call the command again, it shows us the rest of the previous signal, and the beginning of the next signal. Repeat 4 times, and then eventually it somehow continuously lines up the data, but we are unsure if it is offset or not. I am sorry if I am not being specific enough, I can give anymore details necessary. I am attaching images of what I mean for the data and plots if anyone has any intuition on why this issue is happening. We know that each channel has a 401 point cap, but we don't know why it only sends 401 points after about 4 calls. The first array is about length 60, then the next three range randomly from 397-407, then all afterwards are 401.

 

If it is not obvious from the photos, the first is just a short amount of data being pulled, then after it shows the correct screen print but just shifted over due to the overflow of data, over time they start becoming 401 in length and overlapping, and then in the final photo i show the comparisons between the original random ones and the ones that consistently overlap (the brownish line, which again, we aren't sure if the overlapping ones are even correct, or if it just balances out the overflow at that point to some random spot on the x-axis) Also, it seems the photos are in reverse order, so the last of the 6 is the first one. The one with only ~67 points and just a blue squiggly graph

 

If this is something with the coding, help would be appreciated but is not necessary, what I would like to know is if it is either and issue with the network analyzer, it's settings, or me completely misunderstanding how it parses data.

 

Thank you for any help or responses, everything is appreciated, apologies in advance for the unstructured post

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