HELP - HP 3562A DSA - negative spikes below nose floor - Page 1
Forum section: electronics / repair

author: innkeeper
author_rank: Unknown
timestamp: August 25, 2017, 10:39:44 pm
content: I'm looking for help on this, as i am in unfamiliar territory.I scored a HP 3562A in unknown condition.for the most part it seems fully functional, it needed a memory battery replacement, which i've done.didn't know how else to describe what i am seeing thoughpictures below. 2 with no input, and one with the source output connected to the first channel.This happens on both channels.I don't know if this is jsut how this DSA works or if it is a problem, and if it is a problem, what to start looking at.or if its just operator error.It is certainly not behavior as i am used to with my HP spectrum analyzer, and not something that's described in the Service manual or operations manual.no input (noise floor) auto rangeno input noise floor manual range -100 to -180dbsignal from source 500mv peak, 50khz, display 10khz span and manual Y 0.0db to -150db

author: Johnny10
author_rank: Frequent Contributor
timestamp: August 25, 2017, 11:13:13 pm
content: I have the HP 3561A single channel.The linear spectrum display shows this type of display.The range you have chosen may be why it shows those lines below noise floor.After doing some reading of the HP 3562A specs... it has quite different capabilities than the 3561.

author: innkeeper
author_rank: Unknown
timestamp: August 26, 2017, 05:15:06 am
content: do you see artifacts below your noise floor?

author: innkeeper
author_rank: Unknown
timestamp: August 26, 2017, 02:56:19 pm
content: i did a bit more playing today.I was able to get a clean full sweep with only 10mv output from the source, auto y scale, but as i ramped up the source output, the number of artifacts increased until it ended up looking like that last picture about 500mv as i recall.these artifacts go down to about -800db, so i know there truly artifacts of some type. I would find it hard to believe this is normal though, i've never seen a spectrum analyzer behave like this .. but if someone familiar with this DSA says it is i can accept that. if its not i want to find the cause.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 26, 2017, 03:51:31 pm
content: These could be just artificial effects of the logarithmic scale. With very low levels there is slight chance that due to meter internal noise the power reading gets close to 0 or even negative. This can happen if they try to extend the readings to very low levels, e.g. by subtracting offsets. This will result in very negative dB readings.I don't know that instrument, but at level of some - 100 dB I would not be surprised to see such artifacts in such an old instrument. So for me it would not be anything to worry about. The extend of the spikes might depend on the adjustment - less negative spikes might be more convenient, but might be also less accurate - having a higher background noise level.

author: Johnny10
author_rank: Frequent Contributor
timestamp: August 26, 2017, 06:03:29 pm
content: I was thinking... where have I seen a working 3562A and remembered that a youtube video on updating this unit.You might ask this gentleman if he has same experience.D-Lab

author: innkeeper
author_rank: Unknown
timestamp: August 27, 2017, 04:16:14 am
content: I'll see if i can shoot him an email.