8562E DC spur - Page 1
Forum section: electronics / testgear

author: jhenderson0107
author_rank: Regular Contributor
timestamp: May 26, 2015, 02:02:43 pm
content: I recently acquired an Agilent 8562EC spectrum analyzer which measures a substantial DC spur, regardless of input signal, as illustrated below. While some DC component is unavoidable, is this level typical for this instrument?

author: KJDS
author_rank: Super Contributor
timestamp: May 26, 2015, 02:09:46 pm
content: I've got a slightly broken HP8594E on my bench with a very similar DC level. I'll head to the storage lot later today and pick up a couple more and measure these and report back.

author: jhenderson0107
author_rank: Regular Contributor
timestamp: May 26, 2015, 02:13:20 pm
content: Thank you KJDS. All other aspects of the instrument tested so far work well, so I don't believe this is a defect. Perhaps the calibration guide for this instrument provides instructions to minimize the DC offset within the RF chain, to mitigate this effect.

author: jadew
author_rank: Frequent Contributor
timestamp: May 26, 2015, 02:50:33 pm
content: It's due to the fact that DANL changes with frequency and it gets worse as you approach DC.With a decently low RBW you should see that the DANL is in spec startingwith 100 kHz when AC coupled, orwith 30 Hzwhen DC coupled.Edit:I just tested the DANL at 9 kHz, AC coupled, with a RBW of 1 Hz. It's -130 dBm (better than the spec).Edit 2:Looks like AC coupling has no effect on DANL, it only affects the frequency response, which is obviously normal.

author: jhenderson0107
author_rank: Regular Contributor
timestamp: May 26, 2015, 04:19:43 pm
content: jadew -Thank you for your response. I am not questioning the instrument noise floor, which appears similar to the trace you attached in your response.Rather, I am trying to ascertain the typical amplitude of the the DC spur. The instrument trace you posted indicates a DC component in excess of the specified reference level of -50 dBm. What is the amplitude of the DC component if the reference level is raised to 0 dBm?

author: jadew
author_rank: Frequent Contributor
timestamp: May 26, 2015, 04:37:24 pm
content: -10 dBm and goes down to -20 dBm with 0 attenuation.

author: jhenderson0107
author_rank: Regular Contributor
timestamp: May 26, 2015, 04:40:37 pm
content: Thanks, that's exactly what I see on my unit. So, this is probably typical behavior.

author: rastro
author_rank: Frequent Contributor
timestamp: May 26, 2015, 05:15:56 pm
content: This DC signal is mentioned in the Signal Path video on repairing HP8562B starting @ 9:00:https://www.youtube.com/watch?v=hkedOOij8X4The explanation is the LO feed-through signal shows up as DC. This appears to be a normal artifact.

author: jhenderson0107
author_rank: Regular Contributor
timestamp: May 26, 2015, 05:28:13 pm
content: rastro -Thanks for the reminder. I did see that video, but Shahriar comments that the DC component measures -45dBm, which seems "very low". I am trying to determine the "normal" level of this LO feedthrough component.

author: jadew
author_rank: Frequent Contributor
timestamp: May 26, 2015, 05:58:07 pm
content: @rastro,That's very interesting, I always assumed it's not a real signal.

author: The_Praiodanish
author_rank: Newbie
timestamp: May 26, 2015, 07:54:27 pm
content: Hi,on my 8562A and 8560E it looks rather similar to your plot so I would say that's normal.