Sweep time vs screen refresh rate - Page 1
Forum section: electronics / projects

author: videobruce
author_rank: Frequent Contributor
timestamp: June 29, 2013, 09:00:08 pm
content: I'm trying out a Rigol DSA 815 spectrum analyzer and have found using any sweep rate below 100m/sec makes no difference in the refresh rate of the sweep.On a older SA (LCD display), it allows down to 20 m/sec and that clearly shows a difference where when I drop below 100 m/sec the trace jigger (if you can call it that) looks the same.Switching between 100 & 150 m/sec then shows the difference with a slower refresh rate. Note, both scopes were set to the same parameters and were feed the same signals.Comments?

author: w2aew
author_rank: Super Contributor
timestamp: June 30, 2013, 04:22:21 am
content: Quote from: videobruce on June 29, 2013, 09:00:08 pmThere are a lot of factors that influence the update rate on a spectrum analyzer. Many won't allow a sweep speed faster than a certain value for a given RBW setting. The reset time to tune the internal LOs back to the beginning of the sweep will also affect the update rate even if the sweep speed is fast. The LO/IF architecture can vary dramatically between different spectrum analyzers, so the tuning speed will also vary. These are just some of the variables affecting spectrum update rate, there are many more.

author: videobruce
author_rank: Frequent Contributor
timestamp: June 30, 2013, 10:27:30 am
content: Thanks for the reply, but that really isn't answering the question why sweep times slower than 100ms do not show the increase in speed on the display..

author: w2aew
author_rank: Super Contributor
timestamp: June 30, 2013, 10:08:30 pm
content: Quote from: videobruce on June 30, 2013, 10:27:30 amI assume you mean ...sweep times *smaller* than 100ms...Probably one of two reasons:1) The spectrum analyzer may limit how fast the sweep can be for a given resolution BW setting, in order to ensure that the filter is given enough time to charge.Or2) The update rate might actually be being limited by the time it takes to reset the sweep.

author: videobruce
author_rank: Frequent Contributor
timestamp: July 01, 2013, 01:38:54 pm
content: Ok, but then why show rates less than what the display actually shows? Kinda deceiving isn't it? If it can only do 100ms, then that should be the shortest span setting.Anyone else seen anything like this with any scope SA or plain old oscilloscope??

author: w2aew
author_rank: Super Contributor
timestamp: July 01, 2013, 04:43:55 pm
content: Quote from: videobruce on July 01, 2013, 01:38:54 pmI would have thought that if the SA was limiting the sweep speed based on RBW, that it would provide an indication that it was doing that. If it is not limiting the sweep speed, then it may be sweeping as displayed, but the reset to a new sweep might be the major factor in the update rate and why it appears it isn't changing. Unfortunately I can't comment specifically on your SA, because I've never used one of them. Most of the higher end SAs I've used will automatically adjust the sweep speed as a function of RBW and SPAN. They let you manually override this, and will warn you when the sweep speed is too fast for a given RBW setting.

author: videobruce
author_rank: Frequent Contributor
timestamp: July 01, 2013, 05:38:15 pm
content: Attached are screen shots of the two SA's. All the settings including RBW & VBW are the same with the same feed and start/stop frequencies. Unfortunately, I can't show a real time display, but under 150mS the speed of the trace to reflect signal level changes does bot change on the Rigol. It does on the IFR.I doubled checked everything to make it a apples to apples comparison as much as I could considering the two different price classes and time frames involved.

author: w2aew
author_rank: Super Contributor
timestamp: July 01, 2013, 06:31:42 pm
content: Thanks for the screen shots. It's clear that the IFR is advising you that for the given sweep time, it suggests that the RBW needs to be increased for accurate results. I don't see any such indication on the Rigol. So, it is either ignoring the sweep time you asked for, or it is giving you that sweep time and the display update rate is being driven by how fast it can reset the LOs for the next sweep (which may be on the order of 10-100ms). I wonder if you can visually see a difference between a 100ms sweep time vs. a 10ms sweep time, given how fast it updates the LCD screen... It appears really to be a question for Rigol tech support to understand what it is really doing. I would expect that 10ms sweep would be too fast for a 100kHz RBW.On on older HP swept SA that I have here, using a 83MHz span and 100kHz RBW, the auto-coupled sweep time is just over 20ms. If I manually adjust it to be faster, it lets me do it, but also gives the warning that the measurements are uncalibrated.

author: videobruce
author_rank: Frequent Contributor
timestamp: July 02, 2013, 04:38:12 am
content: QuoteI can see each time I change the sweep time on the 2399 by looking at the 'ripple' of the 'haystack' of a 8VSB signal. It clearly shows the difference by the 'activity' of the trace which it does not with the 815.QuoteThat is my next task, but the guy I have been talking to won't be back until next week.QuoteIt's not too fast for the IRF at 20ms.