Programmed frequency switching time - Page 1
Forum section: electronics / projects

author: precaud
author_rank: Frequent Contributor
timestamp: July 04, 2018, 08:06:29 pm
content: (I'm not sure this is the right forum for this question, maybe Metrology or Test Equip would be better?)I'm considering setting up an automated "stepped sine wave" measurement. The simplified process would be:Set new frequency --> allow to settle --> trigger digitizer/DSO --> collect data --> rinse and repeatWhat I'm concerned about is the synthesizer/generator frequency settling time. After sending the new frequency (via GPIB or whatever), how long does it take to make the change and settle? For sake of discussion, lets' assume "phase-continuous switching", where the new freq's first phase point will be calculated from the old freq's last phase point.I looked through some generator data sheets and most companies don't specify it. They specify squarewave settling time but that is something else.The only generator I have here that specifies it is an HP 3325B. It specifies switching time as the time from receipt of command to when the output freq is within 1Hz of the programmed value, as:<10mS for a 100kHz step,<25mS for a 1MHz step<70mS for a 10MHz step.Are there others that spec better?How does one measure this?

author: JS
author_rank: Frequent Contributor
timestamp: July 04, 2018, 09:33:47 pm
content: Run your measurements with your sistem using a short piece of coax as your DUT. Take long samples in the range you expect this to be, analyze with a window till you find what you want...JS

author: precaud
author_rank: Frequent Contributor
timestamp: July 05, 2018, 12:56:46 pm
content: With that approach, there's no trigger from the receipt of command to start the clock.

author: JS
author_rank: Frequent Contributor
timestamp: July 06, 2018, 12:47:23 am
content: Quote from: precaud on July 05, 2018, 12:56:46 pmscope's 2nd channel?JS

author: DaJMasta
author_rank: Super Contributor
timestamp: July 06, 2018, 01:07:15 am
content: This is usually specified in RF generators, and there are definitely those that can do it faster. I think the way to check would be just hook it up to the scope, then maybe single shot capture with a trigger pulse to step the frequency up and measure it there. Add a little buffer and a little delay to process the command and you should have a good approach.I suspect sending individual frequency commands would be more delay than just a settle time (also needs processing time), so I would setup the sweep (if this is available) then trigger it with trigger pulses and just offset your digitizer from taking its reading a fixed interval from the trigger pulse, the delay interval being whatever you found in looking at it with your scope plus some margin.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 06, 2018, 09:19:09 am
content: For testing the manufacturer might look at the internal VCO control. It will show a step there and this step is easy to observe.Modern generators based of DDS have essentially instant frequency steps - maybe some minor time for the filter to settle, but that is usually well below µs.

author: precaud
author_rank: Frequent Contributor
timestamp: July 06, 2018, 01:12:38 pm
content: Quote from: DaJMasta on July 06, 2018, 01:07:15 amYes, and that's what I want to do. Capturing and examining the waveform period is the obvious part. The question is, how to trigger the scope to capture the entire event from the sending (or receipt) of the freq command. And mark it as T=0.QuoteThere are no external signals available that coincide with a freq change. Maybe I'll have to pick off one of the GPIB status lines?Quote from: Kleinstein on July 06, 2018, 09:19:09 amGiven its resolution and stability, I don't think the 3325B is a VCO-based design. But I could be wrong.QuoteThat was my thinking as well. But I haven't seen any numbers to back it up. If marketing an instrument for ATE applications, one would think that would be a really important spec to give. The only instruments (which operate up to 20MHz) I've seen it spec'ed for are the 3325x family and near-relatives (3336x).