Agilent 33500 series AWG and Trueform technology - Page 1
Forum section: electronics / testgear

author: jpb
author_rank: Super Contributor
timestamp: May 11, 2013, 11:10:02 am
content: The Agilent 33500 series of arbitrary waveform/function generators come up on e-bay fairly often - and on Agilent prime and the Trueform technology looks interestingsee here for details:http://cp.literature.agilent.com/litweb/pdf/M8190-91050.pdfThey get around the jitter problems of fixed clock arbs (like the Rigol DG4000 series) by upsampling and then filtering and resampling which allows a fixed clockrate.They claim it has all the advantages of fixed clock DDS (simple and effective image reject filtering and low cost) and of variable clock arbs (low jitter and no missed points).BUT I am slightly cynical and notice the following two things about the specs for the 33500 series:1.) Arbs cannot be swept and can only be amplitude or phase modulated NOT frequency modulated.2.) A settling time of <200 ns to 0.5% of final value is listed.I have a feeling that these two are related but I struggle to understand what this means in practice.Settling time is easy to understand in terms of say a step change, but what exactly does it mean in terms of a constantly changing waveform? I would guess it means that the upsampling and filtering and resampling introduces a 200nsec delay - you essentially have some sort of pipeline (in computer terms) which perhaps needs to be flushed if frequency is changed.If this is true then I'm not sure how great this approach is when compared with say using a RigolDG4062 at step-by-step mode and just presampling the waveform before loading it into arb memory. (I'm ignoring the advantage of sequenced waveforms and the fact that it has a lot more memory than the Rigol - I'm looking to understand the fundamentals.)It also seems to be a bit of a disadvantage compared to a true arb which can rapidly sweep the sampling rate to give frequency changes and modulation.Some unbiased technical assessment from clever forum members would be much appreciated!

author: nctnico
author_rank: Super Contributor
timestamp: May 11, 2013, 12:20:55 pm
content: In a nutshell: A played back waveform must be filtered depending on the samplerate otherwise you would see steps in a sine wave. The output filter is usually taken into account when creating an arb. waveform so the samplerate must be fixed.

author: jpb
author_rank: Super Contributor
timestamp: May 11, 2013, 02:27:37 pm
content: Quote from: nctnico on May 11, 2013, 12:20:55 pmThanks for replying, but I don't understand what point you are making. Yes Agilent (as do others) does pre-distort some waveforms to account for the characteristics of the filter and this is much easier with a fixed sample rate. Also an image reject filter to remove the effects of the steps in the DAC is needed which is true also for variable clock arbs such as the Tabor WW5061, but that doesn't really answer my question - probably I didn't make myself clear enough.To rephrase my queries:In the trueform technology the process is :i.) the arbitrary waveform samples from memory are expanded on the timescale by introducing blanks between and then filtering to essentially produce a lot ofinterpolated points in-between - the memory point separation is dependent on the selected sample rate which is arbitrary but less than or equal to the maximum rate of250MS/secii.) this new waveform (or rather set of sampled points but with a very small time step equivalent to perhaps several GS/s) is resampled at 250MS/seciii.) the output is now filtered (again) to remove images arising from the 250MS/s DAC.So the process allows arbitrary data sampling rates up to 250MS/s but always operates the DAC at 250MS/s - a clever system.My questions are -1.) Given that you can have any sample rate, why can't you sweep through a range of sample rates? (You can manually do runs at different sample rates but youcan't apply a sweep I think - at least not according to the data sheet.)2.) What is this settling time of 200nsecs? Data sheets for other arbs don't mention settling time so it seems to be an intrinsic part of the Trueform process.I guess I ought to address my questions to Agilent but I was wondering if anyone on the forums had experience of the system and its pros and cons versus true arbs with variable clocks and standard DDS systems with fixed clocks.

author: jpb
author_rank: Super Contributor
timestamp: May 17, 2013, 12:15:36 pm
content: Just as an update.I asked this question on the Agilent forum and Agilent has confirmed that you CAN do a sweep of an arb so the data sheets are currently incorrect.I still don't have an answer as to what the 200nsec settling time means in practice in the context of a time varying waveform.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 17, 2013, 01:51:51 pm
content: Quote from: jpb on May 11, 2013, 11:10:02 am0.5% of 10 million is 50,000. Does .05% of the final value mean that a 10MHz signal setting could wind up being 9.950MHz to 10.050MHz?

author: jpb
author_rank: Super Contributor
timestamp: May 17, 2013, 02:09:41 pm
content: Quote from: Electro Fan on May 17, 2013, 01:51:51 pmI think it is amplitude rather than frequency (this is for arbitrary waveforms where frequency may not even make sense in that they need not be periodic).But it puzzles me as it is much longer than the rise time (approximately 10 nsecs). The DAC operates at 250MS/s so 200nsecs is 50 samples so perhaps it needs 50 samples to properly perform the digital signal processing steps it goes through.The Agilent person on the Agilent forum said they would come back on the question, hopefully I'll get some sort of technical answer from them.

author: jpb
author_rank: Super Contributor
timestamp: May 17, 2013, 05:55:56 pm
content: Agilent have now responded to my questions, and if I understand their answers correctly:1.) the settling time only applies to the option where no filter is applied where step changes will take a while to settle at their final value (I guess that this means there is ringing without the filter and it is the decay rate of this).2.) The 33500 initially didn't allow sweep or FM of arbs but this has been allowed since firmware version 1.14, the literature has yet to catch up but I guess now that I've raised the issue the data sheet and manual will be updated.