Siglent SDG2082X THD Measurements - Page 1
Forum section: electronics / testgear

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 02, 2024, 03:14:07 am
content: Just got my SDG2082X and I'm really pleased with it. Most of what I do is audio, and for THD measurements I use the built in low THD oscillators in my analyzers, but I was curious about the 2082, since the specs are a bit limited. Here's a quick measurement at three frequencies, 20, 1000 and 20000 Hz. Levels are 0.1, 0.5, 1.0 and 5.0 VRMS. The sweet spot seems to be about 1V.

author: 2N3055
author_rank: Super Contributor
timestamp: December 02, 2024, 06:53:02 am
content: Those results are similar to SDG6052X I have.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 02, 2024, 11:25:02 pm
content: Had more time to see what the distortion looks like so I took the raw signal and residual signal output from the analyzer (HP 8903B) and put them both on the (new!) scope. Here's the 1 Vpp and 15 Vpp results. Nothing unusual. There's a bit of high frequency stuff on the 1Vpp residual that's not quite 120 Hz. Maybe a display update frequency edge or some other internal clock. I changed the waveform frequency so it wouldn't drift. Minor in any case.

author: KungFuJosh
author_rank: Super Contributor
timestamp: December 03, 2024, 12:26:41 am
content: Quote from: 2N3055 on December 02, 2024, 06:53:02 amIsn't the SDG6000X series basically an upgraded SDG2000X?

author: 2N3055
author_rank: Super Contributor
timestamp: December 03, 2024, 09:50:16 am
content: Quote from: KungFuJosh on December 03, 2024, 12:26:41 amIt looks the same but no..It is 16bit, but much larger BW, so analog part is different,DAC is different..I would think that they would have 16 bit waveform generation part similar though...

author: KungFuJosh
author_rank: Super Contributor
timestamp: December 03, 2024, 01:01:08 pm
content: Yes, hardware upgrades. 😉😉

author: mawyatt
author_rank: Super Contributor
timestamp: December 03, 2024, 03:21:46 pm
content: This brings up a something from way back wrt AWG HD. Could one create a file for the AWG that improves the THD at some fixed frequency and amplitude sine wave by pre-distortion to compensate for the natural AWG induced distortion such that the AWG renders a lower distortion waveform at the fixed frequency and amplitude?Or could one create such a file and use one AWG channel (CH2) to compensate (sorta error correction) the main channel (CH1) which would operate with the builtin sine-wave source. Here CH2 error correction waveform would be "summed" with CH1 sine wave source in the Analog Domain (outside the AWG) with resistive summing (50Ω) to yield a lower result HD sine-wave. Doing this in the Analog Domain is better than having the AWG do the summation (some AWGs can do this) in the Digital Domain as the AD has a higher effective resolution by means of the analog summation than DD summation which is limited by the DAC resolution (14~16 bits).Computing the Harmonic "Correction File" in either case might be difficult, but likely within the grasp of some DSP gurus capability (read not us!!). One could use trial and error means while watching the output with an FFT or SA and "nulling" each harmonic one by one, tedious tho.Anyway just a thought from way back that might allow producing a better quality sine waveform and with some selective post analog filtering might prove useful for some high dynamic range needs/measurements.Best

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 03, 2024, 04:24:52 pm
content: That's a fascinating idea. Can't fix noise, but it might not be hard to sum the residual signal from my THD analyzer with a pure sine and get a better result. That said, I found the THD to be much better than I thought it might be from the 0.075% number in the datasheet.

author: 2N3055
author_rank: Super Contributor
timestamp: December 03, 2024, 05:09:26 pm
content: Looking at the residual in time domain, it is pretty obvious there is direct correlation between waveform and error residual. Error is clearly defined, periodic and repeatable for 1kHz for instance.Those are nonlinearities from DAC itself and distortion from analog part. Combined.Predistortion waveform could be made for specific AWG, channel, frequency and amplitude.That could decrease THD by one decimal point even without mixing 2 channels together.Going further than that would require cleaning up that high frequency noise seen at lower amplitudes.Creating pre distorted waveform, using larger amplitudes, post filtering and attenuation could create better result.What I usually do when I need smaller amplitude signal with lower noise, is use post attenuation on purpose, to keep AWG in a sweet spot... That alone makes notable improvement.

author: Mahagam
author_rank: Regular Contributor
timestamp: December 03, 2024, 06:48:40 pm
content: I`ve measured strange ~100kHz noise from my SDG2042X.One pic is SDG output switched ON. Waveform: DC 0.0VSecond - SDG output switched OFF.SDG and SDS connected by coaxial cable.It looks like it`s noise from power supply.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 03, 2024, 07:01:12 pm
content: Those levels are pretty low, maybe on the order of what I saw in my THD residual. Might be supply but I'd hesitate to assign blame without seeing a schematic. So far I've no complaint about the cleanliness of the waveforms for any practical use.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 03, 2024, 10:06:55 pm
content: May I also play along?I did some measurements on my two Siglent function generators and that is the SDG1032X and the SDG2042X.From both generators I did the Spectrum measurements at 1 and 3V when the generator is terminated with 50 Ohm.The two measurement frequencies are 1KHz and 10 KHZ.The measurements were done with a QuantAsylum 403 Audio Analyzer, softwareThe spectrum window is Blackman-Harris, there is 5x averaging to make the noise floor a little lower and the spurs are more visible as a result.The SampleRate is 192KHz, 384KHz is not possible for now, then I have to modify the configuration file and I have not done that yet on this measuring computer.This higher Sample frequency is also not really necessary for these measurements.The first picture is the noise floor when the SDG1032X output is disabled.I left the markers for the 10KHZ signal for a moment..Let me start with the SDG2042X at 1KHz and 1V RMS at a 50 Ohm load..The SDG2042X at 1KHz and 3V RMS at a 50 Ohm load..Tthe SDG2042X at 10KHz and 1V RMS at a 50 Ohm load..The SDG2042X at 10KHz and 3V RMS at a 50 Ohm load..------------------------------------------------------------------------------------------------------------------------------------------------------------------------The following measurements were made on the Siglent SDG1032X.The SDG1032X at 1KHz and 1V RMS at a 50 Ohm load..The SDG1032X at 1KHz and 3V RMS at a 50 Ohm load..The SDG1032X at 10KHz and 1V RMS at a 50 Ohm load..The SDG1032X at 10KHz and 3V RMS at a 50 Ohm load..And now for a nice picture to show how nice the square wave of the SDG1032X is.This was taken at a 10Hz frequency, Look how beautifully flat the harmonics are..Kind regards,Bram

author: 2N3055
author_rank: Super Contributor
timestamp: December 03, 2024, 11:15:49 pm
content: Quote from: blackdog on December 03, 2024, 10:06:55 pmAnd it is your fault that I have to buy QA403 now....

author: KungFuJosh
author_rank: Super Contributor
timestamp: December 04, 2024, 12:28:03 am
content: Quote from: 2N3055 on December 03, 2024, 11:15:49 pmI've been resisting that for at least a couple years. 🤣

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 04, 2024, 03:34:39 am
content: Same here but I'm sure you've heard that resistance is futile.

author: KungFuJosh
author_rank: Super Contributor
timestamp: December 04, 2024, 03:53:39 am
content: Ha, that's true. What I'm really waiting for is the proper excuse to make the purchase. 😉

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 04, 2024, 12:52:30 pm
content: BTW, I want to thank everybody for their inputs. It's handy to have this sort of info available when contemplating a purchase or to tell if what you see is normal/typical.

author: mawyatt
author_rank: Super Contributor
timestamp: December 04, 2024, 02:36:14 pm
content: Seems you've made another wise choice with the SDG2082X and SDS800X HDNow get ready for the SDG2082X Big BrotherAlso you've noted the great resources here for these Instruments, it's a shame that folks have to sift thru all the BS that's flying around, especially wrt to DSOs which seem to create a war zone every time a DSO is mentioned.Hopefully you've benefited from the EEVblog Discount Code from Rob which is honored at Saelig in your areaBest

author: blackdog
author_rank: Frequent Contributor
timestamp: December 04, 2024, 03:06:56 pm
content: Hi,I want to give you some reference material, this is to see the difference between an “Analog” generator and many function generators.Let me start with an oldie, this is the Heatkit IG-1272 that I bought and built at the end of 1970.Early this year I made it working again, just not tuned for low distortion and this can be clearly seen in the photo below.Many audio devices and transmitters I have measured through with this model..The THD, not so good at the moment...But more important, look at all these harmonics from de 50Hz power frequency....And now a 1KHZ signal from an Audio Precision “Portable One Plus”First a picture of the measuring instrument itself, so you know what I'm talking about..This is a picture to kiss so beautifully.But now what am I seeing below 1KHz....The grid frequency and its harmonics....This is one of the few problems I have encountered with this measuring instrument his purchase in 2000.There is too little shielding applied in the housing.For certain measurements I would sit with the instrument on a few chairs in the middle of a room as far away as possible from other equipment and power connections.If I measure with the nice sound card that is in another computer with Virtins' software and I adjust the level a bit for the lowest distortion,I get a value of 0.00022% THD when I measure up to the 6th harmonic.This Audio Precision has a wonderful sine wave generator..And now for something I built myself, I have more versions but the other one which has an even lower THD is just quick to connect, this one is not quite finished.What I show below is a Wienbrug oscilator based on a design by Jim Williams and consists of two opamps.The second opamp helps keep the commonmode error as small as possible.Both opamps have been buffered, because of this they do not see any load and therefore deliver their maximum performance.The buffer ICs are the LH0002(under the heatsinks) and the dual opamp is the OPA1612.High quality selected parts have been used here, good quality is not free and do not forget about the thick steel housing as shielding..Low spurs are only possible with battery power or an analog power supply with very few commonmode problems.Maybe nowadays it is also possible with the Switcher operating at tens of MHz, however, I have never tested this..And this is what it looks like in the spectrum measurement, there is nothing at all to see the grid frequency and low distortion.This is the best I have been able to make with a simple Wienbrug circuit.Better is possible as the Audio Precision shows, but then with a different construction of the sine generator. and again, with very good shielding!.I hope this is enough comparison material to make trade-offs between modern function generators and analog frequency generation.The spurs and distortion products of modern function generators can be made a lot less by first using a bandpass filter and then a lowpass filter.This only for a limited frequency range, otherwise it becomes quite extensive.What, do I hear something?OK, OK herewith a picture of the generator that is in the Quant Asylum.The third harmonic is the strongest, but still within specifications which are better than -115dB..Best regards,Bram