Author Topic: Siglent SDG1032X sine distortion at 1 kHz ?  (Read 7339 times)

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Offline JBealeTopic starter

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Siglent SDG1032X sine distortion at 1 kHz ?
« on: December 24, 2022, 07:02:14 am »
I'm curious if anyone with a Siglent SDG1032X has measured the sinewave distortion at 1 kHz. I don't have one myself.

On page 7 of the spec sheet at https://siglentna.com/wp-content/uploads/dlm_uploads/2022/06/SDG1000X_DataSheet_DS0201X_E01I.pdf it says the sine distortion spec is -60 dBc from 0-10 MHz, at 0 dBm output.  On p.3 there is a plot of 2nd and 3rd harmonic levels that suggests -65 dB at 100 kHz, but it doesn't show lower frequencies.

I tried building a fixed 1 kHz Wien-bridge oscillator using a NE5534A opamp with the old-school filament lamp for AGC, and that works OK. The sinewave looks unclipped on my scope (which has only 8 bits of resolution I think) but my example barely reaches -60 dB THD, even after adding a few poles of RC low-pass on the output.  Now I suppose the audio-frequency ADC I'm using to measure that with might also be far worse than its spec, or my garden-variety 0.1 uF ceramic caps are somehow nonlinear at that level (??) or something else, but so far I can't prove it.
« Last Edit: December 24, 2022, 07:06:42 am by JBeale »
 

Offline Martin72

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #1 on: December 24, 2022, 08:50:56 am »
Quote
I'm curious if anyone with a Siglent SDG1032X has measured the sinewave distortion at 1 kHz.

Could do this later on..
This threads reminds me of the kit I´ve bought over a year ago and still haven´t assembled yet.
It generates a sinewave of 1khz with a claimed thd of round about appx -150dB, theoretically.

Offline Performa01

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #2 on: December 24, 2022, 12:58:29 pm »
I do not have an SDG1032X, but I stumbled across an old measurement of the SAG1021, which should be the same circuitry as the integrated AWG of an SDS2000X Plus. In any case this is the cheapest AWG solution from Siglent, so the SDG1000X should be at least as good.

The attached image shows the THD from 10 Hz to 1 MHz at 2 Vpp output into 50 ohms. 0.1% equals -60 dBc.
 
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Offline Martin72

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #3 on: December 24, 2022, 01:37:40 pm »
Made FFT measures with both generators I currently got, more in the comparison thread...
Here single pics, showing the SDG1062X and 2122X,  generating a 1khz sinewave with 2.85Vpp.

Martin
 
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Offline MrAl

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #4 on: December 24, 2022, 01:58:06 pm »
Just to note, my earlier remarks were not intended to suggest that small LCR meters were not useful at all, just not useful for some applications.

For example, someone i know talks about getting a small LCR meter for testing inductors.  However, he works mainly on DC to DC converters.
What would happen if he bought one would be he would be disappointed that they do not work well with converter applications and inductors because inductors used for most converter circuits have more variable characteristics that required a certain DC bias.  What this means is the small LCR meter may be of no use at all to him.

For more information take a look at an anisotropic curve for any metal core used for converter circuit inductors and also transformers.  The inductance may look like 10uH at 10ma but 100uH at 100ma and 1000uH at 1000ma.  Then, as the current increases to 2 amps, the inductance may go down to 50uH, then finally down to some very low value maybe almost a short.
What this means is that there is no way you can determine how useful a particular inductor is for a converter circuit using a small LCR meter.

As far as capacitance, that's a different story because they dont usually need a large current to test properly.  Maybe some of the really big ones though like 100000uf and like that, to test for ESR.

We could look at some examples.
 

Offline Martin72

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #5 on: December 24, 2022, 02:00:32 pm »
Hmmm..... ;)

Offline rf-loop

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #6 on: December 24, 2022, 02:01:01 pm »
SDG1000X series sinewave charasteristrics (THD, harmonics, non harmonic spurs) are specified for 0dBm signal level.

Here 1kHz sine out from SDG1000X  and level 0dBm (load 50ohm).
Test using SDS2504X HD
FFT window of course "flat top"
For better visibility time domain average 512 and freq domain average 8. (for reduce random noise level)

I'm lazy and do not want calculate total from harmonics. ;)
Also only 8 markers (latest public FW).



 
ETA: There is NOT visible SDG1000X harmonics alone. Here can see: SDG + oscilloscope front end generated and FFT "lies"!
« Last Edit: January 18, 2023, 12:51:01 pm by rf-loop »
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Offline Martin72

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #7 on: December 24, 2022, 02:09:43 pm »
Ah, dBm... |O

Offline Hexley

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #8 on: December 24, 2022, 06:04:26 pm »
For what it's worth, I measured my SDG1062X with an old HP331A distortion analyzer. The frequency was set to 1 kHz, and the output level was set to 0 dBm with a 600 ohm load (the generator load parameter was set to match this load).

The analyzer's meter read one needle's width below 0.1% THD. Call it 0.095%, maybe.
 
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Offline TimFox

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #9 on: December 24, 2022, 06:08:28 pm »
For what it's worth, I measured my SDG1062X with an old HP331A distortion analyzer. The frequency was set to 1 kHz, and the output level was set to 0 dBm with a 600 ohm load (the generator load parameter was set to match this load).

The analyzer's meter read one needle's width below 0.1% THD. Call it 0.095%, maybe.

That's roughly the "residual distortion" of the 331-334 family.
My 339 can measure down to maybe 0.002% THD.
 

Offline JBealeTopic starter

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #10 on: December 25, 2022, 01:01:26 am »
Thanks everyone for the useful input. Looks like this model has reasonable performance and a decent spec, plus an array of other features that are probably useful in other ways, so I placed an order for one. Somehow I had the idea that -60 dBc was easy to obtain with a simple analog circuit, maybe with the right one but I've found that not just any circuit will necessarily do that.
 

Offline Martin72

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #11 on: December 25, 2022, 11:09:54 pm »
I'm lazy and do not want calculate total from harmonics. ;)

Hopefully the siglent will do it for us someday. ;)

What your measure concerns:
Using flat top makes sense to me, as it got the best amplitude accuracy.
Also using averaging.
I see the values displayed in the screen upper right..
To be honest, in all the years I´ve used fft as it comes, adjusted as long as the output seems plausible to me, without thinking about it.
Something I want to change, so I´ve read the application notes from lecroy about setting up the fft.
Span = half of samplerate, OK got it.
"Delta"-f = 1/time per division *10 divisions, OK got it...
But there must be more when I look at your screenshot.
Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz, but it isn´t, it is 23.84Hz....Why ?
Is it because of the amount of fft points, we can choose up to 2M ?
And what about delta-f in general, the lower the better ?

Offline radiolistener

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #12 on: December 26, 2022, 05:18:34 am »
Here 1kHz sine out from SDG1000X  and level 0dBm (load 50ohm).

Looks much better than Chinese generators which have about -40 dBc
 

Offline David Hess

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #13 on: December 26, 2022, 05:58:55 am »
I tried building a fixed 1 kHz Wien-bridge oscillator using a NE5534A opamp with the old-school filament lamp for AGC, and that works OK. The sinewave looks unclipped on my scope (which has only 8 bits of resolution I think) but my example barely reaches -60 dB THD, even after adding a few poles of RC low-pass on the output.  Now I suppose the audio-frequency ADC I'm using to measure that with might also be far worse than its spec, or my garden-variety 0.1 uF ceramic caps are somehow nonlinear at that level (??) or something else, but so far I can't prove it.

An audio frequency ADC should be a lot better than -60dB THD, so that must be your Wein bridge oscillator.

Starting on page 29 of Linear Technology application note 43, Jim Williams discusses the distortion of various audio Wein bridge oscillator configurations.  The most basic with a single good operational amplifier and a lamp for AGC achieves 0.007% or -83dB at 1 kHz with distortion limited by the lamp itself.

The NE5534A is pretty good so I suspect your capacitors are the problem.  This is definitely a place for C0G or NP0 ceramic capacitors if you do not use film capacitors except for decoupling.
 
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Offline Performa01

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #14 on: December 26, 2022, 08:05:38 am »
Span = half of samplerate, OK got it.
"Delta"-f = 1/time per division *10 divisions, OK got it...
But there must be more when I look at your screenshot.
Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz, but it isn´t, it is 23.84Hz....Why ?
Is it because of the amount of fft points, we can choose up to 2M ?
And what about delta-f in general, the lower the better ?

I have tried to explain the basics of the FFT setup several times before, e.g. here, Reply #23:

https://www.eevblog.com/forum/testgear/rohde-schwarz-rtb2002-vs-siglent-sds2104x-plus/msg3239832/#msg3239832

Quote
• The analysis bandwidth (FFT-BW) is always half the FFT sample rate (FFT-SR).
• The frequency step (Δf or df) is the sample rate divided by the number of FFT points.
• The resolution bandwidth (RBW) is the frequency step multiplied by a factor specific for the
window function in use.
• The maximum number of FFT points depends on the record length, which in turn increases with slower timebase settings, but is ultimately limited by the maximum memory set in the Acquire menu and of course also the specified maximum possible FFT length. Apart from that, the max. number of FFT points can be further limited by the specific setting in the FFT menu.

SR (Sampe rate) = RBW * k, where k is the 3 dB bandwidth factor in bins, depending on the window function: Rectangle 0.99, Blackman 1.74, Hanning 1.62, Hamming 1.64, Flattop 3.73.

Blackman and especially Flattop are the most universal and useful window functions in practice, whereas Rectangle is rather specialized and should be avoided unless you absolutely know that you actually need it (e.g. for short transients).

Thus: df = RBW / 4 (rounded) in case of the flattop window.

To get the proper settings for any given FFT-BW and RBW pair, proceed as follows:

Determine the FFT samplerate: SR = FFT-BW * 2 [Sa/s];
Determine the number of FFT points: FFT-Pts >= SR / df [-];
Determine the timebase: TB >= FFT-Pts / SR / 10 [s/div];

 
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Offline rf-loop

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #15 on: December 26, 2022, 10:26:18 am »
I'm lazy and do not want calculate total from harmonics. ;)

Hopefully the siglent will do it for us someday. ;)

What your measure concerns:
Using flat top makes sense to me, as it got the best amplitude accuracy.
Also using averaging.
I see the values displayed in the screen upper right..
To be honest, in all the years I´ve used fft as it comes, adjusted as long as the output seems plausible to me, without thinking about it.
Something I want to change, so I´ve read the application notes from lecroy about setting up the fft.
Span = half of samplerate, OK got it.
"Delta"-f = 1/time per division *10 divisions, OK got it...
But there must be more when I look at your screenshot.
Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz, but it isn´t, it is 23.84Hz....Why ?
Is it because of the amount of fft points, we can choose up to 2M ?
And what about delta-f in general, the lower the better ?

"Span = half of samplerate, OK got it."
FFT full span (0Hz to FFT max frequency) = sampling frequency / 2
As you can see in my image displayed span is not at all this FFT full span.  There is displayed 20kHz span just because I have adjusted it for this span. This particular FFT full span is in this case 25MHz   
(but if user know what he is doing he can also look higher frequencies...  If I now feed her 50.001MHz signal I can also see 1kHz but lets leave this story to other time and place...)


"Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz.... "
Please try to unlearn this wrong habit...  ;)

Lets look it right way.
"Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz, but it isn´t, it is 23.84Hz....Why ?"

Δf = FFT sampling frequency / FFT length
In this case (my picture)  sampling frequency is 50MSa/s (look FFT sampling not time domain sampling)
FFT length is 2097152 pts.

50000000Hz / 2097152 = 23,84185791 Hz.   So we can display informantion Δf = 23.84Hz

"And what about delta-f in general, the lower the better ?"

There is no single simple answer at all. Because it depends.... better for what?
Reducing FFT points give less resolution (higher Δf) and more speed again depending our othewr settings... t/div, memory length, time domain sampling speed (in some cases FFT use same sampling speed and on other cases it use samplerate what is decimated from time domain samplerate what also can be decimated from real ADC samplerate.
So there is many things what affect each others. What is optimal setup for different needs - simply answer is: Lowest Δf is not always best but some times best is as as low Δf as possible. 


But naturally resolution depends what is Δf (aka Bin width) and what is used FFT window function.
If we think normal spectrum analyzer we talk resolution using RBW (And if we dive to bit more deep we need also know filter shape factors)
In spectrum analyzer RBW is defined (mostly) so that it mean filter -3dBc width. So when you want more or less resolution you adjust RBW
In this kind of FFT here, you adjust resolution by changing Δf and also FFT window function and here need understand that this is bit more complex depending what user need in different cases. Here I have used window "FlatTop" because it give best level accuracy because its top is wide and there is minimal if even any level drop between these single FFT points. (starting from 0Hz there is FFT points just using Δf  interval up to max freq what is sampling freq/2)

Roughly rounded it can say that Flat Top window "RBW" width is 3.7*Δf  (and shape factor is roughly 2.5) (-60 dBc width roughly bit over 9*Δf)
Flattop window scallop loss is roughly 0 so it is level accurate also with signals between FFT points (what exist  Δf intervals starting from 0f).


Other windows have different "RBW" width and different shape factor and scalloping loss 

What are optimal FFT settings depends what we are doing. In some case we need speed but not high resolution... in some case we want look some carrier with weak modulation so we need perhaps maximal resolution and perhaps still good level accuracy for measure modulation levels.

Example if look 7.05MHz 0dBm carrier with 0.2% AM modulation there need "all resolution what can get" (SDS2kX HD) for look and try measure this for get somehow acceptable level accuracy for this modulation level. (-60dBc peaks 120Hz distace from carrier).

So there need select 20MHz sampling freq for FFT (full span is then 10MHz) and 2Mpts FFT length. So we get Δf  9.54Hz. Now we can roughly think Flat Top window RBW is <40Hz  (perhaps near 35Hz (-3dBc))Then set center to carrier and some example 1 or 2k span. With FFT FlatTop window it give roughly -60dBc level for modulation peaks. Of course if modulation depth is more, example 10 or 50% it goes very easy. (as we now know Flat Top window "width" at -60dBc is around 85Hz we have enough resolution for measure these 120Hz modulation peaks.)


ETA: oh yes Performa01 answer allready when I have been slowly writing answer multitasking with many other things with family etc. ;)
« Last Edit: December 26, 2022, 10:35:48 am by rf-loop »
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Offline Martin72

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #16 on: December 26, 2022, 10:36:25 am »
Thank you both !

Online gf

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #17 on: December 26, 2022, 11:30:59 am »
Ah, dBm... |O

No need to bang the head. The unit (dBV, dBm,...) does not really matter. What matters is the relative amplitude of the harmonics w.r.t. the fundamental, which is usually denoted dBc.

BTW, another thing which does influence the SRDF and harmonic distortion is the utilization of the DAC's and ADC's full scale range. SFDR of a DAC or ADC is usually specified in dBFS units, i.e. relative to a full-scale sine wave. I.e. the specified numbers apply to full-scale sine waves. For lower amplitudes the numbers are correspondingly worse.
 

Offline 2N3055

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #18 on: December 26, 2022, 11:34:50 am »
Ah, dBm... |O

No need to bang the head. The unit (dBV, dBm,...) does not really matter. What matters is the relative amplitude of the harmonics w.r.t. the fundamental, which is usually denoted dBc.

BTW, another thing which does influence the SRDF and harmonic distortion is the utilization of the DAC's and ADC's full scale range. SFDR of a DAC or ADC is usually specified in dBFS units, i.e. relative to a full-scale sine wave. I.e. the specified numbers apply to full-scale sine waves. For lower amplitudes the numbers are correspondingly worse.
Respectfully, it matters because distortion won't be same at different amplitudes. Relative dB will be the same of course...
"Just hard work is not enough - it must be applied sensibly."
Dr. Richard W. Hamming
 

Offline markone

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #19 on: December 26, 2022, 11:59:45 am »
SDG1000X series sinewave charasteristrics (THD, harmonics, non harmonic spurs) are specified for 0dBm signal level.

Here 1kHz sine out from SDG1000X  and level 0dBm (load 50ohm).
Test using SDS2504X HD
FFT window of course "flat top"
For better visibility time domain average 512 and freq domain average 8. (for reduce random noise level)

I'm lazy and do not want calculate total from harmonics. ;)
Also only 8 markers (latest public FW).




Just out of curiosity, what are (and where are specified) DANL and Phase Noise numbers for SDS2504X HD in FFT mode ?
 

Online gf

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #20 on: December 26, 2022, 12:02:04 pm »
Ah, dBm... |O

No need to bang the head. The unit (dBV, dBm,...) does not really matter. What matters is the relative amplitude of the harmonics w.r.t. the fundamental, which is usually denoted dBc.

BTW, another thing which does influence the SRDF and harmonic distortion is the utilization of the DAC's and ADC's full scale range. SFDR of a DAC or ADC is usually specified in dBFS units, i.e. relative to a full-scale sine wave. I.e. the specified numbers apply to full-scale sine waves. For lower amplitudes the numbers are correspondingly worse.
Respectfully, it matters because distortion won't be same at different amplitudes. Relative dB will be the same of course...

Agreed. I mean, it does not matter, whether the FFT results are displayed in dBm or dBV units.
The amplitude of the signal does of course matter, due to different distortion at different amplitudes.
 
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Offline rf-loop

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #21 on: December 26, 2022, 02:52:26 pm »

Just out of curiosity, what are (and where are specified) DANL and Phase Noise numbers for SDS2504X HD in FFT mode ?

Nowhere.

Also it is not so straightforward in this case how to setup example for get DANL. It depends... example here in image you can see also one kind of Displayed Average Noise Level in just this used setup.
/window used in this image  is Flat Top. Input Signal is -40dBm ~1MHz

Note: during this long averaging there have been input signal frequency drift so marker signal displayed level is dropped nearly 1dB . True signal is ~-40dBm

« Last Edit: December 26, 2022, 03:07:36 pm by rf-loop »
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Offline Hexley

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #22 on: December 26, 2022, 05:47:36 pm »
I tried building a fixed 1 kHz Wien-bridge oscillator using a NE5534A opamp with the old-school filament lamp for AGC, and that works OK. The sinewave looks unclipped on my scope (which has only 8 bits of resolution I think) but my example barely reaches -60 dB THD, even after adding a few poles of RC low-pass on the output.  Now I suppose the audio-frequency ADC I'm using to measure that with might also be far worse than its spec, or my garden-variety 0.1 uF ceramic caps are somehow nonlinear at that level (??) or something else, but so far I can't prove it.

An audio frequency ADC should be a lot better than -60dB THD, so that must be your Wein bridge oscillator.

Starting on page 29 of Linear Technology application note 43, Jim Williams discusses the distortion of various audio Wein bridge oscillator configurations.  The most basic with a single good operational amplifier and a lamp for AGC achieves 0.007% or -83dB at 1 kHz with distortion limited by the lamp itself.

The NE5534A is pretty good so I suspect your capacitors are the problem.  This is definitely a place for C0G or NP0 ceramic capacitors if you do not use film capacitors except for decoupling.

In case the OP is still looking for a homebrew solution, perhaps this would be of interest:

There is a clever alternative to the Wien bridge -- the "Phase Shift Oscillator" -- that can offer somewhat lower distortion. Roger Rosens published a description in Wireless World in 1982 https://worldradiohistory.com/hd2/IDX-UK/Technology/Technology-All-Eras/Archive-Wireless-World-IDX/80s/Wireless-World-1982-02-S-OCR-IDX-40.pdf of an implementation that uses a distortion cancellation technique to reduce the third harmonic component.

A later version of the circuit may be found here http://www.redcircuits.com/Page82.htm. This one replaces the gain-stabilizing thermistor with an LED/photo resistor setup. I was able to get a measured 0.00031% THD at 2 kHz with this circuit.
 
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Offline Martin72

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #23 on: December 26, 2022, 05:55:59 pm »
Thank you both !

So some things have become clearer to me now, the first result is the re-enactment of the FFT measurement.
Looks like I got higher distortions but still it´s good.
My new SDG2122X looks worse with the same settings. ;)
(Post it in the comparison thread)


« Last Edit: December 26, 2022, 06:23:54 pm by Martin72 »
 

Offline JBealeTopic starter

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Re: Siglent SDG1032X sine distortion at 1 kHz ?
« Reply #24 on: December 27, 2022, 03:55:57 am »
An audio frequency ADC should be a lot better than -60dB THD, so that must be your Wein bridge oscillator. [...]
The NE5534A is pretty good so I suspect your capacitors are the problem.  This is definitely a place for C0G or NP0 ceramic capacitors if you do not use film capacitors except for decoupling.
Words of wisdom! I had paid no attention to capacitor type. I was guilty of using X7R for the RC tuning
KYOCERA AVX SR215C104KAA (MLCC - Leaded 50V 0.1 uF X7R )
and simply swapping those out for
PANASONIC ECW-FD2W104KQ (Polypropylene Film 450VDC 0.1uF)
gives me a far better result: Audacity spectrum plot now shows around -90 dBc, and my cheap laptop audio input may start to become a factor here. This is as good a number as I had hoped to achieve. I had not realized how nonlinear the X7R could get in such an application, but thank you for pointing that out!

« Last Edit: December 27, 2022, 04:07:47 am by JBeale »
 


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