Low frequency modulation in high frequency DDS generated signal

kaz1 said:

It could be just visual artefact of sine wave addition:
f1 = sin(2*pi*(0:1023)*.13);
f2 = sin(2*pi*(0:1023)*.27);
y = f1+f2;
plot ( y)

View attachment 195369

Click to expand...

This can be what undersampled RF looks like

D.A.(Tony)Stewart said:

This can be what undersampled RF looks like

Click to expand...

What do you mean? there is no physical sampling rate.
f1 is normalised 0.13 of Fs of 1
f2 is normalised 0.27 of Fs of 1
both added as they are generated.

do FFT and you should see 0.13 and 0.27 with mirrored copies if frequency axis is set normalised between 0 and 1

Hi,

I guess the root cause of the issue is already validated.

If it was a DAC nonlinearity ... why would this nonlinearity not happen when one changes the frequency by 0.29Hz?
(Relative) Nonlinearities may vary with amplitude ... but why with frequency?

And the magnitude of nonlinearity errors are rather small ... I guess below 1% .. so one would not even recognize them on a scope picture. (but on an FFT)
Mainly because nonlinearities cause overtones.

Klaus

KlausST said:

Hi,

I guess the root cause of the issue is already validated.

If it was a DAC nonlinearity ... why would this nonlinearity not happen when one changes the frequency by 0.29Hz?
(Relative) Nonlinearities may vary with amplitude ... but why with frequency?

And the magnitude of nonlinearity errors are rather small ... I guess below 1% .. so one would not even recognize them on a scope picture. (but on an FFT)
Mainly because nonlinearities cause overtones.

Klaus

Click to expand...

ok so what is the cause called as technical term?

It´s the effect of adding two signals with almost identical frequencies.
Some call it "beating", some call it "envelope" ...

It´s the same effect that is used to tune guitar (strings).

Klaus

OK I can also see that in this basic simulation:
############################
f1 = sin(2*pi*(0:1023*10)*.1300);
f2 = sin(2*pi*(0:1023*10)*.1301);
f3 = sin(2*pi*(0:1023*10)*.1303);
f4 = sin(2*pi*(0:1023*10)*.1309);
f5 = sin(2*pi*(0:1023*10)*.140);

y = f1+f2+f3+f4+f5;
plot ( y)
############################


Clearly this is a time domain issue and is concern for applications like music or the issue of this thread.
It does not apply to frequency domain. The OP presented the issue as low frequency modulation.
Thanks for the clarification.

kaz1 said:

The OP presented the issue as low frequency modulation.

Click to expand...

OK.
I also had my issues with correct terminology in this thread.

--> I read it as: (the amplitude is) modulated with a low frequency. (or looks alike)

I´m with you: this is not "visible" in frequency domain.
The modualtion frequency you see is not a real frequency.

Klaus

Interference patterns are not AM

D.A.(Tony)Stewart said:

Interference patterns are not AM

Click to expand...

I agreed - twice - to it.

but it looks like one.

Or the other way round:

If you see such a pattern on your scope how can one distinguish if it´s frequency interference or AM?

(picture from: https://www.researchgate.net/figure...ies-f-1-and-f-2-blue-and-green_fig1_353337207)

Klaus

Hi,

D.A. (Tony) Stewart : I did check FFT a while ago but it didn't reveal anything specific. However I do have IMD in my signal, IMD3 according to the frequencies generated, but these frequencies carry very low power compared to the desired ones.
I'm not sure what you mean by sensitive to interference pattern, could you please specify ?

Kaz1 : If the root of the problem is indeed IMD, do you know of/is there a systematic way of cancelling the low frequency components that occur in the system ? To be truthful, I don't think I understand the underlying reason behind the intensity modulation. Klaus kindly explained what seemed to be the problem in the case of the specific frequencies I provided, but is this a systematic issue ? Should the differences of the differences of input frequencies always be 0 to avoid the "beating of the beating" (leading to the observable intensity beating) ? If you happen to know any literature references on the subject I would gladly take a look.

Considering the range of f , do the patterns change with different sweep rates to ensure sampling rate is fast enough

Quant01 said:

Hi,

D.A. (Tony) Stewart : I did check FFT a while ago but it didn't reveal anything specific. However I do have IMD in my signal, IMD3 according to the frequencies generated, but these frequencies carry very low power compared to the desired ones.
I'm not sure what you mean by sensitive to interference pattern, could you please specify ?

Kaz1 : If the root of the problem is indeed IMD, do you know of/is there a systematic way of cancelling the low frequency components that occur in the system ? To be truthful, I don't think I understand the underlying reason behind the intensity modulation. Klaus kindly explained what seemed to be the problem in the case of the specific frequencies I provided, but is this a systematic issue ? Should the differences of the differences of input frequencies always be 0 to avoid the "beating of the beating" (leading to the observable intensity beating) ? If you happen to know any literature references on the subject I would gladly take a look.

Click to expand...

I see you are not sure about your problem.
Since laser applications is your area, we may not understand your problem. You need to word the problem "as is" rather than any lower level analysis or beatings or modulation.

The main question: is your application sensitive to time domain or not. I mean if you are just adding up clean tones you will get various patterns and peak-to average will change but frequency domain will be clean. The time domain issues are relevant is some cases such power amplifier's non-linear operation. So does your application fall into this category.
If it is IMD it will show on spectrum and will change in locations and power depending on generated tones though could be very weak in good design. There is plenty literature on IMD on the web.

Hi,

D.A. (Tony) Stewart: for certain different frequencies (other than the ones I specified for Klaus earlier) the beating does occur but usually at 290mHz. From my resent testing of different scenarios based on Klaus' idea, it looks like the beating occurs at the frequency corresponding to the greatest common denominator of the input frequencies. Usually it is 290mHz because this corresponds to the resolution of the DDS, but I guess by pure chance it could happen at another frequency. And if you're referring to the patterns visible on the oscilloscope, apart from phase, they are always similar. I don't know if this answers your question ?

Kaz1: Then yes my application is sensitive to time domain. I'm using an amplifier and the manufacturer of the AWG informed me that such problems could occur if the amplifier was in non linear regime.
So to "state the problem as is" : I'm sending a RF signal comprising multiple sine waves of different, fixed, frequencies into an amplifier. This amplifier is connected to an optical device that diffracts a laser. The intensity of the laser oscillates at 290mHz because of this. By analyzing the output of the AWG on an oscilloscope, we see the 290mHz beating. By using a spectrum analyzer after the amplifier, something that looks like IMD3 is visible, but at significantly lower power than the main tones.
So maybe other types of IMD also appear (other than IMD3).
The laser itself is not a relevant component of the problem, it only serves, in this case, as a visual proof that there is a problem.
I think my next step is to find an amplifier that will operate in linear regime in my application.

Also ADC clock frequency can modulate your tones.
The power amplifier may behave better if you randomise the phase to reduce peak/average, and dither each tone

Quant01 said:

Hi,

D.A. (Tony) Stewart: for certain different frequencies (other than the ones I specified for Klaus earlier) the beating does occur but usually at 290mHz. From my resent testing of different scenarios based on Klaus' idea, it looks like the beating occurs at the frequency corresponding to the greatest common denominator of the input frequencies. Usually it is 290mHz because this corresponds to the resolution of the DDS, but I guess by pure chance it could happen at another frequency. And if you're referring to the patterns visible on the oscilloscope, apart from phase, they are always similar. I don't know if this answers your question ?

Click to expand...

Not quite. My hypothesis is interference patterns from sub-sampling with inadequate 100 MB buffer length to resolve 10's of MHz and 0.29 Hz at the same time.

I made discrete signals on Falstad's Analog simulator site, then it prompted me to increase the BW (or sampling rate as Falstad Sim only has a small buffer for browser simulation. But after, If I reduce the sampling rate 3 orders of magnitude from 3.125 ns to ms, I see the interference pattern as baseband. Only one signal had a 0.254 Hz offset . You can expect up to N multiples of the resolution with N signal sources.

This is the way old sub-sampling GHz scopes would work with fast capture but slow sampling rate.

Does that describe your capture?


sim https://tinyurl.com/2apq4qnq

Low-frequency modulation in DDS-generated signals often arises from DDS resolution limits, amplifier nonlinearity, or measurement artifacts like oscilloscope undersampling. Verifying with a spectrum analyzer and adjusting DDS settings can help. Using a linear amplifier or phase dithering may reduce intermodulation distortion (IMD), especially in precision applications like laser systems

Hi,
Firaconsortium :After another discussion with the manufacturer, it appears that both DDS resolution and amplifier non linearity cause the issue. At this point in time, I don't have a better amplifier and thus cannot test the hypothesis, but in the future I will investigate this idea.

D.A. (Tony) Stewart: I don't think this is the issue. Especially since this would mean that I am seeing a measurement artifact which would then have no physical impact on the real signal. However, thanks to the laser I am using, I can actually see the modulation based on the laser light going on and off. Again the laser is not a relevant component in the origin of the issue, but it definitely helps to confirm that the problem is physical and not only a numerical error of the oscilloscope. Also, the capture you send doesn't exactly look like what I am seeing. Attached is one of the occurrences of the beating I see.

Kaz1: the randomized phase to reduce PAPR was taken care of prior to my initial question. However, I didn't think of adding noise in the system. I'll look into it and see if I get good results.