Low frequency modulation in high frequency DDS generated signal

[Quant01]

Hi all,
Recently, I posted the following message on another forum and was advised to ask around in a DSP forum :

"I've been using an arbitrary waveform generator for one of my projects, and specifically, I'm using a DDS option on that AWG (the waveform generator is an M4i6621x8 from Spectrum Instrumentation). Essentially, I'm sending asking the DDS to output a sum of RF sine waves, all of them being on the order of 10s of MHz, with a delta in frequency of at least 1Mhz. However, on the output signal, if I ask for a certain amount of sin waves (usually 5 or more), I start observing a modulation of the amplitude of frequency a few hundred mHz (orders of magnitude away from my input frequencies). The modulation is sinusoidal.
My questions are : why does this happen and how can I remove this feature ? I would expect some MHz beating because of how sum of sine functions work, but definitely not mHz.
Some additional information :
-the frequency resolution of the DDS is about 290 mHz, so this value seems to fit with the observed beating, but I fail to see how this could be related to the issue.
-I didn't notice such beating when using less than 5 frequencies as an input.
-I am using a ZHL-1-2W+ power amplifier at the output of the AWG, which is then connected to an acousto optic deflector (a device that diffracts a laser beam according to input frequencies). The amplitude modulation becomes visible on camera based on the intensity of the diffracted beams."

If anyone knows how to solve this issue or has any idea, it would be greatly appreciated.
Thanks in advance.

 

[KlausST]

Hi,

There´s something wrong.

I ask for a certain amount of sin waves (usually 5 or more),

which are in the 10s of MHz.

So 5 sine waves @ 5MHz .. is just a signal time of 1us.

But you say:

I start observing a modulation of the amplitude of frequency a few hundred mHz

to see 100mHz .. you need to analyze at least 10 seconds, which means 50.000.000 sinewaves with a frequency of 10MHz.

**
Where am I wrong?

Klaus

 

[kaz1]

Hi all,
Recently, I posted the following message on another forum and was advised to ask around in a DSP forum :

"I've been using an arbitrary waveform generator for one of my projects, and specifically, I'm using a DDS option on that AWG (the waveform generator is an M4i6621x8 from Spectrum Instrumentation). Essentially, I'm sending asking the DDS to output a sum of RF sine waves, all of them being on the order of 10s of MHz, with a delta in frequency of at least 1Mhz. However, on the output signal, if I ask for a certain amount of sin waves (usually 5 or more), I start observing a modulation of the amplitude of frequency a few hundred mHz (orders of magnitude away from my input frequencies). The modulation is sinusoidal.
My questions are : why does this happen and how can I remove this feature ? I would expect some MHz beating because of how sum of sine functions work, but definitely not mHz.
Some additional information :
-the frequency resolution of the DDS is about 290 mHz, so this value seems to fit with the observed beating, but I fail to see how this could be related to the issue.
-I didn't notice such beating when using less than 5 frequencies as an input.
-I am using a ZHL-1-2W+ power amplifier at the output of the AWG, which is then connected to an acousto optic deflector (a device that diffracts a laser beam according to input frequencies). The amplitude modulation becomes visible on camera based on the intensity of the diffracted beams."

If anyone knows how to solve this issue or has any idea, it would be greatly appreciated.
Thanks in advance.

I suggest you try frequencies as multiples of DDS resolution e.g. for 5MHz try 0.29 * 18MHz = 5.22MHz

 

[Quant01]

Hi,

There´s something wrong.


which are in the 10s of MHz.

So 5 sine waves @ 5MHz .. is just a signal time of 1us.

But you say:

to see 100mHz .. you need to analyze at least 10 seconds, which means 50.000.000 sinewaves with a frequency of 10MHz.

**
Where am I wrong?

Klaus

Hi Klaus,
Sorry If I wasn't clear. The frequencies that I ask the DDS to output are 50MHz or so. So for instance I could have A*sin(2pi*f1*t)+B*sin(2pi*f2*t) as a requested output, with f1 and f2 being 50MHz and 52MHz. Then on the oscilloscope, I look at the signals overall shape for 20 seconds or so, and when checking this, I can see an amplitude modulation of frequency 100mHz or so (to be specific, something close to 290mHz). Attached is a screenshot of the observed amplitude. Does this clear up the misunderstanding ?
Best regards,
Quant

 

[Quant01]

I suggest you try frequencies as multiples of DDS resolution e.g. for 5MHz try 0.29 * 18MHz = 5.22MHz

Hi Kaz1,
Thanks for your input. I will definitely try something along those lines. Hopefully I'll be able to extract some interesting information from that test.
Best regards,
Quant

 

[FvM]

Without knowing the exact combination of generated frequencies, it's impossible to determine if it's regular interference or a generator artefact. You should also consider simple oscilloscope undersampling, does the modulation show with a spectrum analyzer or power meter?

 

[KlausST]

Then on the oscilloscope, I look at the signals overall shape for 20 seconds or so, and when checking this, I can see an amplitude modulation of frequency 100mHz or so

I agree with FvM ... I assume it is just a measurement artefact.

Still I don´t know what this means:

if I ask for a certain amount of sin waves (usually 5 or more)

Klaus

 

[Quant01]

Without knowing the exact combination of generated frequencies, it's impossible to determine if it's regular interference or a generator artefact. You should also consider simple oscilloscope undersampling, does the modulation show with a spectrum analyzer or power meter?

Hi FvM,
I am quite confident in the fact that this is not an oscilloscope issue or any hardware issue other than the generator because I captured this beating on camera. I'm using the signal to diffract a laser with an acousto optic deflector, and the beam intensity, which is proportional to the amplitude of the of the input RF signal. When imaging the diffracted beams, I measured a sinusoidal intensity beating, of frequency around 300mHz.
I actually tested the same signal using an AWG without the DDS option (so preprogramming all the sine waves and then sending them to the card) and the beating does not occur, all conditions equal otherwise. However it would be much more convenient for me to solve the beating issue than to forfeit the DDS.

 

[Quant01]

I agree with FvM ... I assume it is just a measurement artefact.

Still I don´t know what this means:


Klaus

The DDS I'm using works with a given number of "cores", which are responsible for generating one waveform. So the output signal is the sum of the outputs of all the cores. Therefore when I say "asking for 5 sine waves" I'm programming 5 DDS cores to output one sine wave each. The DDS will combine them and output the result.

 

[kaz1]

Hi Kaz1,
Thanks for your input. I will definitely try something along those lines. Hopefully I'll be able to extract some interesting information from that test.
Best regards,
Quant

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)