Electronics > RF, Microwave, Ham Radio

Low frequency modulation in high frequency DDS generated signal

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Quant:
Hi all,

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 (see oscilloscope screenshot ).
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 (attached is an occurrence of the beating measured on camera)

Any input would be greatly appreciated. Thanks in advance.

jonpaul:
Bonjour cher Monsieur,

Je vous suggère de poser le problème à l'assistance du fabricant :

https://spectrum-instrumentation.com/support/support_rma.php

Je chercherais également de l'aide dans un forum plus spécifique, par exemple sur les DSP.

Bonne chance,

Jon


I suggest you pose the problem to the manufacturer support:



https://spectrum-instrumentation.com/support/support_rma.php


I would also seek assistance at a more specific  forum eg on DSP

Good luck,


Jon
_____

Quant:
Hi Jon,

Thank you for your reply. I've already contacted the support team but unfortunately the conversation didn't solve my problem. They were the ones who suggested that the modulation could come from the 290mHz frequency step. For now I'll try asking on a DSP forum as you suggested (thank you for the tip).
Best regards,
Quant

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