Amplitude shift keying fails at 10MHz for SDG2042 - Page 1
Forum section: electronics / testgear

author: maelh
author_rank: Regular Contributor
timestamp: December 13, 2022, 09:07:52 am
content: Hi,I have a strange problem with my signal generator SDG2042X (firmware upgraded to 120MHz).I am trying to generator a sine wave that is modulated by a square wave, or in other words a sine wave that is switched on or off in regular intervals.The SDG2042X has an option for amplitude shift keying (ASK). This works fine for a carrier of 10kHz and a modulation of 1kHZ ASK, but it fails already for a carrier of 100kHz with the same modulation of 1kHz ASK.Instead it doesn't seem to modulate at all, and instead I can see a 1kHz square wave modulated by a 330mHz (milliherz, so very low frequency) sine wave.Is this a sign of the mixing limitation of my signal generator? It seems this low frequency sine wave is an artifact in general (just that the low frequency varies depending on the actually desires signal frequency).Before that I had a similar problem, when I wanted to actually create a 10MHz sine carrier modulated by a 1kHz sine wave.Is there any solution for this signal generator?My goal is to create a 10MHz sine wave with a 1kHz square wave modulation.Edit:The signal generator output is connected to the scope with a 50 Ohm BNC cable and a 50 Ohm termination.I also noted that a pure 10MHz sine wave seems to have low frequency noise superimposed. When I zoom out, I can see a 33.3Hz sine wave.For a pure 1MHz sine wave, I can see a 3.3Hz sine wave.

author: wasedadoc
author_rank: Super Contributor
timestamp: December 13, 2022, 11:01:43 am
content: If using a digital scope check that the sampling rate is high enough not to cause aliasing of the carrier frequency. It is not unusual for a scope to automatically reduce the sampling rate drastically when the horizontal timebase is slow.

author: maelh
author_rank: Regular Contributor
timestamp: December 13, 2022, 11:17:49 am
content: Thank you, that's definitely something to keep in mind (It's indeed a digital scope that goes up to 100MHz).Does anybody also have a solution for creating a ASK/square wave modulated 10MHz carrier with my signal gen? Does anybody have similar issue with the "hacked" SDG2042X or did you have success?Because in this case the scope was not the issue, I could see the 10MHz carrier fine, only when adding modulation it didn't work out anymore: the 1kHz square wave got modulated, instead of the 10MHz carrier (and this is at a scale where the unmodulated 10MHz sine wave could be seen well beforehand).I could add a scope screenshot, however it looks quite chaotic.

author: wasedadoc
author_rank: Super Contributor
timestamp: December 13, 2022, 12:04:53 pm
content: Yes, let's see the screenshot.

author: maelh
author_rank: Regular Contributor
timestamp: December 13, 2022, 12:44:14 pm
content: The scope had reset its memory depth to the minimal value, increasing the aquiring memory depth to the maximum possible (and thereby I assume the sample frequency) fixed the issue. Odd since the pure 10MHz sine wave appeared to be mostly fine.Apparently the modulation adds many higher frequency components and possibly they alias to lower frequencies?Either way, it seems fixed. Thanks!Edit: I see that if you keep the aquire menu open (on my Rigol DS1054Z) it will also update the sample frequency used depending on time zoom level and memory depth. That seems to explain the very weird results I saw. The ASK 10MHz sine wave appears fine now.There still seems to be some minor lower frequency noise superimposed, I am not sure if those are further aliasing artifacts or other noise.

author: switchabl
author_rank: Frequent Contributor
timestamp: December 13, 2022, 10:55:04 pm
content: If you suspect aliasing, you can always switch acquisition to "peak detect" to check the signal envelope.