Function Gen (Siglent SDG1010) mV signals?!?! - Page 1
Forum section: electronics / beginners

author: daemonix
author_rank: Contributor
timestamp: June 17, 2014, 04:31:32 pm
content: Hi!Im a newbie so pointers for reading and understanding concepts is good!Im trying to use the f-generator down to milliVolts range in order to test some circuits with op-amps. (so very small mV AC fluctuations in the input for example)On the scope (1074z) I see the the output of the f-gen is really really noisy...Am I doing something wrong?I tried to use a voltage divider so I can use the f-gen in the Volts range but in general I want to know the correct practice for using f-gens down to low mV levels.Thanks a lot!

author: w2aew
author_rank: Super Contributor
timestamp: June 17, 2014, 05:25:48 pm
content: Ideally, you would use an attenuator on the output. This will lower the signal level as well as the noise from the generator.Often though, when measuring these low level signals on a scope, the noise you see is from the scope, not the generator. A quick test - disconnect the generator from the scope and see what the noise level is on the scope at the same vertical setting. If it's about the same, then it's coming from the scope. If it drops significantly, then it's from the generator. To be sure you don't get fooled by triggering on some aspect of the noise, use the same trigger (maybe from the trigger or sync output of the generator, on a different scope channel).

author: Rick Law
author_rank: Super Contributor
timestamp: June 17, 2014, 05:29:37 pm
content: Quote from: daemonix on June 17, 2014, 04:31:32 pmSince you said you are a newbie, I assume you don't know this yet.I hate to tell you this, you can pick up single digit mV anywhere. Just laying two wire side by side, cross talk can occur. Move your hand too close to the circuit, it will pick up your disruption. At times, just tapping the table (not even banging), the breadboard on the table will pick up the noise as the connections loosen and re-seat in their slots or as the components vibrate.If you want your circuit to be single digit mV accuracy, you need careful planning with good shielding and good layout and...Hack, just measuring V+ rail on the left side of the board verses the right side of the board will give you mV difference.

author: daemonix
author_rank: Contributor
timestamp: June 17, 2014, 06:31:03 pm
content: Thank you both. I do know about random mV flying around but in my case i think a 10-20mVpp with with the base on 0v would be great from my f-gen.Ill try the attenuator. Any circuit that i have to study as a good attenuator?

author: w2aew
author_rank: Super Contributor
timestamp: June 17, 2014, 06:43:02 pm
content: Quote from: daemonix on June 17, 2014, 06:31:03 pmHere's a video I did on homemade RF attenuators. Same concepts apply to low frequencies of course, and the layout isn't critical!

author: daemonix
author_rank: Contributor
timestamp: June 17, 2014, 06:52:45 pm
content: Quote from: w2aew on June 17, 2014, 06:43:02 pmI must admit I have watched all of your videosIll watch this one again as a lot of things just skip my memory until I practically use them.Thanks for the great videos btw! I think I got the RF/HAM bug from you and your help via youtube about RF books recentlyO0