Siglent SDG1025 overvoltage++ on channel 1 - Page 1
Forum section: electronics / testgear

author: fuho
author_rank: Contributor
timestamp: February 08, 2014, 04:05:02 am
content: This my first post, so I'd first like to say hi to everyone. I have been lurking here for ages, and last week finally ordered a scope, power supply and a fun gen. This forum and Dave's videos have been a great help for me.Today the last bit (DS1074Z) arrived so I could finally start playing with the signal generator (Siglent SDG1025). And I ran into problems right away, it doesn't seem to regulate the voltage on the channel 1 at all. Except for when it switches the output ranges. It is therefore always clipped to the maximum potential of that range.Here I have measure the whole range, on the left hand side is what Iset the output to be on the generator, on the right hand side is the measured amplitude from oscilloscope:Code:4.0mV -> 7.7mV..6.3mV -> 7.7mV6.4mV -> 27mV..19.9mV -> 27mV20.0mV -> 87mV..63.2mV -> 87mV63.3mV -> 280mV..199.9mV -> 280mV200.0mV -> 890mV..632.4mV -> 891mV632.5mV -> 2.85V..1.999V -> 2.85V2.000V -> 8.59V..6.324V -> 8.59V6.325V -> 26.1VI have a video of what is going on too, but I believe it is more clear from the data above.The question is, am I overlooking something? Channel 2 seems to be working fine. I tried different cables, different inputs on the scope to no avail. It just seems like there is something wrong with the generator. I intend to send it back on Monday, unless someone here can suggest any mistakes I might be making.Thanks for reading and again, I am glad I am finally posting, even though it is under less than amazing circumstances.

author: tautech
author_rank: Super Contributor
timestamp: February 08, 2014, 04:40:25 am
content: Welcome to EVVblog fuho.I have a SDG1010 and will have a fiddle to see if you are doing anything wrong. Just a heads up, are you toggling between channels to do the setup? Got to go out but I will be back.

author: BravoV
author_rank: Super Contributor
timestamp: February 08, 2014, 05:01:15 am
content: Stupid question , hows the termination ?

author: fuho
author_rank: Contributor
timestamp: February 08, 2014, 05:10:15 am
content: Hi tautech,I am not exactly sure what you mean by toggling between channels, but here is what I did:I plugged both output channels from the gen to the scope. When I am adjusting the amplitude for channel 2 on the generator, I can see the amplitude change on the scope and the voltages are ~identical.When I am adjusting the voltage for channel 1though, it doesn't change on the scope, until it reaches the end of that voltage range (this is one internal range of the generator, I know it reached the end of that range because I can hear the relay switch inside)Once it switches, it clips the output voltage on channel 1 to the maximum of the newly selected range, which means there can be for example 26.1V on the output even though the generator shows just 6.32V!!This also means that there is ALWAYS higher voltage on the output then the fun gen thinks there is!I am glad I waited for the scope to arrive before I started using the generator, I'd have fried something with that thing I am sure.BravoV: Hello, I set the generator for High impedance. but I tried with 50Ohm setting too. It's not double the voltage on the scope, it is clearly clipped/glued to the max of the internal voltage range. No matter how much I turn the knob the waveform doesn't move UNTIL I move out of the range, at which point it switches to higher/lower range depending on direction.

author: BravoV
author_rank: Super Contributor
timestamp: February 08, 2014, 05:16:50 am
content: Quote from: BravoV on February 08, 2014, 05:01:15 amQuote from: fuho on February 08, 2014, 05:10:15 amSounds like you don't know what a proper termination is, then read slowly & throughly this thread ->HERE, and learn the OP's mistake, hope this helps.

author: fuho
author_rank: Contributor
timestamp: February 08, 2014, 05:40:09 am
content: Hi again,my 50 Ohm terminators haven't arrived yet, along with some other cables and accessories. But as I said before, I set the SDG1025 to output to HighZ, and since the scope has 1 MOhm input impedance the values on the generator (when using channel 2) and scope are identical. I did read the link you suggested, but am I missing something?EDIT: Attached a second image, showing the measured output from the channel 1 of the generator. Using same settings as CH1.

author: kebogen
author_rank: Contributor
timestamp: February 08, 2014, 06:41:33 am
content: aha, I have a SDG1050, and it is a very nice function generator.I think you have made a very normal mistake. You should set the generator output to 50ohm, and the oscilloscope should be set to 50 ohm, too.

author: cyr
author_rank: Frequent Contributor
timestamp: February 08, 2014, 08:14:59 am
content: I don't see how termination explains the problem in the OP. Yes, that would yield an unexpected voltage but always approximately double.If 6.324V becomes 8.59V and 6.325V becomes 26.1V, the generator must be faulty.

author: tautech
author_rank: Super Contributor
timestamp: February 08, 2014, 12:43:57 pm
content: Quote from: fuho on February 08, 2014, 05:10:15 amThe button CH1/2 to the left of Sine will toggle the waveform setup screen between Ch1 & Ch2.You must do this to set up correctly as setup screens look very similar(with same waveform selected) except for the bar at top of screen changes color from green(CH2) to yellow(CH1)Try a Self Adjust in the Utility menu and or a Set to Default in the Utility- System sub menu.Be aware that CH2 has a max 6 V p/p were as CH1 has 20 V p/p max.I hope it is operator error just as I had when I first got mine.

author: echen1024
author_rank: Super Contributor
timestamp: February 08, 2014, 04:33:04 pm
content: Haha. As Bravo says, this is a trap for young players. I bought a few T connectors and a few 50 ohm terminators, and no more problems.

author: fuho
author_rank: Contributor
timestamp: February 08, 2014, 05:19:26 pm
content: Quote from: cyr on February 08, 2014, 08:14:59 amThis, this is what I thought. I can only get the generator to produce 8 specific voltages on CH1: 7.7mV, 27mV, 87mV, 280mV, 890mV, 2.85V, 8.59V, 26.1V no matter what I set the output amplitude to be.AFAIK DS100Z series doesn't let you lower input impedance to 50 Ohm and even though I have the T-adapters I still don't have the 50 Ohm terminators. This is why I a using the signal generator in the high impedance load mode. (I mean I could hack something together with a 50 Ohm resistor, but I am pretty sure that is not the problem here)Could someone who has a DS1000 series generator similar please plug it in, no terminators, just a simple 50 Ohm RG58 patch directly between the scope and the generator and let me know if you experience the same jumps on the output when you slowly increase the set output say by 10mV all the way to the 20V (or your generators max) output?Here are two images showing the settings on the generator side (one for each channel):

author: commongrounder
author_rank: Frequent Contributor
timestamp: February 08, 2014, 06:47:12 pm
content: I own a SDG1000 series unit, but I don't need to even try it to see that you have a problem. I would guess that the digital step attenuator is not under microprocessor control and is hung at maximum output. You are just getting the maximum values at each range as it is switched in. Sorry....

author: fuho
author_rank: Contributor
timestamp: February 08, 2014, 06:49:13 pm
content: Thanks commongrounder, that was my hypothesis too. Off to UPS with it then...