Is this a Function generator issue? - Page 1
Forum section: electronics / beginners

author: ArnieG
author_rank: Newbie
timestamp: August 25, 2025, 01:15:54 pm
content: I am using a GFG-8020H function generator, set to produce a Sine wave, 3 V peak-to-peak, 500 Hz with no DC offset (knob pressed in).This is the image that was displayed on an oscilloscope:Somehow even with no DC offset selected on the function gen, the sine wave is not centred.Is there some setting or setup on the function gen that I missed, or some adjustment/calibration that I could do to correct this problem? Suggestions welcome!

author: Grandchuck
author_rank: Super Contributor
timestamp: August 25, 2025, 01:24:03 pm
content: Set the FG to max amplitude and post that image.

author: Grandchuck
author_rank: Super Contributor
timestamp: August 25, 2025, 03:55:25 pm
content: Found this on the Internet:DC Offset: Variable +10 to -lOV open circuit, +5 to -5V into 50 ohms

author: bdunham7
author_rank: Super Contributor
timestamp: August 25, 2025, 04:05:15 pm
content: Quote from: ArnieG on August 25, 2025, 01:15:54 pmI haven't seen that model but I had a bunch of it's competitor analog FGs, the Tek CFG-250 and they probably function similarly. Your wave looks like it has some distortion at the top peak as well as the offset. This indicates that it needs at least internal adjustment. On the Tek there are about eleventeen million trimpots that adjust offset, shape, frequency, etc etc. It's very complicated and really requires a service manual. IIRC the sine wave is synthesized or shaped piecewise from a triangle wave fundamental and when that part of the circuit is out of adjustment (or has a failure) then you'll see those sharp-tipped sine waves.https://www.artisantg.com/info/Instek_GFG_8020H_Manual.pdfSo try and dig up a service manual, otherwise you may find this very difficult to correct.

author: TimFox
author_rank: Super Contributor
timestamp: August 25, 2025, 04:47:20 pm
content: Analog function generators, which shape a sine wave through a piecewise linear network from a triangle wave, are prone to a problem at the positive and negative peaks due to a required discontinuity in the first derivative between the original triangle and the desired sine shapes.

author: CaptDon
author_rank: Super Contributor
timestamp: August 25, 2025, 05:32:22 pm
content: Yes, the simple answer is that there is usually an internal D.C. Offset trimpot which sets the offset when the button is not active. It is often a ten or fifteen turn pot. just tweek it and will be fine.

author: ArnieG
author_rank: Newbie
timestamp: August 26, 2025, 02:00:25 am
content: Quote from: CaptDon on August 25, 2025, 05:32:22 pmIt certainly looks like it. Although there is absolutely no DC offset selected on the FG, based on the oscilloscope trace, the sine wave seems to be riding on top of 280mV DC.

author: wraper
author_rank: Unknown
timestamp: August 26, 2025, 02:08:54 am
content: Did you check if there is no offset with signal gen disconnected?

author: CaptDon
author_rank: Super Contributor
timestamp: August 26, 2025, 11:46:07 pm
content: That's why I said "Look for the internal adjustment"!!! The external knob sets the offset when in the active (pulled out) position. Inside the unit is the offset adjust for when the offset is not active (pushed in). 99% of function generators have that internal adjustment or the digital equivalent in the CAL menu if it is a very expensive digitally controlled function generator. Any of this making any sense?

author: ArnieG
author_rank: Newbie
timestamp: August 27, 2025, 02:08:27 am
content: Quote from: Grandchuck on August 25, 2025, 01:24:03 pmNow surprisingly when i tried this, maxing out the amplitude, then dialling it back down to 3V p-to-p - the unwanted "offset" went away.Strange!That was a quick fix, appreciate the suggestion.