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Function Generator Waveform Discontinuity

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jcwtm:
Hi all, I just bought a used BK 4040 function generator and its waveforms are discontinuous at the maximum and minimum. This is most noticeable when outputting the triangle wave and is present to some degree on all frequency ranges. No other features have been enabled. I have tried other function generators on this scope and this function generator on another scope and am certain it's an issue with this function generator. I only have a basic understanding of how a function generator works but if the triangle wave is generated by charging and discharging a capacitor, how can the waveform be discontinuous if it's the same capacitor? Any ideas as to what could be causing this?

james_s:
It isn't discontinuous, something is causing a rapid voltage change just past the upper and lower peaks though which is quite strange. Do you have a schematic? Sounds like someone was able to get one. https://www.eevblog.com/forum/repair/bk-4040-function-generator-issues/

I don't know how the B&K 4040 is designed, it's possible that it is a digital function generator, it could have a stuck bit in the DAC.

Brumby:

--- Quote from: james_s on October 22, 2021, 06:15:01 am ---It isn't discontinuous, something is causing a rapid voltage change just past the upper and lower peaks
--- End quote ---
Agreed


--- Quote ---though which is quite strange.
--- End quote ---
It certainly is.


--- Quote ---it could have a stuck bit in the DAC

--- End quote ---
Looks to me like a DC offset between the positive going half cycle and the negative going half cycle.  Not sure I would go for a stuck bit in the DAC as this would present as a repeated step change.

Interesting fault, though.

jcwtm:

--- Quote from: james_s on October 22, 2021, 06:15:01 am ---It isn't discontinuous, something is causing a rapid voltage change just past the upper and lower peaks though which is quite strange. Do you have a schematic? Sounds like someone was able to get one. https://www.eevblog.com/forum/repair/bk-4040-function-generator-issues/

I don't know how the B&K 4040 is designed, it's possible that it is a digital function generator, it could have a stuck bit in the DAC.

--- End quote ---

I'll try to get a copy and report back.

TurboTom:
It's analog design, based on the MAX038 function generator IC. Since the waveform shaping happens internally in the IC with the timing capacitor charge/discharge voltage as the main signal source, the only external cause for an effect like this may be a too high resistive component in the timing capacitor link, causing a level jump when the current sources/sinks are toggled. You may check the range switches and look for other (partial) discontinuities in this area.

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