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OWON XDM1041 the unknown multimeter...

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Kleinstein:
As far As I see it, there is still a ready made DCDC converter to power the analog part. So the SMPS noise may not be the relevant noise source.
The DMM chip set uses a SD-ADC that could have simultaneous 50/60 Hz supression. It still needs the correct clock frequency.  I remember seeing a change in the clock for the XDM2041 that is internally similar.  Has someone checked if the clock is correct for mains hum suppression ?

Edit:
From the pictures it looks like they have the same 4 MHz clock and may thus have a similar poor mains hum suppression problem as here:
https://hackaday.com/2021/07/14/fixing-noisy-measurements-on-an-owon-xdm2041-bench-multimeter/

Changing the clock could still be tricky as this may also effect seriel speed.

raphaelCoelho:
I already read about it...
What it seems is that the problem happens when the main power frequency is 50Hz, on the 60Hz it doesn't.
Anyway, a linear power supply, with transformer, isolated from the network, could not improve the performance of the DMM?

Kleinstein:
An old style 50/60 Hz transformer would have some advantage, as it does not have as much higher frequency common mode bakcground and could get away without a Y cap. It is still not sure if the improvement would be very visible. It could be worth a try - at least for a quick test with an external power supply.  User kwass may be able to tell how much improvement he sees.

The 4 MHz clock is slightly better for 60 Hz than for 50 Hz, but still not really good. So I don't understand why they don't use the correct crystal - that frequency is available (some 50 cents at Mouser) and the 4 MHz one should be only marginal cheaper, if at all.
It looks like the interface to the DMM chip is SPI and thus not sensitive to the clock. So changing the clock should be OK, though it will invalidate the calibration, not just for the frequency measurement. Chances are the BW for AC RMS would also go up a little.

Johnny B Good:
 From all the teardown videos I've seen and what's been said here, I reckon replacement of the 5v 2.1A smpsu board with a conventional analogue, mains transformer based psu will be a total and utter waste of time and energy (both yours and that of its annual electricity costs).

 As it stands, it only draws a maximum of 1.5W (NOT the claimed 15W which seems to be a typo that lost the decimal point). At a 25% backlight setting, it only draws a tad less than 1W and just a tad over at the 50% setting. Since, unlike a signal generator, all of that input energy lands up heating the bench meter's innards. Indeed, when measuring current, the thermal load sees an increase over and above that basic energy input (another watt when testing a 10A current with (afair) a 0.01 ohm shunt).

 In view of all those downsides, such a psu mod seems doomed to failure in its primary goal of improving the bench meter's noise immunity and performance stability. IOW, I'm strongly suggesting that you don't waste your time (let alone the costs) of replacing the existing smpsu board. :)

BugCatcher:
I did a few simple experiments:

- recorded the voltage of two alkaline cells (5V fixed, slow mode)
  The Owon DMMEasycontrol App is recording the gain-corrected 24bit ADC-data. This
  is equal to a 6.5 digit resolution. That's resolution, not precision! -> see diagram

  -> No visible noise problem!
        (the spacing of the horizontal lines is 0.1mV ... equals the resolution of the displayed measurement results)

- resistance mode
  two 1m long banana-cables are hanging down "flapping in the breeze", not twisted. Cables are a few cm away from a 230V/50Hz cable
  -> reading: 50MOhm-range "overload"
  connected a 20MOhm resistor at the end of the banana-cables
  -> when I'm sitting still   -> stable reading with the normal 1 unit quantization noise
  -> when I'm moving          -> up to 20..30kOhm variation
                                          -> a few kOhm even if I'm a few meters away

  -> Pure electrostatic effect!   No 50/60Hz interference!


 I don't see any problem!

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