Fix the noise measurement on the Owon XDM2041 desktop multimeter | Hacker Day

2021-11-18 08:26:38 By : Ms. Ivy Ye

After purchasing the Owon XDM2041 benchtop multimeter for automatic test setup, [Petteri Aimonen] was disappointed to find that, especially in the higher megohm range, the measured value fluctuates greatly and is usually very inaccurate. Since this is a desktop multimeter worth about $170, and Owon's support staff is uncooperative, [Petteri] set out to solve the problem, starting with a complete dismantling.

As [Petteri] pointed out, there is not much inside one of these multimeters. The motherboard with the entire system contains a GigaDevices GD32F103CBT6 MCU, plus the star of the show: the HY3131 multimeter chip from HYCON Technology Corporation. After reviewing the HY3131 data sheet, the culprit is very obvious: when sampling, the presence of power supply voltage noise is usually suppressed by choosing a suitable crystal.

Unfortunately, unlike the 4.9152 MHz crystal oscillator recommended in the HY3131 reference schematic, Owon’s engineers clearly chose a 4 MHz crystal oscillator, so it essentially confuses the line noise.

[Petteri] believes that the resulting sampling timing may work well at 60 Hz line frequency, but it is obvious that a lot of noise sneaks into the measurement at 50 Hz. As shown in the figure, after replacing the crystal with a 3.072 MHz crystal, the performance has been significantly improved.

The elements of Euclid are still related to the Fourier transform on finite groups after 2300.

OK. But this messes up the frequency measurement because it is based on the CPU clock. Now someone must write an open source firmware to care about this division factor.

here you go. Please continue to develop.

HY3131 accepts an external clock source, and there may even be no need to change ENXI. People may have to sniff SPI commands to detect applicable modes, but I can see how to use fractional-N PLLs to achieve interference-free conversion between different clock frequencies, and possibly even add line synchronization in this way.

I can confirm that the external clock fed through the 30 pF capacitor is working properly. I did this to try it before buying a crystal.

I am glad to see that someone has implemented a hardware solution that not only solves the 50Hz noise problem, but also solves the frequency measurement accuracy...

Well, I want to know if a complete calibration can solve the frequency measurement problem.

Well, this is your problem. Your power almost exceeds the specification by 17%! Where do you have such a jankey grid? Texas? In this situation, how can you expect to accomplish anything?

I think you need to move to a more civilized environment.

No, but one of the first power transformers I accumulated in the early 1970s came from Hammond (a well-known Canadian company) with a rated frequency of 25 Hz.

I think this is Niagra's standard placement for a while.

A 60Hz transformer will generate heat at 50Hz or 25Hz, but in turn means that the transformer does not have to work so hard.

What a rude comment. Again, like the metric system, 50 Hz is the standard in most parts of the world, except for...you guessed it...North America. Maybe read a book first, and then call the rest of the world uncivilized.

60Hz: A smaller transformer core is required. This may not be a problem for household electronic products that now use SMPS, but it is important for large/large power transformers.

I'm fairly certain that this is a joke about the recent drop in frequency and subsequent failures experienced by the Texas power grid.

Don't feed the trolls.

Well, remind me never to buy Owon products... 50Hz power supply is the standard here. The only places I know of using 60Hz are in some areas of the United States and Japan.

I want to know if their little XDM1041 encounters the same problem?

Most likely it is, because the data sheets look very similar. It does not specify the available sampling rate options, but it does show that 15 ms is the shortest time, the same as on the XDM2041. I wouldn't be surprised if they were very similar hardware, but they realized that the box was unnecessarily large.

Can't you just use a toggle switch to select the crystal?

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