01 DM3068 Digital Multimeter
1. DM3068 Digital Multimeter
This is a RIGOL six-and-a-half digit digital multimeter, model DM3068. Yesterday, while using this multimeter in DC mode to test the DC signal in a square wave signal, I encountered a major pitfall. The frequency of this square wave signal is 1kHz, with a low level of 0V, a high level of 5V, and a duty cycle of 50%. It is generated by the DG1062. As we know, the DC component of this signal should be 2.5V. The FLUKE45 voltmeter measured this signal and displayed a DC voltage of 2.5V. What followed was a shocking revelation. The DM3068 actually displayed a DC voltage of only over 145 millivolts. When I modified the peak-to-peak value of the AC signal to 1V, the DM3068 displayed the correct DC voltage of 2.5V. However, when I changed the amplitude of the square wave back to 5V, the DM3068’s measured DC voltage returned to 145 millivolts. When I modified the frequency of the square wave to 100kHz, keeping the duty cycle at 50%, the DM3068 displayed a DC voltage of 2.5V.
2. Measurement Data
Next, by programming, I gradually increased the frequency of the square wave generated by the DG1062 from 50Hz to 25kHz. I measured the DC voltage obtained by the DM3068 at each frequency. The results were astonishing. It can be seen that when the signal frequency is below 5kHz, the DM3068 measures the DC signal of the square wave to be less than 200mV. Then, the measured DC component jumps to over 1.7V. As the frequency increases, the measured value of the DC component also rises. Only after the signal frequency exceeds 11.6kHz does the DM3068 display the correct DC component of 2.5V.
▲ Figure 1.2.1 DC Component Measurement at Different FrequenciesI feel a bit obsessive-compulsive; this time I re-measured, just lowering the frequency of the square wave from 25kHz to 50Hz. It can be seen that at high frequencies, the measurement results are still normal. When the frequency is less than 10kHz, especially below 5kHz, the measured DC voltage starts to soar.
▲ Figure 1.2.2 Voltage Measurement from 25kHz to 50HzWithout comparison, there is no harm; below I used the DC mode of the FLUKE45 multimeter to measure the signal simultaneously with the DM3068. The differences in their measurement results are evident. By the end, the DM3068 began to go haywire. The results are clear. When the square wave frequency is less than 5kHz, the error in the DC voltage measured by the DM3068 is far greater than that of the FLUKE45 digital multimeter.
▲ Figure 1.2.3 Comparison of FLUKE and DM3068 Measurement Results※ Conclusion ※
This article documents a significant pitfall encountered while measuring square wave signals with the Puyuan multimeter. Since I only had this digital multimeter on hand, I had even returned to Puyuan for repairs last year, spending 600 yuan. Now it seems that there are still some issues. It looks like I need to have it repaired again. I wonder if it can really be fixed?
