How do I use the DVM and frequency counter function on the MDO3000?
Hello and welcome to Tektronix! Today I’m going to demonstrate the digital voltmeter and frequency counter functionality of the MDO3000 series mixed domain oscilloscope.
The DVM can use any of the 4 analog scope inputs to feed the signal into a free-running voltmeter with 4-digit precision or into a frequency counter with 5-digit precision. The DVM measurements are free-running in relation to the scope’s acquisition system, so its measurements are always updating even if the scope isn’t actively acquiring data.
For this demo I’m using the MDO3000’s built-in AFG to generate the signals that will be measured by the DVM. I’m using the AFG to generate a 5 kHz 100 mVpp sine wave with a 50 mV offset and acquiring it on channel 1. To activate the DVM, press the Measure button, press the DVM softkey, and use the Multipurpose knob A to change the DVM mode. Let’s begin with AC RMS. Simply scroll down to the AC RMS option and the DVM will automatically measure and display the AC RMS value of the signal from the selected source. Similarly, to measure the DC level of the signal, simply scroll to the DC option in the menu. Here we measure exactly only the DC component or offset of the signal. To measure the AC+DC RMS value of the signal, scroll once again to this option in the DVM menu. This function takes a true RMS measurement of the waveform including the DC offset.
The Frequency Counter can be accessed in the same way as the DVM. Press the Measure button, press the DVM softkey, and scroll down to Frequency. This will measure the zero-crossings of your signal and extract frequency information out of these measurements.
The DVM provides statistical information on its measurements, showing the maximum and minimum measured values and a graphical range of measurements taken over a short period of time. The maximum value is indicated by a red flag, the minimum value is indicated by a green flag, and the current value is indicated by a yellow arrow. A gray bar shows the range of measurements taken within the last five seconds. It can be used as a measurement settling indicator because the bar will shrink down to nothing if the measurement hasn’t changed in 5 seconds. Measurements statistics are shown in the right corner of the DVM display. You can clear the DVM statistics by pressing the Reset DCM Statistics softkey. This will remove the maximum, minimum, and average values on the right side of the DVM display and replace them with the current value.
The DVM and counter depend on the scope’s triggering and scaling system to acquire an accurate measurement. For example, if you’re measuring frequency and the trigger level is outside the waveform, the counter can’t see zero-crossings and won’t be able to calculate frequency. Similarly, if you’re making an RMS measurement and the vertical range is too large or too small, the DVM can’t make the measurement because the signal is either clipping or has insufficient amplitude. You’ll notice that changes in vertical scale changes the DVM range. If the yellow current measurement arrow is visibly within the displayed range, the measurement will have the highest accuracy.
Instead of using a scope to look at waveforms, a DMM to measure AC or DC levels with higher accuracy, and a frequency counter to measure frequency of a signal, you can accomplish all three tasks with one instrument, the highly-adaptable MDO3000.
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