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Implement five common debug tasks with a scope

Posted: 27 Jun 2014     Print Version  Bookmark and Share

Keywords:oscilloscopes  EMI  switching power supply  Logic analyser  Digital voltmeter 

In this example, the noise margin of a CAN bus serial receiver circuit was characterized using an integrated oscilloscope. First, a live CAN signal was captured with an analogue channel on the oscilloscope and loaded into the integrated arbitrary/function generator's edit memory. Then the waveform generator was used to repetitively output the captured serial stimulus signal to drive the receiver circuit's input. The serial output of the receiver circuit was then acquired with channel 3 of the oscilloscope and the decoded serial output displayed. In this example, adding a bus trigger stabilised the display.

Gaussian noise was then added to the serial signal and the decoded output of the receiver circuit was monitored, looking for data packets to begin to change or disappear, indicating bit errors. This is shown in figure 4.

Figure 4: Capturing missing serial packets at the output of the serial receiver can indicate bit errors.

By monitoring the decoded output of the receiver, we found that the receiver design worked well with noise levels up to about 40% of the serial signal amplitude, but demonstrated significant errors when the noise level reached 45%-50% of the signal amplitude. This test method is effective for quickly verifying a receiver's noise margin.

Validate a switching power supply
Oscilloscope-based power measurements let you get the same accurate and repeatable results, even if you rarely deal with power measurements. This example shows how to make common power measurements with an oscilloscope using automatic power measurements, an integrated DVM, a differential voltage probe, and a current probe.

In this example, the input voltage (yellow) and current (blue) from an AC-to-DC converter is shown in figure 5. The integrated 4-digit DVM monitored the DC output voltage. The measurement statistics at the right side of the DVM display indicate that the output voltage is very stable. The graphical readout provides a visual indication of voltage variations. A power-measurement application was then used to take input power quality measurements including power, crest factor, and power factor to characterise the effects of the power supply on the AC power source. From there, current harmonics measurements were used to provide a frequency-domain analysis of the input current, in both graphical and tabular formats.

Figure 5: The DVM monitors the power supply's DC output voltage while the oscilloscope displays AC input voltage waveform (yellow) and the current waveform (blue).

Another key power measurement is switching loss in power devices, a major limitation to a power supply's efficiency. In figure 6, the differential voltage across the MOSFET (yellow trace) was measured, as was the current flowing through the switching device (blue trace). Then the instantaneous power waveform was generated (red trace) and switching loss power and energy measurements were displayed.

Figure 6: The screen displays voltage, current, and power loss in a MOSFET.

Finally, a measurement called safe operating area allows automatic monitoring and pass/fail testing of switching behaviour over various input and load conditions. By comparing the switching device's voltage, current, and instantaneous power levels relative to the device's maximum ratings, this measurement is used to assure that device reliability will not be compromised by exceeding specifications.

About the author
Scott Davidson is Marketing Manager, Tektronix, MidRange Scope Product Line. With more than 30 years of experience at Tektronix, Davidson has held a variety of engineering and marketing positions, as well as manufacturing and engineering management roles. He holds BSEE and MSEE degrees from Montana State University.

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