Pre-compliance EMC Testing Using a Real-time Oscilloscope

Figure 1. Formula for calculating radiated power from electric
field measured at a given distance. Only a few nanowatts of
radiated power can cause a product to fail an EMC certification test.

When designing an electric circuit board, we always start with a schematic. All it tells us is the components in use, how they are connected, and what the functionality of the system is. The schematic tells us absolutely nothing about signal integrity, power integrity or electromagnetic interference (EMI). All the schematic tells us about is the connectivity.

Problems with signal integrity, power integrity and EMI all come to life when we turn that schematic into a physical implementation, because once we have connectivity established by the interconnects, the only thing interconnects are going to do is screw up our beautiful design. They're going introduce noise, and that noise is going to cause some combination of signal integrity, power integrity and EMI problems. The best we can do is to minimize its appearance and impact using best design practices.

In this series, we'll focus on design issues that affect EMI, and how you can use a real-time oscilloscope to find the root causes of EMI that negatively affect a product's electromagnetic compatibility (EMC).

Testing Power Rail Sequences in Complex Embedded Systems

Figure 1. Four power rail signals on a single grid, with cursors
measuring the time delay between the first pair in the sequence.

Embedded computing systems generally require multiple supply voltages to deliver power to the microprocessor, memory and other on-board devices. There is usually a 12- or 15-volt DC primary supply, and numerous buck or boost converters working off the primary to help provide various voltages throughout the embedded system. Most microcontrollers have a prescribed order in which the voltages must be applied to prevent problems like lockups, so an area of concern when designing deeply embedded systems is the proper sequencing of power rails as they power up or down. Power management IC’s (PMIC) or power sequencers perform many of the sequencing tasks, but during validation and when troubleshooting, the order and timing of the power sequence should be verified.

Correlating Sensor and Serial Data in Complex Embedded Systems

Figure 1: Voltage output of a temperature sensor.
As the temperature rises, the output voltage falls. 

The microcontrollers/microprocessors in deeply embedded systems often are set up to monitor and control operational parameters.  Take, for example, a deeply embedded system where a microcontroller is used to control temperature that has been sensed by a temperature sensor.  The sensor is read through one of the microcontrollers analog interfaces.  As temperature changes are sensed, the microcontroller adjusts the speed of a cooling fan, which is driven by a pulse width modulated signal. The microcontroller uses a program algorithm to convert the DC level of the sensor into a PWM signal with an appropriate duty cycle to set the fan speed to correct any changes in temperature. 

Where it is possible to probe the temperature sensor, the output is a DC signal that changes very slowly over time.  Figure 1 shows a direct measurement of the temperature sensor using a heavily filtered oscilloscope channel to minimize noise pickup.

Correlating Low to High-Speed Events in Complex Embedded Systems

Figure 1: A challenge when testing embedded systems
is to correlate events in a low-speed interface like SPI
to events in a high-speed interface like PCIe.

A common requirement when testing embedded systems is to measure the timing between signals with low data rates and those with high data rates. Looking at the functional block diagram of our typical deeply embedded system in Figure 1, we see low-speed serial interfaces like SPI and I2C along with high-speed serial links, like PCIe (often serving as the high-speed serial PHY in our diagram).

Take for example testing the initialization of the system. When power is first turned on, the ROM bios and flash memory initialize program elements that are required by the embedded system’s microprocessor. Once the initialization is complete, the microprocessor has to notify the motherboard via PCIe that it is active and ready to receive data via the high-speed serial bus. This all has to happen within 200 milliseconds.