DAILY NEWS Nov 23, 2012 7:00 AM - 0 comments

Using a mixed domain oscilloscope to take wireless system-level debug to the next level

MDO troubleshooting

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By: Gina Bonini, worldwide technical marketing manager, Tektronix Inc.
2012-11-23

The trend toward integrating wireless radios into a multitude of different products, from livestock tracking tags to smart energy home appliances, is causing a profound change in the responsibilities of engineers as they struggle to incorporate unfamiliar RF technology into their designs. System-level designers frequently find themselves needing to solve problems without equipment capable of performing the full range of required tasks.

Designers have historically struggled to make the measurements necessary to design, troubleshoot, and verify systems that contain analog, digital and RF signals. Until recently, no single instrument has been optimized for measuring all of these signal types. That changed last year with the emergence of the mixed domain oscilloscope (MDO) that combines a mixed signal oscilloscope with a modern spectrum analyzer. The key breakthrough provided by the MDO for troubleshooting and verification tasks is its ability to make time-correlated measurements across two domains: the time domain and the frequency domain. In addition, it can make these measurements across 20 analog and digital signals, and an RF signal.

Time-correlated means that the mixed domain oscilloscope can measure timing relationships between all of its inputs. It can, for instance, measure the time between a control bus sending a ”change frequency” command and the radio reaching the desired frequency, measure the turn-on time of a radio, or look at radiated emissions from a switching power supply. A power supply voltage dip during a device state change can be analyzed and correlated to the impact on the RF signal. Time correlation is critical for understanding the complete system operation: cause and effect.

Now let’s take a closer look at several applications areas where the MDO offers advantages compared to traditional equipment.

EMI detection

Electromagnetic Compatibility (EMC) continues to be an important element of modern electronics design. Compliance to some level of Electromagnetic Interference (EMI) testing is required for every electronic device. It is often difficult, however, to debug and troubleshoot problems in this area. Diagnostics become even more difficult if the source of interference is intermittent. It can be almost impossible to correlate emissions with specific events in an electronic device using traditional measurement techniques.

Examples include:

* Correlating impulse emissions with specific memory access or diagnostic state of a device under test

* Characterizing emissions during the device power-up or state changes such as going in and out of low power mode

* Understanding the relationship between emissions and higher order harmonics

The MDO is well-suited to these types of measurements. It combines a wide capture bandwidth of up to 3 GHz in the frequency domain with the ability to make measurements at a specific point in time, correlated with events of interest in the time domain.

The noise from a power supply can be measured with an EMI current probe. For the example in Figure 1, a switching power supply is being loaded by a resistor and small capacitor. The automated marker function of the MDO is used to show the frequency and amplitude of the most prominent signals radiating from the supply. The highest value is represented as the red reference marker. The fundamental frequency and the second harmonic are about the same level at around 30 dBuA. The upper half of the screen shows the waveform at the switching transistor in the device under test.

(*See Figure 1)

Identifying noise

When integrating a radio chip or module into a typical embedded system, a common and often frustrating task designers face is tracking down and eliminating noise and spurious signals. Potential noise sources include switching power supplies, digital noise from other parts of the system, and external sources. Noise considerations also include any possible interference generated by the radio as well as the need to avoid interfering with other radios while meeting regulatory requirements.

One use for the MDO is to understand the effects of powering a radio IC with a switching power supply. This is aided considerably by the use of what is known as Spectrum Time. This allows the user to move through an acquisition in the time domain to investigate how the RF spectrum changes over time. In Figure 2, Spectrum Time is placed to show the spectrum of a transmitted signal during several symbols of a preamble of a packet.

(*See Figure 2)

To see the packet transmission from the radio, RF vs. time traces have been added to the time domain view. The orange trace marked with “A” shows the instantaneous RF amplitude vs. time. While the orange trace marked with “f ” shows the instantaneous RF frequency vs. time, relative to the center frequency of the spectrum display. The green trace (Channel 4) shows the current into the module. As can be seen, the current rises from close to 0 between packets, to about 40 mA during transmission. The yellow trace (Channel 1) shows the AC ripple on the power supply voltage at the module. There is only a small dip in the voltage during transmission.

The previous capture was taken with the radio module powered by a clean laboratory power supply. In Figure 3, the same RF signal is shown but with a boost type switching supply powering the radio module. Boost regulators are notorious for generating noise, but are valuable to allow the use of a battery with one or two alkaline or NiCad cells and relatively few components, lowering the cost.

As shown, the noise has increased at the base of the modulated signal. Near the transmitted signal, there is noise at least 5 dB higher than with the clean power supply. The noise is also readily apparent in the current and voltage waveforms. The additional noise would degrade the signal-to-noise ratio of the signal at the receiver used to gather the data from this transmitter, reducing the effective range of the radio system.

(*See Figure 3)

ZigBee integration

As ZigBee and similar smart automation standards become more commonplace in all types of embedded systems and applications, engineers need the ability to quickly and efficiently validate and verify ZigBee module performance. This system level task is made more complex with the presence of RF and the need to look at analog, digital and RF signals in concert with each other.

Some of the critical tests to verify radio operation include RF and power supply measurement, digital commands, spurious signals and interference. Radio ICs and modules need to be set up to meet the operating requirements of the specific application and any protocol-specific setups. The MDO allows decode of the SPI commands to the ZigBee module. In addition to SPI, decode can be obtained for additional protocols including I2C, RS-232, USB, Ethernet, CAN, LIN, and others.

Figure 4 shows the digital capture of the SPI commands. In this case, the analog, digital, and RF acquisitions have been set to trigger on the drain current of Trace 4 occurring above the 130 mA level. All the time domain measurements in the upper display left of center show the events that occurred prior to the current exceeding this level at RF turning on. This includes digital decode, analog (voltage and current), and RF vs. time. This shows that a digital command occurred roughly 600 microseconds before the RF event turn-on.

(*See Figure 4)

The traces in purple show where the decoded data is in the time domain. Pan and zoom functions can be used to read digital waveforms and the decoded data. Other commands and data read back on SPI (MISO) can be read or triggered on to confirm correct commands and verify operation of the radio.

The MDO architecture simplifies measurements between SPI commands and correlated RF events. In Figure 5, the trigger event is now changed to the SPI command {37}, the radio transmit command. Markers on the time domain display show the SPI command to current draw (at the beginning of the RF Tx turn-on) is now 1.768 ms.

(*See Figure 5)

In the previous example from Figure 4, the command delay to turn-on was about 600 µs. The actual event in Figure 5 is almost three times longer. This demonstrates the behavior of the ZigBee radio is actually complying to one of the PHY layer performance requirements of IEEE 802.15.4. The ZigBee radio uses a pseudo-random delay between command and turn-on event to enable the radio to listen for other ZigBee radio transmitters or other radio interference channels.

The need to look across digital, analog and RF signals at the same time on the same design has become an almost universal requirement for system level debug, whether to debug timing issues across serial and RF, integrate and verify wireless components, or to identify unwanted sources of EMI. The mixed domain oscilloscope represents a key breakthrough for troubleshooting and verification of today’s wireless-enabled designs because of its ability to make time-correlated measurements in both the time domain and the frequency domains.

Photos

FIG. 1: Power supply switching noise.
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Caption: FIG. 1: Power supply switching noise.
FIG. 2: Radio transmission using a clean laboratory power supply.
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Caption: FIG. 2: Radio transmission using a clean laboratory pow...
FIG. 3: Radio transmission showing increased noise with a switching power supply.
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Caption: FIG. 3: Radio transmission showing increased noise with...
FIG. 4: Packet decode of SPI Digital signals.
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Caption: FIG. 4: Packet decode of SPI Digital signals.
FIG. 5: Subsequent acquisition triggered on SPI command shows delay between command and radio turn-on.
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Caption: FIG. 5: Subsequent acquisition triggered on SPI command...

Companies in This Story

Tektronix Canada Inc.



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