Use An MDO To Debug Your Wireless Embedded Designs

Fig 1. The Tektronix MDO4000 enables designers, for the first time, to capture time-correlated analogue, digital, and RF signals for a complete system view of their device.

Fig 2. The spectrum shown in the frequency domain view is taken from the period of time indicated by the short orange bar in the time-domain view, known as the spectrum time.

Fig 3. In this time- and frequency-domain view showing the turn-on of a VCO/PLL, channel 1 (yellow) is probing a control signal that enables the VCO. Channel 2 (cyan) is probing the PLL voltage. The SPI bus that’s programming the VCO/PLL with the desired frequency is probed with three digital channels and automatically decoded. Notice that spectrum time is placed after the VCO was enabled and coincident with the command on the SPI bus telling the VCO/PLL the desired frequency.

Fig 4. Spectrum time is moved about 60 µs to the right. At this point, the spectrum shows that the VCO/PLL is in the process of tuning to the correct frequency (2.400 GHz). It has made it up to 2.3168 GHz.

Fig 5. Spectrum time is moved another 120 µs to the right. At this point the spectrum shows that the VCO/PLL has actually overshot the correct frequency and gone all the way to 2.4164 GHz.

Fig 6. The VCO/PLL eventually settles on the correct 2.400-GHz frequency about 340 µs after the VCO was enabled.

Fig 7. To view how the RF amplitude varies over time, the RF amplitude versus time trace is added to the time domain view. Using cursors, it is simple to measure the time between the RF signal and the corresponding control signal.

A recent report by IHS iSuppli predicted that shipments of electronic products with embedded wireless local-area networking (WLAN) technology will surpass 1 billion units for the first time in 2011 and then rise to more than 2 billion in 2015 as wireless connectivity becomes a standard feature.1

Demand for wireless connectivity is fuelled by low component cost. For around $2.50, engineers can add an off-the-shelf wireless module to their design. Research conducted by Tektronix indicates that almost 40% of current embedded system designs now include wireless connectivity. 

While consumer demand is strong and component costs are low, designing and validating the performance of embedded systems with wireless functionality can be very challenging. Today, design engineers must analyse analogue, digital, and RF signals when troubleshooting their wireless-enabled designs. To make these measurements, they typically use an oscilloscope to view time-domain waveforms and a spectrum analyser to look at the frequency domain signals.

Both instruments do a good job in their respective domains, but they don’t work well together. So, it’s difficult to analyse the interaction between time-domain control signals and output signals in the frequency domain and vice versa. Designers need the ability to capture time-correlated analogue, digital, and RF signals with a single instrument to see how the RF spectrum is changing over time.

The Mixed-Domain Oscilloscope

A mixed-domain oscilloscope (MDO) can display both time- and frequency-domain information in a single glance. The RF spectrum can be viewed at any point in time to see how it changes over time or with device state. In fact, the Tektronix MDO4000 enables designers, for the first time, to capture time-correlated analogue, digital, and RF signals for a complete system view of their device (Fig. 1).

An MDO can be used as a standalone oscilloscope, or it can function as a full-function spectrum analyser. When both the RF channel and any analogue or digital channels are on, the oscilloscope display is split into two views. The upper half of the display is a traditional oscilloscope time-domain view. The lower half of the display is a frequency-domain view of the RF input.

The frequency-domain view is not simply a fast Fourier transform (FFT) of the analogue or digital channels in the instrument, but is the spectrum acquired from the RF input. The spectrum shown in the frequency-domain view is taken from the period of time indicated by the short orange bar in the time-domain view—known as the spectrum time (Fig. 2).  Spectrum time can be moved through the acquisition to investigate how the RF spectrum changes over time while the oscilloscope is live and running or on a stopped acquisition.

Advanced Triggering

To deal with the time-varying nature of modern RF applications, the MDO provides a triggered acquisition system that is fully integrated with the RF, analogue, and digital channels. This means that a single trigger event coordinates acquisition across all channels, so the spectrum can be captured at the precise point in time where an interesting time-domain event is occurring. 

Time domain triggers include Edge, Sequence, Pulse Width, Timeout, Runt, Logic, Setup/Hold Violation, Rise/Fall Time, Video, and a variety of parallel and serial bus packet triggers. In addition, the MDO can trigger on the power level of the RF input. For example, it can trigger on an RF transmitter turning on.

Advanced RF triggering enables the RF input power level to be used as a source for Sequence, Pulse Width, Timeout, Runt, and Logic trigger types. For instance, the instrument can be set to trigger on an RF pulse of a specific length or use the RF channel as an input to a logic trigger, enabling the oscilloscope to trigger.

To illustrate the benefits of time-correlated analogue, digital, and RF display, consider a voltage-controlled oscillator (VCO) with phase-locked loop (PLL) control. Figure 3 shows the setup for this test, where channel 1 of the oscilloscope is used to probe the VCO enable line, channel 2 is connected to the PLL voltage signal, the serial peripheral interface (SPI) control bus is monitored and decoded using the digital inputs, and the VCO output is connected directly to the MDO RF input.

First, the SPI bus sends a command to VCO to set the frequency to 2.4 GHz. Based on the command received, the PLL output voltage starts to increase as expected. The higher voltage tunes the VCO to a proportional frequency. After some time delay, the PLL output frequency adjusts to the desired value.

On the MDO4000 display, the spectrum time bar can be moved to see the output frequency and characteristics at any point during the startup sequence. The single instrument shows the serial bus commands, analogue outputs, and measurement of the turn-on time of the PLL (Figure 4, Figure 5, Figure 6). 

Another situation when time-correlated analogue, digital, and RF can really simplify the debug process might be when we need to characterise performance of a digital wireless servo system. The goal is to measure the time delay from when the control signal is received to when the RF signal reaches its new state.

The centre frequency is set to match the transmitter carrier frequency. To view how the RF amplitude varies over time, the RF amplitude versus time trace is added to the time-domain view (Fig. 7). The MDO RF pulse-width trigger can be used to capture the command, and both signals are displayed. Using cursors, it is simple to measure the time between the RF signal and the corresponding control signal. 

Correlated Measurements

Correlating events in the frequency domain with time-domain signals is simple with an MDO. Whether you’re integrating Bluetooth, ZigBee, or other wireless technologies, you can use an MDO to quickly troubleshoot system-level issues. And with the MDO4000, correlating events, observing interactions, or measuring timing latencies between the two domains is exceptionally easy, giving you quick insight into your design’s operation.

Reference

1. IHS iSuppli report: “More Than 1 Billion Devices to Have Embedded Wireless Networking Capability,” by Jagdish Rebello, 15 April, 2011