Hunting noise sources in wireless embedded systems

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.

 Hunting down noise sources has never been easy. However, as embedded systems have become more complex with the addition of wireless, designers face bigger hurdles than ever before to track down noise sources. And let’s face it – wireless is everywhere. It’s estimated that there are more than 1 billion wireless devices in use today and more than 30 percent of embedded designs now include wireless, and that number continues to grow daily.

In adding wireless capability to embedded systems, there are a number of issues typically encountered in the integration. For battery powered systems, a switching regulator is typically used to have the highest practical efficiency at the lowest cost. The size of the power supply is also often an issue. This can lead to the use of high switching frequencies to minimize the size and requirements of output filtering. These power supplies often have ripple on the output voltage which can show up on the RF transmitter output, especially when under load or under low battery conditions. To avoid this, additional power supply filtering may be needed to avoid unwanted impairment of the radio signal, even though the cost or size is undesirable.

The hardware circuits and the software configuration of the radio chip or module can affect the quality of the transmitted signal. If not properly set up and filtered, the radio can cause interference to other radio systems and/or fail to conform to applicable agency regulations. Some radio systems will need channel filters, RF Surface Acoustic Wave, or other relatively expensive filters to meet agency regulations for out-of-channel and out-of-band emissions.

The tool of choice for the embedded designer, the oscilloscope, is optimized for making time domain measurements only. An MSO (Mixed Signal Oscilloscope) can measure both analog and digital signals, but it remains difficult to effectively measure RF signals with an oscilloscope at the RF carrier. It is also quite difficult to adequately correlate events in the time and frequency domains – something critical for finding system-level problems.

While spectrum analyzers are available for making measurements in the frequency domain, these are not the tool of choice for most embedded designers. Using spectrum analyzers to make time correlated measurements with the rest of the system is virtually impossible.

In this article, we will explore tips and techniques for hunting noise sources using a new type of instrument called the mixed domain oscilloscope, or MDO. Tektronix recently introduced the world’s first MDO and the examples provided here are based on the MDO4000 Series. The oscilloscope has the ability to simultaneously display four analog signals, 16 digital waveforms, up to 4 decoded serial and/or parallel buses, and one RF signal. All of these signals are time correlated to show the effects of control signals on the analog and RF domains.

Before diving into a hands-on example on the use of the MDO, it might be helpful to first review some of the key concepts behind this oscilloscope. The primary value of a mixed domain oscilloscope for hunting noise sources 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 multiple analog, digital, and RF signals.

When we talk about time-correlated what this means is the MDO can measure timing relationships between all of its inputs. It can, for instance, measure the time between a control
signal and the beginning of a radio transmission, measure the risetime of a transmitted radio signal, or measure the time between symbols in a wireless data stream. 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 important for understanding the complete system operation or cause and effect.

Time domain signals are signals that are best viewed as amplitude vs. time. These are the signals traditionally measured with an oscilloscope. Viewing signals as amplitude vs. time helps answer questions like; “is this power supply really DC,” “is there sufficient setup time on this digital signal,” “is my RF signal on,” or “what information is currently being sent over this wired bus?” Time domain signals are not limited to analog inputs. Seeing RF amplitude, frequency, and phase versus time can enable a study of simple analog modulations, turn-on, and settling behavior of RF signals.

Frequency Domain signals of the other hand are signals that are best viewed as amplitude vs. frequency. These are the signals traditionally measured with a spectrum analyzer. Viewing signals as amplitude vs. frequency helps answer questions like; “is this transmitted RF signal within its allocated spectrum,” “is the harmonic distortion on this signal causing problems in my device,” or “are there any signals present within this frequency band?”