Test equipment eases hunt for noise sources in wireless embedded systems

4 mins read

When integrating a radio chip or module into a typical embedded system, designers often face the frustrating task of tracking down and eliminating noise and spurious signals. Potential noise sources include switching power supplies, digital noise from other parts of the system and noise from external sources.

Hunting down noise sources has never been easy. However, as embedded systems have become more complex with the addition of wireless, designers face even bigger hurdles. And wireless is everywhere; it's estimated there are more than 1billion wireless devices in use and more than 30% of embedded designs now include wireless. When 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 provide 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 minimise the size and requirements of output filtering. These power supplies often generate ripple on the output voltage, which can show up on the output of the rf transmitter, 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 may be undesirable. The hardware circuits and the software configuration of the radio chip or module can also affect the quality of the transmitted signal. If these are not set up and filtered properly, the radio can interfere with other radio systems and/or fail to conform to applicable regulations. Some radio systems will need channel filters, rf surface acoustic wave or other relatively expensive filters to meet regulations for out of channel and out of band emissions. Oscilloscope: the tool of choice The tool of choice for the embedded designer, the oscilloscope, is optimised for making time domain measurements. While a mixed signal oscilloscope (MSO) can measure analogue and digital signals, it remains difficult to measure rf signals effectively with an oscilloscope at the rf carrier. It is also quite difficult to correlate events in the time and frequency domains – something critical for finding system level problems. While spectrum analysers make measurements in the frequency domain, these are not the tool of choice for most embedded designers. Using spectrum analysers to make time correlated measurements with the rest of the system is virtually impossible. A new type of instrument – the mixed domain oscilloscope, or MDO – has recently been launched by Tektronix. The device can display four analogue signals, 16 digital waveforms, up to four decoded serial and/or parallel buses and one rf signal. All signals are time correlated to show the effects of control signals on the analogue and rf domains. When hunting noise sources, the MDO can make time correlated measurements across the time and frequency domains. It can also make these measurements across multiple analogue, digital and rf signals. This means the MDO can measure the timing relationships between all of its inputs; for example, a power supply voltage dip during a device state change can be analysed and correlated to the impact on the rf signal. Time correlation is important in order for engineers to understand the complete operation of the system or to examine causes and effects. Time domain signals are best viewed as amplitude versus time; signals traditionally measured with a scope. This helps to answer questions like 'is this power supply really dc?' or 'what information is currently being sent over this wired bus?'. Time domain signals are not limited to analogue inputs; seeing rf amplitude, frequency and phase plotted against time can enable a study of simple analogue modulations, turn on and settling behaviour of rf signals. Frequency domain signals, on the other hand, are best viewed as amplitude versus frequency and are traditionally measured with a spectrum analyser. Viewing signals in this way helps answer questions like 'is this transmitted rf signal within its allocated spectrum?' and 'are there any signals present within this frequency band?'. The MDO's display can be split to show the frequency domain view of the rf signal and a traditional oscilloscope view of the time domain. Since the horizontal scale of the time domain display is independent of the amount of time required to process a Fourier Transform for the frequency domain display, it is important to represent the actual time period that correlates to the rf acquisition. The MDO enables separate time correlated acquisitions of all inputs (digital, analogue and rf). Each input has separate memory and, depending on the horizontal acquisition time of the time domain display, the rf signal can be moved within the analogue time. Spectrum time can also be moved through the acquisition to investigate how the rf spectrum changes with time. Spectrum time is the time required to support the desired resolution bandwidth of the spectrum display. Noise generated by the radio When adding a radio to an embedded system, there is also the potential problem of the radio generating noise and either interfering with other parts of the system or it failing to meet regulatory limits on radio signals. Measurements such as occupied bandwidth and total power transmitted can also help evaluate regulatory compliance. The first step is to examine the spectrum of the desired signal, as well as the spurious transmissions in neighbouring frequencies. Using an MDO allows the spread of spurious signals to be viewed either side of the fundamental and for their power to be measured. The MDO also allows the user to determine signal power and occupied bandwidth, to ensure both figures are acceptable. The next step is to look at the second harmonic, using the same measurements as the fundamental frequency. Once that has been examined, the user should move to the third harmonic: often the most troublesome in radio systems. It is also possible to take measurements in this band up to the sixth harmonic. However, by then, the emissions will be insignificant. Summary There are many new issues to watch for when including wireless communications in an embedded system. These include the effects of power supply switching noise, correctly setting the operating parameters of the radio integrated circuit and assuring the transmitted output conforms to applicable regulations. The MDO helps designers to diagnose and test for power supply and other noise effects. It can confirm the data commands to the radio are being set correctly and can check for spurious emissions from the transmitter and other circuits. It can not only be used to measure rf signals up to 6GHz, but is also valuable when looking at lower frequency noise from switching power supplies and from digital circuits with time correlated acquisitions. Darren McCarthy is worldwide rf technical marketing manager for Tektronix.