How to Eliminate Noise and Spurious Signals in RF Embedded Systems

When integrating an RF chip or module into a typical embedded system, a common task that designers must 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 noise sources. When considering noise, any possible interference generated by the RF circuit should also be considered, which is an important consideration to avoid interference with other radio equipment and meet regulatory requirements. In this application note, we will introduce techniques and tips for finding noise sources using the MDO4000 Series Mixed Domain Oscilloscope series.

Figure 1. Tektronix MDO4000 Series Mixed Domain Oscilloscope and Microchip RF Test Board Module.

Integrating RF Communications into Embedded Systems

When adding RF functionality to an embedded system, many problems are typically encountered during integration.

For battery-powered systems, switching regulators are typically used to achieve the highest practical efficiency at the lowest cost. Power supply size is often an issue. This requires the use of high switching frequencies to minimize output filtering specifications and requirements. These power supplies typically have ripple on the output voltage, which can appear on the RF transmitter output, especially under high workloads or when the battery is low. To avoid this, additional power supply filtering may be required to prevent the RF output signal from being affected, although this will result in increased cost or size.

The hardware circuit and software configuration of the radio chip or module may affect the quality of the transmitted signal. If not set up and filtered properly, the RF output signal may cause interference to other radio systems or fail to meet the corresponding regulatory standards. Some radio systems require channel filters, RF surface acoustic waves and other relatively high-cost filters to meet regulatory requirements for out-of-channel and out-of-band emissions.

Figure 2. Test connections between the device under test (Microchip Technologies MRF89XA 868 MHz radio) and the MDO4000 Series Mixed Domain Oscilloscope.

Application example: Embedded system with switching power supply and wireless support

In the following discussion, the device under test will use a flexible RF communication IC that has been integrated into an RF test module, the Microchip Technologies MRF89XM8A. This module uses the MRF89XA IC along with filtering and antenna matching. For demonstration purposes, this module is mounted on a Microchip Explorer 16 board and used with a PC to program the RF parameter settings.

To demonstrate the effect of using a switching power supply to power the radio, we use a boost converter IC, the Microchip MCP1640, which is integrated onto the MCP1640EV evaluation board. This converter switches at about 500 kHz, a frequency that is quite common for switching regulators. It can provide the 3.3 V output voltage required by the radio module and supports input voltages as low as 0.8 V. This means that the radio can be powered from a single battery cell, reducing the battery size of the product.

To debug this device, we used a Tektronix MDO4000 Series Mixed Domain Oscilloscope. The MDO4000 Series has the unique ability to simultaneously display 4 analog signals, 16 digital waveforms, up to 4 decoded serial and/or parallel buses, and 1 RF signal. All of these signals are time correlated to show the effects of control signals on both the analog and RF domains. Figure 2 illustrates the setup used for the following tests.

Figure 3. Viewing the time and frequency domains.

Identifying the Source of Noise

For reference, we measured the RF spectrum centered at 868 MHz, which has a fairly low FSK modulated data rate of 2 kbps. The reference spectrum is shown in Figure 3. Note that the MDO4000 Series displays both the time domain view and the frequency domain view, with all signals time-correlated.

The lower half of the screen shows a frequency domain view of the RF signal, in this case the RF transmitter output, and the upper half of the screen is a traditional oscilloscope view of the time domain. The spectrum displayed in the frequency domain view is from the time period indicated by the short orange bar in the time domain view, called the Spectrum Time.

Since the horizontal scale of the time domain display is independent of the amount of time required to process the Fourier transform (FFT) of the time domain display, it is important to represent the actual time period associated with the RF acquisition. The unique architecture of the MDO4000 Series oscilloscope allows all inputs (digital, analog, and RF) to be acquired separately in a time-related manner. Each input has a separate memory, and the RF signal acquired in the memory supports spectrum time and can be moved within analog time depending on the horizontal acquisition time of the time domain display, as shown in Figure 4.

Figure 4. Occupancy power measurement results during multiple symbols of the preamble using a clean experimental source.

With the MDO4000 Series, you can move Spectrum Time in the acquisition data to examine how the RF spectrum changes over time. In Figure 4, we adjust the position of Spectrum Time to show the spectrum of the signal transmitted during multiple symbols of the packet preamble.

The spectrum time is the amount of time required to support the resolution bandwidth (RBW) desired for the spectrum display. It is equal to the window factor divided by the RBW. The default KaiserWindow shaping factor is 2.23, in this example, the spectrum time is 2.23/220 Hz, which is approximately 10 ms.

FSK modulation has only one RF signal frequency at a time, and we use a longer acquisition time for the spectrum to measure occupied bandwidth and total power.

Figure 5. Spectrum during packet data. The frequency vs. time plot shows that the acquired spectrum time is dominated by the lower frequency Tx ON time.

To easily visualize the transmission of packets over the radio, we have added RF vs. time plots to the time domain view of the MDO4000 Series. The orange plot labeled “A” shows the amplitude of the instantaneous RF vs. time. The orange plot labeled “f” shows the frequency of the instantaneous RF signal relative to the center frequency vs. time.

The green waveform (channel 4) shows the current input to the RF module. It can be seen that the current rises from nearly 0 between packets to about 40 mA during transmission. The yellow waveform (channel 1) shows the AC ripple on the module supply voltage. Note that there is only a small voltage drop during transmission.

Figure 5 shows the same signal acquired during the data portion of the packet. Note that most of the energy is at the lower frequencies. Figures 4 and 5 were acquired with the module powered from a clean lab power supply.

Figure 6. Spectrum and power supply measurement results for a switching power supply.

Figure 6 shows the same RF signal, but using a boost switching power supply to power the RF module. Boost regulators are notorious for generating noise, but they allow the use of one or two alkaline or NiCad batteries and relatively few components, reducing cost. Note the increased noise at the bottom of the modulated signal. Near the transmitted signal, the noise is at least 5 dB higher than the clean supply. The noise is clearly visible in the current and voltage waveforms. The additional noise also degrades the signal-to-noise ratio from the transmitter to the receiver, reducing the effective operating range of the RF system.

Figure 7. Power supply switching noise to an equivalent load.