2 Technical Overview

One of the most common circuits in modern electronic systems is the switch-mode power supply (SMPS), which are used to regulate voltage to a certain level. These regulators provide significant improvements in efficiency over linear regulators in most applications. But this efficiency comes at a price, as the switching frequency of the SMPS may interfere with the frequency response of the circuit being supplied.

One example of a SMPS that can cause interference is a buck converter, which is used in this application. A buck converter produces an output voltage that is directly proportional to its duty cycle. It converts power when its switch node switches between connecting to the input voltage and connecting to ground. This inherent switching action causes the currents and voltages in the circuit to switch and fluctuate, resulting in an output voltage that also has ripple on top of the regulated steady-state DC value. The frequency of this ripple can interfere with signals in the circuit, eventually returning an undesirable signal or frequency output.

In a recent wireless smoke detector application, the CC1120 transceiver was powered by a 3.3V input regulated from the LM5166 buck converter. When testing radiated emissions performance in this system, it had been observed that the radiated emissions testing fails FCC standards in the 915MHz band, corresponding to FCC regulations FCC 15.247 and FCC 15.249. Spurious emissions arose when the CC1120 was set to continuous transmission and powered directly by LM5166. This was also seen when powering the CC1120 with various DC-DC layouts, including the LM5166 evaluation module, and various DC-DC converters. The issue was solved after adding LC filters to the 3.3V output or adding an LDO after the DC-DC converter.

In order to further investigate these results, a radiated emissions test board was designed to determine if there is a buck converter available that passes radiated emissions tests using the CC1120 without additional external filters.