Power-supply noise is ubiquitous in most Wi-Fi boards. This noise might impact the phase-noise performance of the XO that’s important for Wi-Fi EVM performance. The TDMA-based (time-division multiple access) RF power amplifier from a mobile phone switches on and off at 217 Hz and generates severe noise for the power supplier.

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Typically, 1.7-A current is drawn from the power supply. The voltage drop can reach up to 500 mV. If some of this noise is injected into the power plan for the XO, the XO with the best power-supply rejection (PSR) will have the lowest spur noise modulated from this 217-Hz power-line noise.

XO PSR Benchmark 

A powerful benchmark methodology is developed for the crystal oscillator for the demanding 802.11n application. PSR performance significantly impacts the EVM performance of the 802.11n systems. Noise present on the supply voltage and ground may be converted to the spurs of the reference clock.

To measure the relative sensitivities of the reference clocks with respect to the supply noise, a sinusoidal noise source with –30 dBm was added to the supply voltage, and the incremental increase in the spur degradations (dBc) of these reference clocks is measured. This test was repeated for a number of different sinusoidal frequencies (Fig. 5 and the table). Here, it can be seen that the FN40-P3 XO is much less susceptible to supply noise than the other two devices.

The TX EVM is correlated to the phase noise and frequency offset of the reference clock. The performance of the reference clock source is sensitive to the external noise injection from power supplies. Some of the Wi-Fi receivers with digital-controlled crystal oscillator (DCXO) and Fractional-N phase-locked loop (PLL) can track the preambles of the packet frame and minimize the frequency offset impact to the EVM. The PSR capability of the crystal oscillator has a strong impact on the EVM in a noisy environment.

According to Pericom Semiconductor Corp., its FN40-P3 XO has the best EVM margin against other devices in the benchmark. By using the toughest 5G EWC-40 test vectors, it can deliver –29.25-dB EVM under a noisy system environment (Fig. 6). The 802.11n chipset used in this benchmark has good frequency and phase tracking capability. The EVM performance is only sensitive to phase jitter and the power-supply noise rejection capability of the reference clock source.

Most Wi-Fi systems must operate in a hostile environment. So, the reference clock source must feature low phase jitter as well as solid power-supply rejection capability to mitigate the performance degradation from the noisy power supply (Fig. 7).