What are the factors that affect phase noise? Can phase noise be reduced?
The three main factors that affect phase noise are update rate, phase detector gain and the phase noise characteristic of the reference frequency. Other factors such as the temperature and supply voltage level have a minimal effect on phase noise.
The update rate is the frequency at which the synthesizer phase detector is operating. Mathematically, it is represented as Update Rate = FREF/R, where FREF is the reference frequency. Generally, the higher the update rate, the lower the phase noise. Figure 1 below shows the phase noise characteristic for Si4133 (IF = 550 MHz) under typical conditions. The update rates used are 200 kHz and 1000 kHz.
Fig 1: Si4133 Phase Noise for Fphi = 200 kHz and 1000 kHz
From figure 1 above, it is apparent that the 1000 kHz update rate results in lower phase noise. This is more noticeable at lower offset frequencies.
The update rate also affects the loop bandwidth. This effect can also be observed in figure 1. The loop bandwidth of the synthesizers is proportional to the update rate.
Phase Detector Gain:
The phase detector gain setting can also be used for optimizing phase noise performance. Figure 2 below shows the phase noise characteristics for various RF1 phase detector (KP1) settings.
Figure 2: Si4133 Phase Noise for KP1 = 1/8, 1/4, 1/2 and 1
For offset frequencies much less than the loop bandwidth, higher phase detector gains lead to lower phase noise. However, it is important to point out that a very large phase detector gain can also result in the loop being unstable. It is recommended that the phase detector gain be set according to the value programmed into the N register. This information is available in the synthesizer device datasheets.
At offset frequencies much less than the loop bandwidth, the reference source largely determines the phase noise of the output. For this reason, it is recommended that a stable reference such as a temperature compensated crystal oscillator (TCXO) be used in applications requiring very good phase noise performance.
In general for synthesizers, the phase noise at a given offset frequency reduces as the offset frequency increases. For integrated phase noise therefore, the lower offset frequencies dominate the integration. As a result, the frequency reference plays a dominant role in the value of the integrated phase error.
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