What are the effects of changing the update rate?
The parameters dependent on, or affected by, the update rate include loop bandwidth, settling time of the PLL, resolution of the channel spacing, and the location of reference related spurs on the phase noise characteristic. Additionally, the loop bandwidth determines the dominant source of phase noise at a given frequency offset.
The update rate determines the loop bandwidth of the PLL. Specifically, the loop bandwidth is approximately one tenth of the update rate. Changing the update rate affects the overall phase noise characteristic.
For offset frequencies much less than the loop bandwidth, the overall phase noise is dominated by the reference source. On the other hand, the voltage-controlled oscillator (VCO) phase noise dominates the overall phase noise for offset frequencies much larger than the loop bandwidth.
The settling time of the PLL is also dependent on the update rate. A higher update rate leads to a shorter settling time. Specific details about the relationship between update rate and settling time are contained in the synthesizer device data sheets. As an example, a 500 kHz update rate on the Si4133 corresponds to a settling time of 80us.
Channel Spacing Resolution:
Resolution of the channel spacing is determined by the update rate. The output frequency can be written as FOUT = (FREF/R)*N, where FREF is the reference frequency. This can be re-written as FOUT = (Update Rate)*N. Therefore, the output frequency can be changed in steps equivalent to the update rate. This is achieved by simply incrementing or decrementing the value of the N register.
Channel spacing is an important consideration in applications where the synthesizer is used to generate different frequencies. A lower update rate is necessary for more closely spaced channels.
Reference Related Spurs:
Reference related spurs are present in the output frequency spectrum of the synthesizer and occur at integer intervals of the update rate. These spurs can be seen in figures 1 and 2 below. In figure 1, update rates of 200 kHz and 1000 kHz are used. In the case of the 200 kHz update rate, the spurs are located at 200kHz, 400kHz, 600kHz and 1000kHz. For the 1000kHz update rate, the first spur is located at an offset of 1000kHz.
Generally, the MLP packages offers better reference related spur performance than TSSOP packages.
Fig 1: Si4133 Phase Noise for Fphi = 200 kHz and 1000 kHz
Figure 2: Si4133 Phase Noise for KP1 = 1/8, 1/4, 1/2 and 1
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