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You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser. Thread starter ReubenMijares Start date Jan 20, Status Not open for further replies. ReubenMijares Newbie level 6. But I don't know actually what's the use of it. Some registers are associated with it which you need to program. May I know what's the use of this PLL and why are the registers need to be configured?
I know how PLL works. It takes 2 input frequencies, one is from a crystal oscillator which is stable, while the other one is from a VCO. What I don't understand are the following: 1. The crystal oscillator is already stable. Why not just use that signal for the clock? No need to compare 2 signals, right? What are its used for digital circuits? The information about the error in phase or the phase difference between the two signals is then used to control the frequency of the loop.
To understand more about the concept of phase and phase difference, it is possible to visualise two waveforms, normally seen as sine waves, as they might appear on an oscilloscope. If the trigger is fired at the same time for both signals they will appear at different points on the screen. The linear plot can also be represented in the form of a circle.
The beginning of the cycle can be represented as a particular point on the circle and as a time progresses the point on the waveform moves around the circle. The instantaneous position on the circle represents the phase at that given moment relative to the beginning of the cycle.
The concept of phase difference takes this concept a little further. Although the two signals we looked at before have the same frequency, the peaks and troughs do not occur in the same place. There is said to be a phase difference between the two signals. This phase difference is measured as the angle between them.
It can be seen that it is the angle between the same point on the two waveforms. In this case a zero crossing point has been taken, but any point will suffice provided that it is the same on both. This phase difference can also be represented on a circle because the two waveforms will be at different points on the cycle as a result of their phase difference. The phase difference measured as an angle: it is the angle between the two lines from the centre of the circle to the point where the waveform is represented.
When there two signals have different frequencies it is found that the phase difference between the two signals is always varying. The reason for this is that the time for each cycle is different and accordingly they are moving around the circle at different rates.
Figure Care needs to be taken with fractional-N PLLs to ensure that spurious tones do not degrade system performance. Integer N PLL. Fractional-N PLL.
Integer N PLL in-band phase noise. Fractional-N PLL in-band phase noise. EVM is similar in scope to integrated phase noise, which considers the noise contribution over a range of offsets. For the 5G system listed earlier, the integration limits are quite wide, starting at 1 kHz and continuing to MHz. EVM can be thought of as a percentage degradation of a perfectly modulated signal from its ideal point expressed as a percentage Figure In a similar manner, integrated phase noise integrates the noise power at different offsets from the carrier and expresses this noise as a dBc number compared to the output frequency.
Modern signal source analyzers will also include these numbers at the push of a button Figure As modulation schemes increase in density, EVM becomes critical. However, since EVM is comprised of various other nonideal parameters due to power amplifier distortion and unwanted mixer products, the integrated noise in dBc is usually defined separately.
Phase error visualization. Signal source analyzer plot. VCO blocking specifications are very important in cellular systems that need to account for the presence of strong transmissions.
If a receiver signal is weak, and if the VCO is too noisy, then the nearby transmitter signal can mix down and drown out the wanted signal Figure The illustration in Figure 19 demonstrates how the nearby transmitter kHz away transmitting at —25 dBm power could, if the receiver VCO is noisy, swamp the wanted signal at — dBm.
These specifications form part of a wireless communications standard. The blocking specifications directly influence the performance requirement of the VCO. VCO noise blockers. The next PLL circuit element to be considered in our circuit is the voltage controlled oscillator. With VCOs, a fundamental trade-off between phase noise, frequency coverage, and power consumption is necessary.
The higher the quality factor Q of the oscillator, the lower the VCO phase noise is. However, higher Q circuits have narrower frequency ranges. Increasing the power supply will also lower the phase noise. This increases PLL circuit complexity, as most PLL charge pumps can only tune to 5 V, so an active filter using operational amplifiers is used to increase the tuning voltage of the PLL circuit on its own.
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