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Base stations use phase locked loop (PLL) systems to generate either RF or IF local signals to up-convert baseband signals to RF signals or vice versa. There are different ways of integrating PLL system blocks, including voltage-controlled oscillators (VCO), based on system requirements, performance and material cost. Why PLL performance is critical in wireless system design will be determined first, followed by a comparison of high level integrations. Detailed analytical approaches will be used to predict the baseline performance for synthesizer systems, using linear models of phase noise analysis in a closed loop, as well as overall system performance optimization, including loop filter designs.

How PLL Performances Affect Wireless Systems

Phase noise is one of the most critical impairments of radio systems, as it corrupts the information carried in the phase of the carrier due to its non-ideality. There are two different types of corruptions that phase noise can make in the systems. One is from in-band (modulated signal band) phase noise, which directly affects down converted or up converted signals, as shown in Figure 1. The amount of in-band phase noise, other than the one that is very close to the carrier, which is slow varying enough for the data to be affected, can be represented as a phase error or error vector magnitude (EVM). Another key aspect of phase noise impact can be explained with an interferer, which is often called a blocker, as shown in Figure 2. When a blocker gets up or down converted, the LO's phase noise will override its phase noise onto the blocker and parts of the phase noise will stay on top of the signal band. These types of non-ideal phase noise can impact any type of radio system such as wideband code division multiple access (W-CDMA) and global system for mobile communications (GSM). Therefore, a certain level of phase noise performance will be expected in wireless infrastructure systems.

What about long term evolution (LTE) systems? What is needed to know about LTE systems in terms of phase noise, as it becomes the next generation mobile standard? LTE uses a multi-carrier modulation technique that transmits data over a number of orthogonal sub-carriers, which consist of data and pilot within various formats of frame structures. Since orthogonal frequency division modulation (OFDM) signals transmit multi-carriers in the frequency domain, multi-carriers with the same phase noise on each carrier can be analyzed as one single-carrier with the same phase noise. However, the far-out phase noise can cause inter symbol interference (ISI), which means there is going to be a system requirement on phase noise for this system as well.

Comparison of High Level Integration for PLL Systems

Both system and component designers know why good phase noise is important in wireless systems. In this section, an approach to designing the best fitting PLL systems will be considered by demonstrating the pros and cons for each approach in designs. Table 1 shows three different integration types of PLL systems. Type 1 has one die that covers the VCO as well as PLL. This approach can be suitable for low to mid performance systems, as VCO phase noise performance will be greatly limited by low tank Q. In general, the tank Q will be limited by the Q of the inductor. Also, if a CMOS VCO is used, the 30 dB/decade slope of phase noise can be extended above 100 kHz offset, assuming the device requires high operating current and voltage swings to meet far-out phase noise performance. This will limit the usage of this type of device in low RMS phase error required systems such as higher modulation systems. It will also require off-chip loop filter components that could require great tunings to maximize the performance. Even with these performance disadvantages, the benefit comes with lower cost and smaller package size.