3.3.1 Overview

Figure 3-3. Fractional-N PLL

The preceding figure shows the block diagram of the fractional-N PLL. It consists of a phase frequency detector (PFD), a charge pump (CP), a loop filter (LF), a voltage controlled oscillator (VCO), an RF divider, a fractional divider and a sigma delta modulator (SDM). The reference frequency fXTO, which is used for the PLL, is the output of the XTO.

In RXMode, the PLL is used to generate the local oscillator signal with the frequency of fLO for the mixer. In TXMode, the TX frequency, fTX, for the power amplifier is generated. The PLL is able to cover the frequency bands of 310 MHz to 318 MHz, 418 MHz to 477 MHz (Low-Band) and 836 MHz to 956 MHz (High-Band).

The tolerances of the PLL are minimized by means of calibration. The calibration data is stored in the factory-locked EEPROM section and written by firmware into the front-end registers as shown in Fractional-N PLL Register Overview. The dynamic transfer function of the PLL is, thus, very stable. High data rates (>20 kBit/s) can be modulated by activating a digital preemphasis filter in the TX DSP (see TX DSP) that compensates for the low-pass behavior of the loop filter. The locking time of the PLL is very constant due to calibration.

During start-up, an additional speed-up mode can be activated (register bit FEEN1.PLSP1) to achieve shorter locking times.

A lock detect circuit, enabled with the register bit FECR.PLCKG, delivers a lock status in the FESR.PLCK register bit. Based on this information, the software can ensure that the ATA8510/15 does not transmit if the PLL is not locked. Both the lock detect feature and the AVCC voltage supervision in the power management (see Power Management) ensure that the ATA8510/15 complies with out-band radiation standards.

A sigma delta modulator (SDM) is used to dither the fractional spurious in a way that the resulting pseudo random noise is below the phase noise of the VCO. The frequency resolution of the output frequencies (fLO or fTX) is fXTO / 218 = 92.7 Hz in the Low-Band frequency range and fXTO / 217 = 185.4 Hz in the High-Band frequency range when using a 24.3 MHz crystal.