An Analysis and Performance Evaluation of a Passive Filter Design Technique for Charge Pump PLL's

The high performance of today's digital phase-lock loop makes it the preferred choice for generation of stablee low noisee tunable local oscillators in wireless communications applicationss This paper investigates the design of passive loop filters for Frequency Synthesizers utilizing a Phase-Frequency Detector and a current switch charge pump such as National Semiconductor's PLLatinum TM Seriess Passive filter design for a TYPE II third order phase-lock loop is discussed in depthh with some discussion of higher order filters includedd Specific test results are presented for a GSM synthesizer designn Optimization of phase-lock loop performance with respect to different parameters is discussedd The basic phase-lock-loop configuration we will be considering is shown in Figure 1 The PLL consists of a high-stability crystal reference oscillatorr a frequency synthesizer such as the National Semiconductor LMX2315TMM a voltage controlled oscillator (VCO)) and a passive loop filterr The frequency synthesizer includes a phase detectorr current mode charge pumpp and programmable frequency dividerss A passive filter is desirable for its simplicityy low costt and low phase noisee In most standard PLL's there are several design parameters which can be treated as constant valuess This linear approximation provides a good estimation of loop performancee The values of the PLL filter design constants depend on the specific applicationn For examplee Kw is determined by the synthesizer charge pump output current magnitudee The notation and definitions for these values along with standard units used throughout this paper are given in Table I beloww TABLE II PLL Filter Design Constants Kvco-(MHzzVolt) Voltage Controlled Oscillator (VCO) Tuning Voltage constantt The frequency vs voltage tuning ratioo Kw-(mAA2qrad) Phase detectorrcharge pump constantt The ratio of the current output to the input phase differentiall RFopt-(MHz) Radio Frequency output of the VCO at which the loop filter is optimizedd Fref-(kHz) Frequency of the phase detector inputss Usually equivalent to the RF channel spacingg N Main divider ratioo Equal to RFopttFreff