MAX038 High-Frequency Waveform Generator

High-Frequency Waveform GeneratorCENTERFREQUENCYR DSYNC DV+ V+ V- A113REFA07DADJ+5V -5V16 17 201019IINOUT8 MAX038FADJR PD13PDI512CCOSCPDOPD C FGND GND GND GND GND DGND2 6 9 11 18 15EXTERNAL OSC INPUT14PDO is a rectangular current-pulse train, alternatingbetween 0µA and 500µA. It has a 50% duty cycle whenthe MAX038 output and PDI are in phase-quadrature(90° out of phase). The duty cycle approaches 100%as the phase difference approaches 180° and conversely,approaches 0% as the phase differenceapproaches 0°. The gain of the phase detector (KD)can be expressed as:KD = 0.318 x RPD (volts/radian) [16]where RPD = phase-detector gain-setting resistor.When the loop is in lock, the input signals to the phasedetector are in approximate phase quadrature, the dutycycle is 50%, and the average current at PDO is 250µA(the current sink of FADJ). This current is dividedbetween FADJ and RPD; 250µA always goes into FADJand any difference current is developed across RPD,creating VFADJ (both polarities). For example, as thephase difference increases, PDO duty cycle increases,the average current increases, and the voltage on RPD(and VFADJ) becomes more positive. This in turndecreases the oscillator frequency, reducing the phasedifference, thus maintaining phase lock. The higherRPD is, the greater VFADJ is for a given phase difference;in other words, the greater the loop gain, the lessthe capture range. The current from PDO must also4C11µFC21µFR OUT50ΩRFOUTPUTFigure 3. Phase-Locked Loop Using Internal Phase Detectorcharge CPD, so the rate at which VFADJ changes (theloop bandwidth) is inversely proportional to CPD.The phase error (deviation from phase quadrature)depends on the open-loop gain of the PLL and the initialfrequency deviation of the oscillator from the externalsignal source. The oscillator conversion gain (Ko) is:KO = ∆ω o ÷ ∆VFADJ [17]which, from equation [6] is:KO = 3.43 x ωo (radians/sec) [18]The loop gain of the PLL system (KV) is:KV = KD x KO [19]where:KD = detector gainKO = oscillator gain.With a loop filter having a response F(s), the open-looptransfer function, T(s), is:T(s) = KD x KO x F(s) ÷ s [20]Using linear feedback analysis techniques, the closedlooptransfer characteristic, H(s), can be related to theopen-loop transfer function as follows:H(s) = T(s) ÷ [1+ T(s)] [21]The transient performance and the frequency responseof the PLL depends on the choice of the filter characteristic,F(s).When the MAX038 internal phase detector is not used,PDI and PDO should be connected to GND.External Phase DetectorsExternal phase detectors may be used instead of theinternal phase detector. The external phase detectorshown in Figure 4 duplicates the action of the MAX038’sinternal phase detector, but the optional ÷N circuit canbe placed between the SYNC output and the phasedetector in applications requiring synchronizing to anexact multiple of the external oscillator. The resistor networkconsisting of R4, R5, and R6 sets the sync range,while capacitor C4 sets the capture range. Note thatthis type of phase detector (with or without the ÷N circuit)locks onto harmonics of the external oscillator aswell as the fundamental. With no external oscillatorinput, this circuit can be unpredictable, depending onthe state of the external input DC level.Figure 4 shows a frequency phase detector that locksonto only the fundamental of the external oscillator.With no external oscillator input, the output of the frequencyphase detector is a positive DC voltage, andthe oscillations are at the lowest frequency as set byR4, R5, and R6.MAX038______________________________________________________________________________________ 13