MAX038 High-Frequency Waveform Generator

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High-Frequency Waveform GeneratorMAX038When the MAX038’s frequency is controlled by a voltagesource (VIN) in series with a fixed resistor (RIN), theoutput frequency is a direct function of VIN as shown inthe above equations. Varying VIN modulates the oscillatorfrequency. For example, using a 10kΩ resistor forR IN and sweeping VIN from 20mV to 7.5V produceslarge frequency deviations (up to 375:1). Select RIN sothat IIN stays within the 2µA to 750µA range. The bandwidthof the IIN control amplifier, which limits the modulatingsignal’s highest frequency, is typically 2MHz.IIN can be used as a summing point to add or subtractcurrents from several sources. This allows the outputfrequency to be a function of the sum of several variables.As VIN approaches 0V, the I IN error increasesdue to the offset voltage of IIN.Output frequency will be offset 1% from its final valuefor 10 seconds after power-up.FADJ InputThe output frequency can be modulated by FADJ,which is intended principally for fine frequency control,usually inside phase-locked loops. Once the fundamental,or center frequency (F o ) is set by IIN, it may bechanged further by setting FADJ to a voltage other than0V. This voltage can vary from -2.4V to +2.4V, causingthe output frequency to vary from 1.7 to 0.30 times thevalue when FADJ is 0V (Fo ±70%). Voltages beyond±2.4V can cause instability or cause the frequencychange to reverse slope.The voltage on FADJ required to cause the output todeviate from Fo by Dx (expressed in %) is given by theformula:VFADJ = -0.0343 x Dx [5]where VFADJ, the voltage on FADJ, is between-2.4V and +2.4V.Note: While IIN is directly proportional to the fundamental,or center frequency (F o ), VFADJ is linearly related to% deviation from F o . V FADJ goes to either side of 0V,corresponding to plus and minus deviation.The voltage on FADJ for any frequency is given by theformula:V FADJ = (F o - Fx) ÷ (0.2915 x Fo) [6]where:Fx = output frequencyFo = frequency when VFADJ = 0V.Likewise, for period calculations:VFADJ = 3.43 x (tx - to) ÷ tx [7]where:t x = output periodt o = period when V FADJ = 0V.Conversely, if VFADJ is known, the frequency is givenby:Fx = F o x (1 - [0.2915 x V FADJ ]) [8]and the period (tx) is:tx = to ÷ (1 - [0.2915 x VFADJ]) [9]Programming FADJFADJ has a 250µA constant current sink to V- that mustbe furnished by the voltage source. The source is usuallyan op-amp output, and the temperature coefficientof the current sink becomes unimportant. For manualadjustment of the deviation, a variable resistor can beused to set VFADJ, but then the 250µA current sink’stemperature coefficient becomes significant. Sinceexternal resistors cannot match the internal temperature-coefficientcurve, using external resistors to programVFADJ is intended only for manual operation,when the operator can correct for any errors. Thisrestriction does not apply when VFADJ is a true voltagesource.A variable resistor, RF, connected between REF (+2.5V)and FADJ provides a convenient means of manuallysetting the frequency deviation. The resistance value(RF) is:RF = (VREF - VFADJ) ÷ 250µA [10]VREF and VFADJ are signed numbers, so use correctalgebraic convention. For example, if VFADJ is -2.0V(+58.3% deviation), the formula becomes:RF = (+2.5V - (-2.0V)) ÷ 250µA= (4.5V) ÷ 250µA= 18kΩDisabling FADJThe FADJ circuit adds a small temperature coefficientto the output frequency. For critical open-loop applications,it can be turned off by connecting FADJ to GND(not REF) through a 12kΩ resistor (R1 in Figure 2). The-250µA current sink at FADJ causes -3V to be developedacross this resistor, producing two results. First,the FADJ circuit remains in its linear region, but disconnectsitself from the main oscillator, improving temperaturestability. Second, the oscillator frequency doubles.If FADJ is turned off in this manner, be sure to correctequations 1-4 and 6-9 above, and 12 and 14 below bydoubling Fo or halving to. Although this method doublesthe normal output frequency, it does not double theupper frequency limit. Do not operate FADJ open circuitor with voltages more negative than -3.5V. Doingso may cause transistor saturation inside the IC, leadingto unwanted changes in frequency and duty cycle.10 ______________________________________________________________________________________
High-Frequency Waveform GeneratorFREQUENCYC11µFC31nF1REF–5V +5V20 17 4V- V+ A13AOC21µF–2.5VPRECISION DUTY-CYCLE ADJUSTMENT CIRCUITR4100kR3100k+2.5VREFMAX038R IN20kR112k7DADJ MAX038R250Ω1019IINOUTSINE-WAVE8OUTPUTFADJ16DV+N.C.15DGND14SYNCN.C.13PDI5122 x 2.5VCOSCPDOF o =R IN x C FGND GND GND GND GNDC F 6 2 9 11 18R7100kR65kR5100kDADJMAX038ADJUST R6 FOR MINIMUM SINE-WAVE DISTORTIONFigure 2. Operating Circuit with Sine-Wave Output and 50% Duty Cycle; SYNC and FADJ DisabledWith FADJ disabled, the output frequency can still bechanged by modulating I IN .Swept Frequency OperationThe output frequency can be swept by applying a varyingsignal to IIN or FADJ. IIN has a wider range, slightlyslower response, lower temperature coefficient, andrequires a single polarity current source. FADJ may beused when the swept range is less than ±70% of thecenter frequency, and it is suitable for phase-lockedloops and other low-deviation, high-accuracy closedloopcontrols. It uses a sweeping voltage symmetricalabout ground.Connecting a resistive network between REF, the voltagesource, and FADJ or IIN is a convenient means ofoffsetting the sweep voltage.Duty CycleThe voltage on DADJ controls the waveform duty cycle(defined as the percentage of time that the outputwaveform is positive). Normally, V DADJ = 0V, and theduty cycle is 50% (Figure 2). Varying this voltage from+2.3V to -2.3V causes the output duty cycle to varyfrom 15% to 85%, about -15% per volt. Voltagesbeyond ±2.3V can shift the output frequency and/orcause instability.DADJ can be used to reduce the sine-wave distortion.The unadjusted duty cycle (V DADJ = 0V) is 50% ±2%;any deviation from exactly 50% causes even order harmonicsto be generated. By applying a smalladjustable voltage (typically less than ±100mV) toV DADJ , exact symmetry can be attained and the distortioncan be minimized (see Figure 2).The voltage on DADJ needed to produce a specificduty cycle is given by the formula:V DADJ = (50% - dc) x 0.0575 [11]or:V DADJ = (0.5 - [t ON ÷ t o ]) x 5.75 [12]where:V DADJ = DADJ voltage (observe the polarity)dc = duty cycle (in %)t ON = ON (positive) timet o = waveform period.Conversely, if V DADJ is known, the duty cycle and ONtime are given by:dc = 50% - (V DADJ x 17.4) [13]t ON = t o x (0.5 - [V DADJ x 0.174]) [14]______________________________________________________________________________________ 11
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MAX038 High-Frequency Waveform Generator

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