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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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74HC4046 Fiches technique(PDF) 8 Page - Fairchild Semiconductor |
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74HC4046 Fiches technique(HTML) 8 Page - Fairchild Semiconductor |
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8 / 17 page  www.fairchildsemi.com 8 Detailed Circuit Description VOLTAGE CONTROLLED OSCILLATOR/SOURCE FOLLOWER The VCO requires two or three external components to operate. These are R1, R2, C1. Resistor R1 and capacitor C1 are selected to determine the center frequency of the VCO. R1 controls the lock range. As R1’s resistance decreases the range of fMIN to fMAX increases. Thus the VCO’s gain increases. As C1 is changed the offset (if used) of R2, and the center frequency is changed. (See typical performance curves) R2 can be used to set the offset fre- quency with 0V at VCO input. If R2 is omitted the VCO range is from 0Hz. As R2 is decreased the offset frequency is increased. The effect of R2 is shown in the design infor- mation table and typical performance curves. By increasing the value of R2 the lock range of the PLL is offset above 0Hz and the gain (Hz/Volt) does not change. In general, when offset is desired, R2 and C1 should be chosen first, and then R1 should be chosen to obtain the proper center frequency. Internally the resistors set a current in a current mirror as shown in Figure 1. The mirrored current drives one side of the capacitor once the capacitor charges up to the thresh- old of the schmitt trigger the oscillator logic flips the capaci- tor over and causes the mirror to charge the opposite side of the capacitor. The output from the internal logic is then taken to pin 4. VCO WITHOUT OFFSET R2 = ∞ VCO WITH OFFSET FIGURE 1. Comparator I Comparator II & III R2= ∞ R2≠∞ R2= ∞ R2≠∞ •Given: f0 •Given: f0 and fL •Given: fMAX •Given: fMIN and fMAX •Use f0 with curve titled •Calculate fMIN from the •Calculate f0 from the •Use fMIN with curve titled center frequency vs R1, C equation fMIN = fo − fL equation fo = fMAX/2 offset frequency vs R2, to determine R1 and C1 •Use fMIN with curve titled •Use f0 with curve titled C to determine R2 and C1 offset frequency vs R2, C center frequency vs R1, C •Calculate fMAX/fMIN to determine R2 and C1 to determine R1 and C1 •Use fMAX/fMIN with curve •Calculate fMAX/fMIN from titled fMAX/fMIN vs R2/R1 the equation fMAX/fMIN = to determine ratio R2/R1 fo + fL/fo − fL to obtain R1 •Use fMAX/fMIN with curve titled fMAX/fMIN vs R2/R1 to determine ratio R2/R1 to obtain R1 |
Numéro de pièce similaire - 74HC4046 |
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Description similaire - 74HC4046 |
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