1、晶振和电容的匹配Load CapacitanceThe Capacitance external to the crystal contained within the feedback loop of the oscillator circuit. For “parallel“ resonant crystals (see below for discussion on parallel vs. series resonant crystals), the value of load capacitance needs to be specified by the customer to i
2、nsure initial Frequency Tolerance. For “series“ resonant crystals, load capacitance is not used. Load capacitance can be measured as the amount of capacitance across the crystal terminals on the PCB. A parallel resonant mode crystal needs to have a load capacitance (CL) specified when ordering (this
3、 was already said above). The approximate crystal loading for a given circuit can be determined from the formula (SEE below for more useful equations):CS is the stray capacitance of the circuit and the input/output capacitance of the inverter or microprocessor chip at the Crystal 1 (C1) and Crystal
4、2 (C2) pins, plus any parasitic capacitances. CS may be assumed to equal 5 pF. Most crystal manufacturers will specify standard parallel resonant load capacitances of either 18 or 20 pF. These values have been found to provide for good Frequency Tolerance in most circuits. Changes in load capacitanc
5、e will result in changes in the output frequency. . Series vs. Parallel:Parallel resonant crystals are intended for circuits which contain reactive components (capacitors) in the oscillator feedback loop. These circuits depend on the reactive components and the crystal to achieve the phase shift nee
6、ded to start and maintain ocsillation at a specified frequency. Series resonant crystals are intended for circuits that contain no reactive components in the oscillator feedback loop. Please see Figure A for Series and Figure B for Parallel resonant crystals:Figure A - Series Figure B - ParallelUsef
7、ul Equations:Equations DefinitionsfS = (Series) frequency = C0 = Static Capacitance in faradsCL = Load capacitance = C1 = Motional capacitance in faradsCo = Shunt capacitance = CL = Load capacitance in faradsC1 = Motional capacitance = f = Nominal frequency in HzL1 = Motional inductance = fL = Anti-resonant frequency in HzR1 = Series resistance = fS = Series resonant frequency in HzQ = Quality factor = L = Inductance into Henrys fL - fS = f = PL = Pullability (ppm/pF)PL = Pullability = Q = Quality factor. R1 = Series resistance in ohms
