Using a VCXO (voltage-controlled crystal oscillator) as a clock (CLK) generator - AN2097
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چکیده
A VCXO (voltage controlled crystal oscillator) is an oscillator whose frequency is determined by a crystal, but can be adjusted by a small amount by changing a control voltage. VCXO clock (CLK) generators have been used in various applications, such as digital TV, digital audio, ADSL, and STB. This application note provides the general structure for a VCXO CLK generator, key performance measurements, guidelines for PCB design, and a test setup for MAX9485, a VCXO CLK Gen chip for MPEG2 and AC-3 audio applications. Structure and application of a VCXO CLK generator A VCXO, or voltage controlled crystal oscillator, oscillates with a frequency determined by a crystal, but which can be adjusted over a narrow range by a control voltage, usually in the range of 0V–2V or 0V–3V. The tuning range of the VCXO is about ±100ppm to ±200ppm. Figure 1 shows the structure of a typical VCXO CLK generator and the crystal's circuit model. Figure 1. Block diagram of a typical VCXO CLK generator. As the capacitance of the varactors CV1 and CV2 changes, the crystal's model is affected and the oscillation frequency changes. The two shunt external shunt capacitors CS1 and CS2 are used to adjust the tuning range and offset the center frequency. Mathematically, according to the crystal circuit in Figure 1, the resonating frequency of the crystal can be expressed as where CL is the equivalent load capacitor that is a lumped sum from CV1, 2 and CS1, 2. More precisely, CL = (CV1 + CS1) || (CV2 + CS2). Taking the first order approximation and considering the fact that C1 << C0 and CL, we can obtain the frequency incremental around fC: Figure 2 shows a typical plot of fC in terms of the value of CS1, where CS1 = CS2.
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Using a VCXO (Voltage-Controlled Crystal Oscillator) as a Clock (CLK) Generator - AN2097
A "VCXO" (Voltage Controlled Crystal Oscillator) is an oscillator whose frequency is determined by a crystal, but can be adjusted by a small amount by changing a control voltage. VCXO clock (CLK) generators have been used in various applications, such as digital TV, digital audio, ADSL, and STB. This application note provides the general structure for a VCXO CLK generator, key performance measu...
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تاریخ انتشار 2010