Crystal oscillator

Posted on Feb 5, 2014

An oscillator is an amplifier with a feedback loop from output to input. Barkhausen criteria states that for oscillation to occur the product of the gains around a loop must be equal to or greater than unity and that the sum of the phase shifts around the loop must be a multiple of 360 °. Before beginning a design or purchase of an oscillator ther

Crystal oscillator
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e are system parameters, which the oscillator will need to conform to. Below are questions which need to be determined by your system. These parameters will determine the type of oscillator you will require (TCXO, TCVCXO, VCXO, Clock, Ovenized). C0 is the capacitance formed by the crystal electrodes plus any holder capacitance. The L1, C1, R1 branch is called the "motional arm". The motional capacitance, C1, controls the "pullability" of the crystal. The shift of a crystal can be calculated by the following formula: Knowing two different loads on the crystal, we can look at the differences between each shift from series to calculate the total trim range. C1 and R1 can be specified on any crystal. Typical values of R1 are 10 to 25 ohms on the fundamental mode and higher on overtones. Typical motional capacitance values are between 0. 018 pf and 0. 024 pf for a fundamental crystal. Motional capacitance is divided by the overtone squared. Static capacitance (C0) is about 213 times C1 on the fundamental mode. Example: given a 0. 020 pf C1 and a C0 of 4. 26pf the shift from series of a 20pf load is 412. 2 ppm and the shift of a 27 pf load is 319. 9ppm. This gives us a tune range of 92. 3ppm between 20 pf and 27 pf loads. The oscillating loop C1, C2 in parallel with RE, Ctrim in parallel with Cpad and the crystal can be isolated from load effects in the colpitts oscillator by keeping RC «RE. The output waveform at the collector...

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