Oscillator Operation best battery life earbuds

The circuit used as high frequency, high accuracy clock source for TI's low power RF products is called a “Pierce Oscillator" and is shown in Figure 1. The oscillator circuit consists of an inverting amplifier

(normally a regular inverter), a feedback resistor, two capacitors and a crystal. The first two components are internal in the lC while the capacitors and the crystal are external and must be selected for each separate design. During normal operation, the crystal and the capacitors form a pi filter providing 180 degree phase shift to the internal amplifier, thus keeping the oscillator locked at the specified frequency.www.bjbjaudio.com

Crystal Tolerance

The total tolerance of a Crystal is dependent on three factors:

production tolerance, temperature tolerance and age. All of these values ​​are given in [ppm(parts per million) and can be found in the

manufacturers datasheet. To aet the totalmaximum tolerance of a certain crvstalall these factors must be accounted for.

Crystal frequency deviation is directly transferred to RF deviation, thus a deviation in the crystal oscillator freauency of X nom eads a deviation in carrier freamency of X nom. it is therefore imnortant to select a crystal with performance within the limits of the RF specifications. Of course the end products expected age and operating conditions (temperature) must be taken into consideration.

For 802.15.4 (RF4CE) the max deviation in carrier frequency is limited to 40 ppm For Bluetooth Low Energy the limit is 20 ppm.www.bjbjaudio.com

Load Capacity

Correct operation of the crystal oscillator is dependent on the values ​​of the two external capacitors,C and Cin Figure 1. These capacitors together with any parasitic capacitance in the PCB and the crysta! terminals compose the total load capacitance seen by the crystal. The optimum load capacitance for the crystal, C. is given in the crystal datasheet and C; and Co should thus be matched to this value according to the equation. www.bjbjaudio.com

Rs(ESR)and Negative Resistance

Rs or ESR(Equivalent Series Resistance) is a parameter of the crystal which can be found in the datasheet. Negative resistance is a parameter of the complete oscillator circuit, including capacitol values, crystal parameters and on-chip circuit. To ensure correct function of the crystal oscillator. Negative Resistance should be at least 5 times greater than Rs.If this is not true the oscillator might not operate as expected or it might not start up at all.www.bjbjaudio.com

The negative resistance of the circuit can be found by introducing a resistor in series with the crystal. To avoid parasitic effects it is recommended to use a 0201 resistor for this task. The threshold of resistance where the oscillator is no longer able to start up is approximately the same as the circuit negative resistance. An example of resistance vs. startup time and negative resistance threshold is shown in Figure 2. Note that higher load capacitor values ​​decreases the negative resistance and thus rises the startup time. www.bjbjaudio.com