What should be included in the load capacitance calculation?
Besides the oscillator transconductance, almost ALL IC makers do not specify the oscillator input and output capacitances. Those should really be included the load capacitor calculation and can make a quite a difference.
How many pins does a crystal sine wave oscillator have?
Many oscillator modules have only 3 pins for +Vcc Ground and output, though some have extra pins for control options and typically drive loads such as 50 ohms with an output of 500mV to 1V as shown in Fig. 2.5.5.
How can I find out my oscillator capacitance?
Naturally, PCB capacitances should also be accounted for, and those are easier to estimate using various PCB design tools (Saturn PCB toolkit is a good free app). The only real way to guarantee full oscillator functionality (after doing the design calculations) is to test it over the full temperature range and operating voltages.
How much power does a crystal oscillator take?
The older crystals could often take 1mW of power, but smaller new ones have maximums that are much lower. Often a series resistor (from the output) is useful to reduce the drive power, but of course it will reduce the gain, so at some value the oscillator will fail to start.
How does a capacitor and crystal oscillator work?
The inverter produces effectively 180 degrees phase shift so, for oscillation to begin, the two capacitors and the crystal together must form an extra 180 degrees phase shift AND there must be an overall voltage gain greater than 1. Look at this response – it mimics a crystal and one capacitor but it doesn’t quite reach 180 degrees: –
How does load capacitance affect the loop gain?
Load capacitance affects loop gain since the feedback voltage is obtained in both configurations from the voltage divider formed by C1 and the crystal, so it is very important to account for stray capacitance when calculating the value of C1 and C2. In the Pierce configuration, adding load capacitance will reduce loop gain in some cases.
What happens when capacitance is increased to 20 pF?
Here’s a picture showing the effect of increasing input and output capacitance from 1 pF up to 20 pF: – The X axis is at 9.9 MHz FYI. As you can possibly see, only when capacitance is 10 pF or 20 pF does the circuit produce 180 degrees of phase shift.
When to ignore load capacitance in LC tank circuit?
In parallel-resonance oscillation mode, the crystal’s inductance (motional inductance) is in parallel with the oscillator’s load capacitance, thereby forming an LC tank circuit. This LC determines the oscillator frequency. When specifying a series-resonant crystal, you can ignore load capacitance.
What should the capacitance of a crystal be?
If you want the crystal to oscillate as close as possible to the marked frequency, on average, you match up the total load capacitance to what the xtal maker has specified by specifying the two capacitors (usually they are both the same value, so you have only one degree of freedom).