How do you select a capacitor for a crystal oscillator?
The capacitors should have twice value of loading capacitance minus stray capacitance (trace and pin capacitance). I.e. if loading capacitance is 20pF, stray capacitance per each uC pin 2.5pF, C44 and C45 on your circuit should be 20*2-2.5=37.5pF. So 33 or 39pF should work.
Why two capacitors are used in crystal oscillator?
Capacitors C1 & C2 are usually chosen so that oscillator frequency is very close to the crystal manufacturer’s target frequency, since frequency is often of prime importance to the circuits that this oscillator drives.
Do crystals need capacitors?
There are a number of misconceptions about load capacitance, which we want to clarify today. In addition to that, we will show you how to choose the right capacitors for your circuit. Most quartz crystals are used in a Pierce oscillation circuit (Figure 1). Therefore, two external capacitors are needed.
Why load capacitor is used in crystal?
The answer is: Crystals must be connected in a feedback loop of some amplifier, in some way or another. As such, the crystal will be subjected to parasitic circuit and board capacitances, aka “loaded”. One can’t avoid this “load”.
What is load capacitor in crystal oscillator?
Load capacitance is the amount of external circuit capacitance in parallel with the crystal itself. In this example, we see that the crystals parallel-resonance mode is always above the series resonance frequency and is characterized by inductive reactance.
What is shunt capacitance in crystal?
: The shunt capacitance of a crystal is due in part to the thickness of the wafer. This is the mea- sured capacitance while not vibrating. Shunt capacitance ranges from 1-7pF. It is not typical to exceed 7pF due to compatibility with oscillator circuit.
What is stray capacitance in crystal?
Stray capacitance is the unavoidable capacitance between nearby conductors or on the microcontroller pins to ground.
What is crystal capacitance?
What is shunt and stray capacitance?
The stray capacitance is the parasitic capacitance present on the pcb board (due to proximity to components) plus the capacitance of the pins. As a rule of thumb, a value for Cstray is on the order of ~5pF. The shunt capacitance is another parameter and it’s mainly a concern for the internal oscillator circuit.
What is load capacitance in crystal?
What does TCXO stand for?
Temperature Compensated Xtal Oscillator
The letters TCXO stand for Temperature Compensated Xtal Oscillator – Xtal is short from crystal and implies a quartz crystal resonator. The TCXO module is able to provide considerably improved performance over that of a standard crystal oscillator, especially in terms of frequency stability over a temperature range.
Which type of crystal is used in crystal oscillator?
quartz
The most common material for oscillator crystals is quartz. At the beginning of the technology, natural quartz crystals were used but now synthetic crystalline quartz grown by hydrothermal synthesis is predominant due to higher purity, lower cost and more convenient handling.
What size capacitors do I need for my Crystal?
Fortunately, it’s trivial to calculate the right capacitors for your crystal. A 12MHz crystal that I use quite a bit is the NX3225SA-12.000000MHZ from NDK. It’s a good size, stable (+/-15 ppm), and easy to find.
What is the best 12MHz Crystal?
A 12MHz crystal that I use quite a bit is the NX3225SA-12.000000MHZ from NDK. It’s a good size, stable (+/-15 ppm), and easy to find.
What is the load capacitance of a crystal?
Every crystal datasheet lists something called the Load Capacitance (CL). In the case of the crystal above, it’s 8 pF. C1 and C2 need to match this Load Capacitance, with the following formula being the key: C1 and C2 are the two capacitors that you see attached to the crystal, but you might be wondering what Cstray is.