There's a lot of info and guides on how to select and calculate load capacitors for the parallel resonant circuits. They all assume that people do already possess crystal, and know its CL. That CL must be balanced within the circuit to allow crystal oscillating at its rated frequency.
Well, I have another problem. I have a list of identical frequency and ppm rated crystals, rated for CL being 16 pF, 18 pF, 20 pf and 30 pF. I do not have these crystals yet. Given resources I mentioned above I can calculate capacitive loads and series resistor. But which crystal must I select from this list, and why?
Simply saying, do I select crystal first and then buy capacitors for it, or I select capacitors first (including other load in the circuit), and then buy a crystal?
I also read that this capacitive load of crystal is adjusted by the manufacturer by simply cutting it to the proper size during its tuning. Is CL just some given number, which does not matter for the circuit out there (outside of Rs/C1/C2 in the oscillation circuit)?
The only circumstance I see load capacitance of the crystal is generally important is ensuring that it is not overloaded - e.g. if I take crystal with CL=16 pF, but (imagine) load from the inverter gate in Pierce oscillator appears to be 20 pF, the circuit will never start.
If I am correct in the statement above, what is better to choose in general cases - crystal with high CL or as small CL as possible for the environment it operates in?
Update: guided by the comment of @analogsystemsrf I started reading various literature on the negative resitance and load capacitance. Found the following useful source:
C0 is proportional to the physical size of the crystal, electrode size. ... A larger metal can crystal will have a larger C0 than the a smaller SMD type crystal.
Proposes that shunt capacitance of the HC-49 must be bigger than HC-49S.
- Crystal Oscillator Basics and Crystal Selection by Microchip.
If the circuit load capacitance does not equal the crystal CL, the operating frequency of the Pierce oscillator will not be at the specified crystal frequency. ... Care should be used in selecting values of C2 and C3. Large values increase frequency stability but decrease the loop gain and may cause oscillator start-up problems.
Proposes that crystals with lower load capacitances are easier to start, but less stable in their operation. Well, that's in line with common sense - anything around the crystal circut may change capacitance (the most rude example is a finger on the terminals), and of course introduced capacitance will have higher percentage for 16 pF load than for 30 pF load.
- AN0016.0: Oscillator Design Considerations by Silicon Labs
Crystals with small load capacitance would typically start faster than crystals requiring a large CL. Large load capacitors also increase power consumption.
Crystals with low ESR and load capacitance typically have the shortest startup time and consume the least amount of power.
Confirms previous conclusion - if startup time is critical, smaller load capacitance crystal is required. In addition, it states that smaller load capacitance crystals consume less power (and this is logical).
It is possible to slightly alter the oscillation frequency of a crystal by adjusting the load capacitance (CL1 and CL2). The pullability of the oscillation system refers to which extent it is possible to tune the resonance frequency of the crystal by changing these values. The crystal sees these capacitors in series through ground, parallel to the closed loop. They will therefore slightly alter the anti-resonance frequency of the crystal.
In my opinion it would be easier to fine-tune Pierce oscillator frequency using trimming caps with higher values of the load capacitors.
If the negative resistance is not high enough to satisfy this criterion, another crystal with lower ESR and/or load capacity requirements should be chosen. The XO Configurator in [Hardware Configurator] in Simplicity Studio is able to calculate the Rneg value for your design based on the load and shunt capacitance, internal loss and frequency.
This subchapter 4.6 sets clear formula with negative resistance (ability to start/oscillate) depends on the load capacitance.