I am looking pretty silly now... the footprint (I created, unfortunately) of the crystal was incorrectly pinned, and so I've rotated the crystal 90 degrees and put the 39pF caps back on and its now correctly oscillating. I should have checked this before bothering you guys!
Checked with 10X probe.
Sorry for wasting anyones time!
Is there a one true answer to the question? It all seems very
frustrating to me. Why doesn't an oscillator start?
A crystal oscillator will fail to start when the crystal and the capacitors attached either side do not fully produce a 180 degrees phase shift back to the input of the inverter inside the chip.
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: -
V1 is the driving voltage source and R2 (100 ohms) represents the output impedance of the gate involved in the oscillator. Look carefully, the phase angle doesn't quite reach 180 degrees and this will mean NO OSCILLATION.
The extra few degrees of phase shift come from the output capacitor on the invertor - the 100 ohms (or whatever the output impedance of the inverter has) AND this extra capacitance push the phase shift past 180 degrees and the oscillator will then oscillate.
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. This means the oscillator will oscillate at the left hand point on each phase curve that the response crosses 180 degrees (parallel resonant point tuned by the external capacitance).
So, you need capacitors to make this type of oscillator work and the manufacturer tells you what to use but, in my humble opinion, there are a lot of subtleties around that some manufacturers maybe either don't fully know or won't tell you. I'll also add that there appear to be very few web articles about what really is going on and the true importance of each capacitor.
Why does removing the external load capacitors make it start?
Maybe the self capacitance of the tracks and gate input capacitance are sufficient. It depends also on the Q of the crystal and is hard to speculate on. Maybe the inverter's slew rate is too slow?
Best Answer
An internal capacitance appears inside the crystal case (Co). It is the capacitance between the crystal surfaces, with quartz as the dielectric. This capacitance is shown in your crystal model circuit as "Cp". This capacitance appears in parallel with the external capacitance (Cp) that your circuit adds. Stray capacitance is included in Cp.
Where you see a crystal specifying a load capacitance, it is generally expected that you would use it in a parallel-resonant circuit, where crystal impedance rises to a maximum, and voltage across its terminals peaks at the desired operating frequency. The operating frequency can be adjusted by varying the external parallel capacitance Cp.
Shown below is the voltage across two identical crystals. For both, the internal parallel capacitance is 3.5 pf between the crystal terminals. The green plot shows parallel resonance at 4.5096 MHz of this bare crystal. It would be difficult to operate this crystal above this frequency.
The red plot shows the same crystal loaded with a parallel 12pf. Its parallel resonance has dropped to 4.5034 MHz.
Note that series resonance for both crystals is 4.5016 MHz. Operating below this frequency would be difficult - some extra inductance would be needed in series. When operated as a parallel-resonator, you can adjust operating frequency in the range of 4.5016 - 4.5096 MHz. by adding an appropriate external Cp. It is a very easy and cheap way of tweaking frequency.
For an oscillator, a sustaining amplifier is required. It will likely add some extra phase shift that also affects frequency. An adjusted Cp can compensate.