Hi cippy, marked area in the right picture (a snapshot of post#10) is for TA31136. See the picture below..
All the oscillator pictures you already have uploaded are variants of colpitts oscillator. So at first, you need to understand designing colpitts oscillator. Therefore, at the begining of post#10 some necessary formulas (for calculation of capacitive divider) are given. See the picture below..
The above formulas are difficult to use due to the lack of information of amplifier and crystal. But without the information of amplifier and crystal, and assuming several terms as frequency independent, we can obtain results for one frequency if we have results for another frequency. We have results for 21.25MHz so we have found results for 72MHz. See the picture below..
You may ask how a complex oscillator circuit (Third overtone oscillator circuit of T31136) will behave as a simple colpitts oscillator circuit.
At third overtone the LC network made by L1 and C51 will behave as capacitive, as a result the circuit will look like a colpitts oscillator and it will oscillate as a colpitts oscillator. On the other hand, at fundamental frequency the LC network made by L1 and C51 will behave as inductive, as a result the circuit will not look like a colpitts oscillator, and due to phase related problem in feedback network it will not oscillate. Note that as the reactance of C3 is too low (because it is a DC block capacitor), neglect the effect of C3 for this explanation.
Now imagine that the third overtone oscillator circuit of TA31136 is oscillating at the third overtone (72MHz). So we will imagine the parallel combinanion of L1 and C51 as a single capacitor. Let its name is Cx. Now neglecting C3, if we draw the picture, it will look like the picture below..
Now what should be the value of C2 and Cx? We already have calculated these in the 9th and 10th line of post#10. These are C2=15pF and Cx=22pF.
To calculate C51, first we need to find the capacitance value which resonates with L1(220nH) at 72MHz using the formula f=1/(2*pi*sqrt(LC)). The capacitance is
found 22pF. So, at 72MHz susceptance of this 22pF will be cancelled by the susceptance of L1(220nH). So to get additional 22pf for Cx, C51 should be 22pF+22pF=44pF.
If you test (C51||L1) with found values, you will find that it is capacitive at 72MHz and inductive at 24MHz. If the value of L1 was not given, then it would be relatively harder to find the values of both L1 and C51.