how is the 680uH figure arrived at?
From the data sheet.
There is a cap of 180pF from pin 3 to 1. (they made this cap 5pF small to make up for loading on the outside)
The transformer should resonate at 455khz.
Inductance is 680uH. (What pin????)
455khz=2 pi squart(180pf * 680uH)
By reading between the lines; I assume 680uH must be from pin 3-1 because it must be across the 180pF.
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More reading between the lines:
Pin 1-2 = 70Turns, Pins 4-6 = 7Turns. That is a 10:1 turns and 100:1 impedance.
Primary = 60k ohms. Secondary = 600 ohms. So secondary must be pins 4-6. Primary must be pins 1-2.
The "resonant" part must be pins 1-3.
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I know I guested on some of this. Reading a data sheet is an art-form.
I have written data sheets. Usually only 50% of the information in the engineer's head makes it to paper. The other 50%, he assumes you know. Every one in the factory knows the primary is pins 1-2. So it did not get written down.
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Another example of 50% of the information.
Romans had "Century", a group of 100 men. Every one knows this.
During the 2nd Punic wars, they had problems getting enough men so Century=60 men. A historian wrote that they went out with 60. BUT We know there are 4 men that don't fight. (trumpet and flag, etc) So did they have 60 men that included the 4 non-fighters or did they have 60 fighters +4 =64. Those who know directly are all dead. Those who wrote about this assumed that every one knows. OR Those who wrote about this did not really know. OR Those who wrote, did not realize that there would be a question.