Most of the time I see oscillator datasheets or datasheets for parts requiring external oscillators, the oscillator output is AC coupled to the part it's driving through a 1000pF MLCC.

This datasheet: https://www.iqdfrequencyproducts.com/products/pn/LFTCXO073006Reel.pdf as an example says "A DC-cut capacitor is not built in to the oscillator; a 1000pF DC-cut capacitor is therefore required in the output line."

So my question is where does this value of 1000pF come from? In particular the above datasheet says "Drive Capability: 10kΩ//10pF".

I'm certain I must be missing something, but if the drive capacity is only 10pF then surely the 1000pF is too large to effectively couple of the 52MHz (in this case) output?


The crystal's output capacitance is 1000pF and the maximum input capacitance it can drive is 10pF.

Capacitor impedance is: \$ Z_{capacitor} = \frac{1}{j \omega C }\$

In other words, the larger the capacitance the lower the impedance (the opposite of a resistor's resistance-impedance relationship, and the opposite of an inductor's inductance-impedance relationship).

Therefore, the output impedance is 100x lower than the input impedance. Sound familiar?

As for the 10pF, I think it's just a round number they chose as a standard. Similar to how 1MOhm is used on oscilloscopes probes.

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