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?


1 Answer 1


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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