# Finding capacitive load driving capability of a Sine wave crystal oscillator

I will be using Crystek CCSS-945X-25-100.000 sine wave oscillator to drive the reference CLKin pins of TI PLL IC LMK04832

This sine wave oscillator will be on a separate PCB which we are calling as "CLOCK DISTRIBUTION BOARD".

Sine wave clock from "CLOCK DISTRIBUTION BOARD" will be connected to main board which has LMK04832 PLL through a SMA cable.

I have checked the CMOS version of the oscillator : CVHD-950.

In datasheet of CMOS oscillator ,they mentioned the load as 15pF.

Now I need to know how much load in capacitance, this sine wave oscillator version can drive because this SMA cable will be having some capacitance and also LMK04832 refclock inputs will be having some capacitance.

Is there any way to know the value of load capacitance, this sine wave oscillator can drive ,from the available oscillator specification in datasheet like 5dBm into 50 Ohms i.e, how to convert 5dBm into 50 Ohms to 5dBm into ... pF?

A sine Xtal Osc will have a 50 Ohms source impedance and designed for a 50 Ohm Load.

+5dBm @ 50 Ohms = 1.125 Vpp ~ 3 mW

A "CMOS" Xtal Osc will be a logic level Square wave and thus rise time (3ns) depends on rated load Capacitance (15 pF typ) and internal RdsOn. This is because RC = T for 64% swing so they are saying @ 15 pF, 3 ns, with 60% swing that RdsOn = 3ns/15pF = 333 Ohm (worst case) but actually it is ~ 25 Ohms. +/-50% for most 3.3V CMOS logic.

+/- 24 mA max out does not state output level but if "25 Ohm" logic is used then the voltage will drop 33% with 50 Ohm load. which is more like 12dBm.

I picked a random 3.6V max logic inverter which I expect to be "25 Ohm logic" is closer to 22 Ohm... but close enuf for government work.

But you probably need the square wave logic source.

However for spurious harmonic EMI reasons , 3ns rise time will have harmonics up to 100MHz or so , you may want controlled impedance with guard grd tracks.