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Programmable crystal oscillator as PLL reference

Question asked by magoo on Feb 10, 2017
Latest reply on Feb 21, 2017 by DonY

Does anyone have some experience with using programmable (instead of fixed frequency) crystal oscillator ICs as the clock reference for Analog Device's PLLs?

 

I'm looking for a clock oscillator with a frequency that is a binary multiple of 10.7 MHz (e.g. 10.7 / 21.4 / 42.8 MHz) to use as the reference for a PLL, because I want to use the same oscillator also as a coherent 10.7 MHz source for some related IF circuitry.

It appears that fixed XCOs with 21.4MHz or 42.8MHz are not easily available as off the shelf components.

However, a number of manufacturers offer programmable clock oscillators that can be ordered preset to these frequencies.

For example, this Digikey part: XLH730021.400000X-ND

 

My question:

Is it advisable to use a configurable clock oscillator as the reference for a PLL?

Such configurable clock sources themselves are internally based on some sort of PLL circuit and therefore have a different (probably worse) phase noise and jitter than a conventional fixed frequency crystal oscillator, and they certainly have spurs.

I fear that the overall phase noise of my PLL circuit will suffer. I remember having had problems with a Fast Ethernet chip's internal frequency multiplier when clocking it from a programmable 25 MHz clock oscillator (but that was a long time ago, so the programmable clock oscillators might have improved in the meantime).

 

What is your experience on this?

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