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PLL System Reference Frequency

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faisal78

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Hi, in RF systems, GSM, CDMA etc, how is the system clock reference choosen.
or example, some systems uses 26MHz reference clock, 13MHz, 16.8MHz and 19.2MHz.

What exact parameter determines this and how was it chosen?

I have always wondered why every RF system uses difference clock reference.

Can't they all just use the same and use a different Frac-N value to achieve the same RF output?
 

That is determined by PDF(Phase detector freq), which is coming from freq plan. Before designing a system, one must first determine the freq plan. Such as one RF channel is 650KHz, the XTAL maybe 13M (divide-by-20), or 26M (divide-by-40). This used interger synthesizer such as ADF4113.

It's possible to use the same XTAL and use fractional synthesizer to merge the output freq, but interger synthesizer has some advantages, such as: cost, spurs, noise floor, etc.

XTAL is not only determined by freq, and also by system, such as if already have a 10M, then the XTAL freq maybe different.
 

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