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Alternate crystal frequency for AD9575

I need to synthesize a 135 MHz very low-phase noise differential (LVDS or LVPECL) clock for my present application.

The AD9575 looks like a convenient choice, though the datasheet indicates that it wants one of three specific reference clocks/crystals as input. From my calculations, in its 6.25 or 6.375 multiplier ratios, it will generate 135 MHz given a 21.6 MHz or 21.17 MHz clock input.

Question: will this work?

Second question: this device is specified for a maximum reference input of 25.78 MHz -- would it tolerate 27 MHz? Used the times 5 mode, this would also meet my requirements.

Advice is appreciated.

[Data Sheet attached for your convenience]

AD9575.pdf
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  • Hello Alex,

    The key parameter here is the VCO frequency, which is rated at 2488 to 2550 MHz over process, temp, and voltage (PVT).

    Unfortunately, 135 MHz would require either a 2430 or 2565 MHz VCO frequency that is outside the allowable range. Now, the differences between those two numbers and the datasheet are small enough such that it would probably work most of the time over the PVT extremes, but we're not able to guarantee it.

    I can tell you that I'd be shocked if a 2160 or 2700 MHz VCO frequency worked with this part.

    All that said, you might have a look at the AD9553 or AD9550, as the allowable VCO range is much larger.

    -Paul Kern

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  • Hello Alex,

    The key parameter here is the VCO frequency, which is rated at 2488 to 2550 MHz over process, temp, and voltage (PVT).

    Unfortunately, 135 MHz would require either a 2430 or 2565 MHz VCO frequency that is outside the allowable range. Now, the differences between those two numbers and the datasheet are small enough such that it would probably work most of the time over the PVT extremes, but we're not able to guarantee it.

    I can tell you that I'd be shocked if a 2160 or 2700 MHz VCO frequency worked with this part.

    All that said, you might have a look at the AD9553 or AD9550, as the allowable VCO range is much larger.

    -Paul Kern

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