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Design >> RF Design >> Fractional spur in pll output spectrum
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Message started by shadow_cuk on Mar 16^th, 2016, 6:31pm

Title: Fractional spur in pll output spectrum
Post by shadow_cuk on Mar 16^th, 2016, 6:31pm

Dear all

A fractional-N pll with its reference frequency of 10Mhz and divide ratio of 10.3 will get a output frequency of 103Mhz. It's known that the fractiional spur will occur at 0.3x10Mhz offset the center frequency, but in real test spurs at 1M/2M/4M also exist. Could any body explain that why there are so many harmonic frequency appeared?

Title: Re: Fractional spur in pll output spectrum
Post by raja.cedt on Mar 17^th, 2016, 1:03am

Think about coupling spurs!!! Refer the following!

A New Mechanism Producing Discrete Spurious Components in Fractional- N Frequency Synthesizers

Best Wishes,
Raj!

Title: Re: Fractional spur in pll output spectrum
Post by shadow_cuk on Mar 17^th, 2016, 6:19pm

Thank you raja, I've read the paper. It mentioned in Fig.11 that a 105Mhz reference with a fractional divide ratio of 255/1024 could have
spurs at 410khz offset, with the author‘s equation I can't get a result of 410kHz，indeed 105M/410k is not a integer.