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pll reference spurs in vco harmonics

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kurtulmehtap

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Dear All,
I have PLL reference spurs -60 dbc at 1 Mhz away from the 1 Ghz carrier. Will I observe the same reference spurs at the VCO harmonics at 2,3,4 Ghz?
Thanx in advance..
 

Yep, same frequency spacing, but different levels. From FM theory, your modulation rate is constant (1 MHz) but the actual frequency deviation is changing at the different harmonics.

60 dBc spurs aren't bad, but if you want to reduce them more, you might get away with adding an elliplitcal lowpass filter with a notch at 1 MHz at the VCO tune port. Just make sure you compensate the added phase shift with a lead-lag network.
 

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