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PLL Reference Frequecy

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Stef83

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Hi,
i have a question that torment me a long time. The reference signal of a PLL (or synthesizer like the ADF4350) must be a sinusoid or can have arbitrary waveform with minimum jitter?

Thank for replies,
Ste
 

It could either be a sine wave or a square wave. If the reference frequency is very low (i.e. less than a few MHz) a square wave is better. It can have harmonics, as long as they are somewhat lower than the fundamental.

An arbitrary waveform will have various phase and amplitude modulations, which could look like random time jitter. This will get multiplied in the PLL, causing all sort of trouble at the output frequency.
 

thanks biff44,
square wave have odd harmonic (3rd -9.5dBc,5th -14dBc) but if they are in phase with the fundamental (as has to be) they don't to made jitter, it's so?
For arbitrary waveform i have not been specific. I intended periodic waveform (not modulated).

But i phase detector can managed any kind waveform as input? And in particular, the divider output waveform can be different from the other input without to cause problems?

Ste
 

For the Reference Input, you need a waveform with a fast rising/falling edge, and only one threshold crossing upwards, and one threshold crossing downwards per period. The threshold is where the input signal is recognized as either a "1" or a "0"

Dividers have rectangular outputs with fast rise/fall times, and satisfy that criteria.

For signals with harmonics, you have to look at them on an oscilloscope and see what those harmonics are doing. If they line up at the right phase to make a rectangular pulse, you are all set. If they line up to make a random set of superimposed sine waves, all bets are off.
 

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