[time-nuts] Locking 100 MHz to 10 MHz

Matt Ettus boyscout at gmail.com
Wed Dec 19 14:53:10 EST 2007

> in my opinion, you would probably introduce some thermal noise through this
> 50KOhm equivalent resistor while the PLL is not operating. Noise could maybe
> be  reduced if there is a cap to ground on this divider (this cap being part of
> the  loop filter).

As long as you don't think it will affect stability when in lock, then I am ok.

> The PLL bandwidth depends solely on the phase noise of your 10MHz source
> versus your 100MHz oscillator. If the 10MHz source is better at say 100Hz  offset
> (better by more than 20dB) then the loop bandwidth should  be more than
> 100Hz, so that the PLL can actually reduce the phase noise of  your 100MHz
> oscillator.
> At 100Hz offset, you say you have -68dBc/Hz at 100MHz. This calculates to
> -88dBc/Hz at 100Hz offset for the 10MHz source (excluding the PLL chip and
> loop-filter noise). You may want to check the noise performance of the PLL on  the
> ADI website's PLL simulator.
> So if you have much better than -88dBc/Hz at 100Hz on your 10MHz oscillator
> (not hard to achieve, many oscillators have <-140dBc/Hz at 100Hz already)
> then you would be wasting performance with a <100Hz loop filter, and you  may
> want to do a 1KHz or even wider loop filter or so. But if you don't  know the
> 10MHz source's performance, it is probably best to be safe and use  10Hz, or
> 100Hz loop filter BW.
> A good spectrum analyzer (such as HP 8560B/E etc with the phase-noise
> software option) should allow you to measure <-68dBc/Hz noise at 100Hz offset  at
> 100MHz, so you can check what BW results in the overall lowest noise.
> In short, if you want to maximize your systems performance, then loop
> bandwidth depends on the performance of the 10MHz versus the 100MHz  oscillator for
> close-in phase noise.
> To do the math, subtract 20log(100/10) = -20dB from the noise of the 100MHz
> oscillator to get the equivalent noise energy for the 10MHz oscillator (at the
>  same frequency offset).
> Hope this makes sense,

Yes, I understand all the math.  I just don't know what to expect from
the 10 MHz references people will plug in.  Clearly if they plug in a
Fury, I should use a wide bandwidth, but who knows what else they'll


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