[time-nuts] Noise contribution of PLL on measuring phase noise

Bob kb8tq kb8tq at n1k.org
Wed Jun 28 12:08:09 EDT 2017


Ok, let’s back up a bit:

There are two basic regions when measuring a phase locked oscillator. You 
can be “inside” the loop bandwidth or “outside”. Yes it’s a bit more complex 
than that, but go with the idea for now. Inside the bandwidth, everything about
the PLL can / may matter. It can and often does contribute noise to the result. 
That noise might be from the detector, a divider, or even the loop amplifier. Outside
the loop bandwidth you just see the oscillator phase noise. (There is a third region
in-between the two where things get even more messy).

Based on that, you will see some noise from the loop amplifier. Exactly how much
(is it 0.001 db?) or how far out will always be a “that depends” sort of thing. Without
measurement (and possibly modeling) there is no single answer to the question. There
are many noise sources in any phase noise setup so this issue is hardly unique to the
loop amp.


> On Jun 28, 2017, at 6:29 AM, Thomas S. Knutsen <la3pna at gmail.com> wrote:
> Hi.
> When measuring Phase Noise using PLL, what noise would be contribuded from
> the PLL into the amplifier?
> Lets say we disconnect the control voltage from the oscillator, shouldn't
> the input stage of the PLL contribute some, altough miniscule part to the
> total noise seen?
> Would this noise be masked by the noise from the phase detector?
> The reason for asking is that I'm trying to improve my PN measurement
> setup.
> Currently the PLL consist of a unity gain buffer made of TL072 before the
> PLL filter, not unlike what Wentzel did in his amplifier. The PLL filter is
> a low noise op-amp (OP27).
> BR.
> Thomas.
> -- 
> With Best regards, Thomas S. Knutsen.
> Please  avoid sending  me  Word  or  PowerPoint  attachments.
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