[time-nuts] Noise contribution of PLL on measuring phase noise
Thomas S. Knutsen
la3pna at gmail.com
Wed Jun 28 15:23:00 EDT 2017
Thank you Bob.
I was thinking of the contribution outside the loop, as for most part my
PLL loops are around 1Hz and resolving closer than 10Hz isn't easy with a
analyzer that stops at 10Hz.
I guess what I'm asking is: should I take any care in the unity gain
buffers or would any normal garden variety op-amp work?
Wentzel used the 310 in his buffers, and those are a bit special, being a
bipolar amp with 2nA input bias current.
2017-06-28 18:08 GMT+02:00 Bob kb8tq <kb8tq at n1k.org>:
> 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
> than that, but go with the idea for now. Inside the bandwidth, everything
> the PLL can / may matter. It can and often does contribute noise to the
> 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
> > 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
> > 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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With Best regards, Thomas S. Knutsen.
Please avoid sending me Word or PowerPoint attachments.
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