Fast VCOs/V->F converters

[Douglas R. Kraul]

>Hmm, are we speaking of noise from your input signal, or from parts
>of the PLL ?
>
>Parts of the PLL can be more or less noisy. You always have a certain
>amount of noise in any system - the questions here are will it be
noticable,
>and will it be much higher than on a "straight" VCO without the loop.
>
>The PLL also acts as a filter for noise from the input signal (i.e. the
>frequency you want to track), and is in fact often used to clean noisy
>carriers. Apart from the frequency dividers in the loop, it's just a sort
>of (auto tracking) BP filter, after all.
>
>It's clear that one can build a PLL (think of a DPLL with high quantisation
>error and low bandwidth) where the internal noise is so high that it will
>never lock - but is this valid for the musical applications we were
>talking about ?
>


What I am referring to is the inherent error in the loop itself.  No
feedback loop is perfect.  Everyone has a certain amount or error.  In a PLL
the phase detector outputs a signal that is related to the phase difference
between the reference osc and the incoming signal.  This output is then
integrated(low pass filtered) and applied to the ref osc control.  This loop
has a certain "gain" that determines the phase error that one will see at
any instance at the ref osc.  The loop also has a time constant determined
by the integrator (the cut-off of the low pass).  This not only determines
how fast the PLL response to a step change in incoming freq but also how
much "hunting around" the PLL does.  The more "loop gain" the lower the
error but it is finite.  Loop components also contribute to the error though
the feedback tends to minimize the effects.  However, feedback can only fix
errors that have predictable characteristic and "good behavior" like
monotonically.

BTW, if you have the inclination and means I suggest putting together a PLL
and trigger a good analog scope (100Mhz or better) with the input signal and
look at the PLL OSC on the second trace.  You will see that the edges of the
PLL VCO jitter.  the "envelop" of the jitter is a good estimate of the error
(represents about 1 sigma of std dev.).  This is the error that I am
referring to.  It shows up as sideband modulation in the signal being
produced by this oscillator.  The two signals are locked and they will
track, subject to the PLL's limits, but there is jitter.

One last thought about PLL.  To be useful musically the PLL's osc needs a
dynamic range that spans the useful musical range.  10 Octaves used to be
the figure of merit used.  Most PLL VCO's that I am aware of struggle to
track over a 10:1 range.  The higher the frequency, the worse it gets.
Finding a PLL to give you 10 octaves will be tuff.

All of this may sound like splitting hairs, and in normal PLL applications
they almost never show their nature.  But musical applications of osc to
produce pitches tend to be very unforgiving.

Douglas Kraul