In a message dated 2/4/01 2:16:24 PM Eastern Standard Time,
[email protected] writes:
<< Allow me to quote from the book "Phase Locked Loops, Principle and
Practice", McGraw Hill 1996, by P.V. Brennan University College, London.
Page 22: "...As far as the control loop is concerned, it should be noted that
frequency dividers act equally as phase dividers, so that a factor of 1/N
must be allowed for in the loop equations". >>
This is certainly true. It must be taken into account in designing a loop,
but it does not represent a source of _frequency_ error so long as lock is
maintained.
The example cited from page 128 is such an extreme case that, were the
controlled oscillator sufficiently unstable or noisy as to experience
short-term excursions beyond 360 degrees at its own frequency--or if the
detector were occasionally unable to resolve with the 0.35 degree precision
specified--the entire system would be susceptible to coming out of lock
entirely! Until that point, the output is still on-frequency, but might
exhibit serious phase noise.
This is why one must perform phase comparisons at a reasonable division
ratio, even if it means sacrificing extremely narrow frequency steps; or
else, one has to be much more meticulous in the design of the VCO and the
phase comparator, and even then, lockup times may be rather long.
73,
John
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