Patrick J wrote:
About PLL:
I know that a PLL is used to lock on to a special freq
but everyone here seems to agree on that it can be used
to multipy by 8 ?!?


Patrick,
A phase locked loop is usually used to lock one frequency to another.
Consider a simple block diagram of a classical analog PLL.  It will have a
phase comparison block, sometimes in phase or in quadrature (90 degrees out
of phase), which produces an error signal.   This error signal indicates
whether the feedback signal edge lags or leads the original signal, i.e.,
whether the feedback signal is a little too fast or a little too slow
relative to the original.  This error is fed into a filter block - the loop
filter - whose output is put into a block containing the voltage to
frequency converter (VtoF).  The output of the voltage to frequency
converter can either be used directly as the feedback to the phase
comparison block, or it can be fed into a counter, say a divide by eight,
and the output of the counter fed back to the comparison block.  But we know
that for the comparison block to work, the comparison frequencies need to be
the same.  The only way for them to be the same is if the output of the VtoF
is exactly 8 times higher frequency than the original.  Tap off the VtoF to
get a frequency exactly 8 times higher than the original.  The biggest
problem with your design is the very low frequency part.  Getting an analog
VtoF to operate that low would be difficult.  You could try a much larger
divide ratio and tap out at the appropriate counter output or you could do
the PLL in software.

Today most low frequency PLLs are done in software, but you will see very
high frequency PLLs used in time measurement circuits for mass spectroscopy
time of flight measurements, in fiber optic communication systems, or
anywhere that precise time locked signals are required.  For example, most
DSP's use PLLs to multiply the oscillator to get 2 or 4 times internal clock
speeds.

Attached is a link to the CD4046 PLL.  They will do a much better job
explaining its operation.  Yes, many companies are obsoleting it, but look
at the various 74XXX4046 devices available, then tell me its obsolete.

http://www.fairchildsemi.com/pf/CD/CD4046BC.html

Regards,
Ed

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