7 Sep
2013
7 Sep
'13
2:55 p.m.
On 09/07/2013 12:57 PM, shadoooo wrote:
As the 4/3 ratio is not an integer multiple, only 1
every 4 pulses
will be aligned with the hardware pulses, thus we would have a phase
uncertain of 1/3 of physical hardware sector. Probably, if the
cartridge is started and synchronized, it could continue to work
correctly until the whole stuff is stopped and restarted. In this
situation it could work in 33% of the cases,
I think perhaps that you do not understand how a PLL really operates.
In every case that I've ever worked with, the PLL center frequency is a
multiple of the reference. (That, for example, is why having a 3GHz CPU
clock on your personal computer that the reference frequency is always
much lower than that 3GHz.
Basically, a PLL operates by comparing the *phase* of the reference
frequency with the PLL oscillator and correcting the PLL oscillator
(almost always a VCO) accordingly.
So, running a PLL at 4x the 12-sector frequency and dividing it by 3
will give you 16 sector timing that closely follows the speed variations
of the spindle motor.
Wikipedia has a good section on "Phas Locked Loop", including an example
of one coded in software.
--Chuck