[time-nuts] PLL Math Question
hmurray at megapathdsl.net
Thu Mar 13 22:11:18 EDT 2014
lists at rtty.us said:
> Timing at the Time Nuts level is about precision.
What's the term for a time-nut that's trying to be not-very-nutty?
bob at evoria.net said:
> includes a 10-bit PWM dithered to 14 bits
When you get it all working, that's going to be one of the weak links, at
least for some applications. As far as I can see, your only application is
entertaining Bob, so it won't be a problem for a while.
The problem is that there is a lot of low frequency noise in that sort of
signal, and it's hard to filter out with typical analog components because
the frequency is so low. On a spectrum analyzer, they turn into spurs.
The typical PWM is on for x counts, then off for N-x counts. You can make
the spectrum easier to filter if you distribute the on bits throughout the N
counts (rather than clumping them together). Sometimes you can do that
easily with a synchronous serial setup. You need the "synchronous" vs
"asynchronous" because the async mode puts in start/stop bits that you don't
want. That may not work with a PIC, but I've used it on an ARM. We just
setup the bits in memory and turned on a DMA channel.
If you really get sucked into time-nuttery, you will upgrade the PIC to an
ARM and see how good you can make things.
These are my opinions. I hate spam.
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