[time-nuts] PLL Math Question

Hal Murray 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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