[sdiy] Tonewheel relics

[rburnett at richieburnett.co.uk]

> @roman and Richie: thanks! From my limited experience with bldc's I
> feel they would be costly, especially a 12-way driver system for them.
> Would they need hall sensor feedback to get the tuning accurate
> enough?

Yes, probably not cheap.  I don't think you'd need hall sensors to 
detect the field, but you would need to ramp the frequency up at a 
controlled rate to allow the mechanical bits time to accelerate up to 
operating speed.

> The pll sounds nice and classic. I'll look into that. At present I'm
> not sure if the pick-up  coils of the generator could handle more
> loading, e.g. As required for closing the loop. But a high impedance
> buffer would probably be ok.

Take the feedback for the PLL from the highest frequency that you can.  
The loop filter in the PLL has to filter out the ripple in the compared 
reference and positional feedback signals.  The higher you can make the 
reference and feedback signal frequencies, the larger the bandwidth of 
the loop filter can be, and the quicker the PLL speed controller will 
settle.  i.e.  Don't use a reference signal that is just one pulse per 
revolution, as this can mean you end up with a very sluggish control 
loop!

-Richie,