[time-nuts] FPGA based TDC (was: Time nuts, off line questions.)
attila at kinali.ch
Sat Dec 17 07:33:54 EST 2016
On Sat, 17 Dec 2016 12:44:55 +0100
Attila Kinali <attila at kinali.ch> wrote:
> > As I understand, the GP2x chips have a true ring oscillator, and it's
> > period PLL'd to the main crystal. I'm not completely sure how control of
> > the period of the ring oscillator is achieved, but I believe it's by
> > drive voltage, and/or varactor diode type components in the ring.
> There is no direct control. The ring defines the oscillation frequency
> by its length. Yes, all changes in temperature and voltage affect the
> delay line and thus the TDC.
In ASICs, the inverter in the delay line is directly modified to
control the switching speed. Most methods work by changing or limiting
current through one part of the inverter to slow it down.
A good overview over many ways how to achieve that with differential
inverters can be found in 
 "CMOS Differential Ring Oscillators", by Jalik, Ibne Reaz, Mohd Ali, 2013
Any simple idea will be worded in the most complicated way.
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