Hi > On Oct 19, 2014, at 5:00 PM, Jim Lux <[email protected]> wrote: > > On 10/19/14, 1:08 PM, Bob Camp wrote: >> Hi >> >>> On Oct 19, 2014, at 3:35 PM, Charles Steinmetz >>> <[email protected]> wrote: >>> >>> Bob wrote (alluding also to something Poul-Henning wrote): >>> >>>> The phase comparison part of the PLL is pretty straightforward if >>>> you are looking at two RF frequencies. An XOR gate is one >>>> solution, there are many others. Getting something like 100 to >>>> 200 ns full scale on the phase comparator makes the rest of the >>>> gizmo much easier. >>> >>> All true. However... >>> >>>> A 12 bit ADC on a MCU will get you to 100's of ps per bit. That >>>> is more resolution (it's < 1 ns) than you need for this. >>> >>> Getting an ADC to sample fast and accurately enough to provide that >>> honest resolution is not trivial. And if you have that, you'll >>> almost certainly have the resources to do the phase comparator >>> digitally, too, which brings many advantages -- so I see no reason >>> to use an analog PC. >> >> If you take a look at some of the newer ARM MCU’s they are getting >> 13+ solid bits out of their ADC’s at a > 10 KHz rate. That’s more >> than good enough for anything you are trying to do with this design. >> There’s no need to make it any more complex. > > I'm using the Freescale Kinetix K20 parts, which have 16 bit differential > input ADCs, and built in averaging. The raw ADC can sample at about 400kHz. > > You can easily get 14 bit performance from these at tens of kHz rates. > I need I/Q, so I sample two inputs at 50 kHz (read one, then the other) > without averaging (so they're about 2.5 microseconds apart), and then > decimate them through a 2 stage CIC and a 13 tap FIR filter down to 200 Hz. > This takes about 60% of the processor running at 48MHz.
I’m using parts from the same family, but not doing the whole DDS thing. Single input and control loop - the part sleeps about 98% of the time. The demo boards (Freedom boards) are all below $15 and free if you go to one of their (often free) classes. Bob > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
