We did the same using a 1 KHz out of the $ 14 ubolx M7 and a Morion . Results better than 1 E-10. Some time nuts are now assembling and testing the same. Total cost less than $ 10 not counting OCXO or GPS. Most expensive item is the filter capacitor. Bert Kehren In a message dated 10/19/2014 6:15:06 P.M. Eastern Daylight Time, albertson.ch...@gmail.com writes:
At the low end of the spectrum, I tried to make the simplest possible GPSDO what would still work. Assuming you have a GPS with 1PPS output, an OCXO and a small DC power supply I was able to get the entire parts for the controller, board, hookup wire and all for under $5. I purposely took the lowest cost solution at each decision point just to see what you'd end up with. Part were from eBay. The result is not bad. but I don't have a really good way to test it. I'm using a Thunderbolt for the 1PPS and a pretty decent OXCO part. Why build a low-end GPSDO when yo have a Thunderbolt? It's and experiment. The way I test is to place the sine output from the TB and from my GPSDO both on a dual channel scope and adjust it so the two sine waves are superimposed. Then I wait for them not to be superimposed. What I see is that over 1/2 hour or so they get slightly out of phase but then drift back in phase, This happens cyclically. It is because of the VERY simply controller. I tried to minimize lines of C++ code. It's running about 16 lines of code, more or less. Using my counter I think the GPSDO is good to 1E-10. Rather than using a $15 ARM MCU board I used a $3 AVR board and used 100% 16-bit integer math in a very simple control loop. There is one external chip because the little AVR could not deal with the 10MHz signal from the OCXO so I used a divider chip. I use two 8-bit DACs to control the EFC on the OCXO. One is curse adjustment, one fine. Added with a resister network and an RC filter with almost a 1 second time constant. If you can spend $35 you can build a very sophisticated controller that logs internal diagnostic data to a computer over USB and displays it's internal status on a graphic LCD panel. Well, actually my controller has an LCD status display and logs data to a PC. But with those parts plugged in the cost is closer to $10. On Sun, Oct 19, 2014 at 2:13 PM, Bob Camp <kb...@n1k.org> wrote: > Hi > >> On Oct 19, 2014, at 5:00 PM, Jim Lux <jim...@earthlink.net> wrote: >> >> On 10/19/14, 1:08 PM, Bob Camp wrote: >>> Hi >>> >>>> On Oct 19, 2014, at 3:35 PM, Charles Steinmetz >>>> <csteinm...@yandex.com> 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 -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. -- Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.