On Fri, Mar 7, 2014 at 10:28 AM, Lars Walenius <lars.walen...@hotmail.com> wrote: > Chris, about using one Arduino for two GPSDO controllers: > > Even if a microcontroller has lots of capacity I would recommend to use > separate controllers for each oscillator. One of the reasons is what Tom van > Baak said about using only one interrupt to avoid jitter and even if you > trigger both channels from the same PPS and have just one interrupt you will > have a problem that you can´t read two ADC´s at the same time.
You don't have to read both at the same time. All you need is to have a constant time between the interrupt and when you read the ADC. That constant can be any reasonable number so long as it remains constant > > Even the HC390 I wouldn´t use for two different oscillators to prevent > crosstalk. Both the processor and HC390 is so cheap it isn´t worth the risk > IMO. Risk? It's easy to measure. Risk is when you don't know what is going to happen. But in this case we can test. > > Actually I would also recommend to put them in separate boxes even if it is > more work (and I´m lazy ) to get best performance. I think you might be addressing pico seconds on a system that works in the few nano seconds range. A serial commanded Rb oscillator moves in such large steps that I'm 100% sure the step quantization error will dominate everything. The step size is something like 5E-11. But the stability I expect will be very good. > > Having two GPSDO´s that you can compare is very nice as long as you > understand how they correlate , if that is not what you want to test. Of > course you can also set one or both in hold mode to test them freerunning. > > > I have thought of connecting the M12 to the Arduino and if someone can help > with code to get the sawtooth correction value into the Arduino and decoded I > would be glad to have it. I'm looking for an OCXO. Not much reason to start before I find one. People are over bidding on eBay for 30 year old salvage parts. eventually I'll win one at a reasonable price. Then I'll write up my results. In the mean time I've started a wholesale refactoring of the posted Arduino code. I need t make it a bit more modular and "testable." I have an Motorola Oncore UT+ type GPS. I think it might have the same sawtooth. I'm pretty sure there is code in the standard NTP distribution to read the Oncore type data and (maybe sawtooth???) I plan to read the NTP drivers and borrow whatever is usable. I did just build and finish testing a serial interfaced LCD display. Now I can display states using just two Arduino pins. (Without the serial interface an LCD takes 6 to 10 pins) I'm using I2C so I can add other devices to the same serial interface, like a DAC or whatever > > > > Lars > > > > > > From: Chris Albertson > > > > On Wed, Mar 5, 2014 at 5:43 PM, Didier Juges <shali...@gmail.com> wrote: >> Tom and Bob, >> It is not obvious to me that it is "easier" to simply apply a correction in >> nS increments with a range as wide as 100nS. How is this done? Using >> switched delay lines or delay gates? > Here is my plan for processing saw tooth data. If it's not going to > work I'd rather hear about it now then a month from now after I've put > in some effort. > This is going into Lars' Arduino based GPSDO. Every second I read the > voltage on a TIC capacitor. This tells by the phase in nanoseconds > between the PPS and the OCXO. Then I add whatever the current GPS > sawtooth value is to whatever my TIC said. I compare this to a set > point. This is the phase error. The OCXO is adjusted based on a > filtered version of this error. > So in short, I don't correct even try to delay the pulse. I don't see > any need to do that. I measure the pulse and get a number in > nanoseconds. then I use sawtooth to correct the number. > It seems way-hard and with no purpose to correct the pulse and then > measure it. Better to correct the measurement. I think it is more > accurate too a delay could never be perfect. > The controller has LOT of spare capacity so I don't see way I can't > add one of more TIC channels and a few more DACs I should be able to > discipline an OCXO and my Rb oscillator from the same GPS PPS input. > The 74HC360 is only 1/2 used an Arduino has enough spare pins. Any > one more 74HC4046 and some passive parts would be required to build a > dual channel GPSDO. > It will be interesting to look at andompare the 10MHz outputs of two > oscillators that are being disciplined by the same controller and GPS > receiver. > > -- > > 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. -- 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.
