Luke I would believe that any of the GPS receivers would be great for what you are doing. (Most likely just opened a bag of worms.) Indeed they have jitter and all the things mentioned. But if the discussion is about a mechanical clock then its accuracy is many decades lower than any of this discussion. Then you have the temperature coefficients s of the clock mechanism that limits your overall accuracy. In you post you are measuring nano-seconds. Seems far far to fine grain as I picture a pendulum clock in my mind. I would speculate 1/100-1000 of a second is reasonable. Because the pendulum has mass and only varies over significant time the funnies with gps should generally go away.
By the way purchase a RB reference on ebay. Fire it up and set it in the bottom of the clock. Now you have a RB clock. ;-) Anyhow now I am in trouble. Regards Paul WB8TSL On Sun, Dec 1, 2013 at 9:46 AM, Mike Garvey <[email protected]> wrote: > Luke, > Seems that you might indeed become a time nut, though most of those on the > list are focused on atomic clock topics and the time precision that atomic > clocks provide. > That said, sounds like you have a good start. While I have not used the > GPS > module you mention, GPS timing sources typically show timing accuracies of > 20 to 100 ns. The short term noise arises from the signal-to-noise of the > "as received" signals, so the 99% accuracy of 60 ns likely means that you > can assume that all of the pulses are within +/-100 ns. The time noise is > "white" for GPS signals. Note also that this is time accuracy, not > frequency accuracy. Frequency accuracy is maintained by GPS Ground Control > Segment with reference to the US Naval Observatory Time Scale. > At the risk of unraveling your existing measurement architecture, you might > consider measuring the clock phase (pendulum passing) with respect to the > GPS pulse. This would be a time interval measurement with an accurate > (GPS) > reference pulse and a time interval based upon your TCXO. This reduces the > burden of calibrating the TCXO and will eliminate concerns about the > frequency accuracy and stability of the TCXO. For example, if your TCXO is > off frequency by 1 ppm, you would get a non-cumulative error of 1 ppm in > the > time interval measurement rather than a 1 ppm error in the clock rate. The > 1 ppm TCXO error drops out of the solution for rate of the clock; only > changes in the TCXO frequency enter and they are typically much smaller. > > Hope this is useful; others on the list will likely have other input. > Mike > > -----Original Message----- > From: [email protected] [mailto:[email protected]] On > Behalf Of Luke Mester > Sent: Sunday, December 01, 2013 2:54 AM > To: [email protected] > Subject: [time-nuts] Using a UBlox NEO-6 GPS module for calibrating a PIC > microprocessor based timer. > > One of my hobbies is collecting and repairing mechanical clocks. I was > looking at buying one of the specialized electronic timers used to measure > the performance of mechanical clocks. I really couldn't justify the cost > just for hobby use. > > Since I have electronics and programming skills I decided to build my own > timer using a PIC chip. This became a much bigger project than I expected! > > I have my clock timer running and have most of the software features that I > need working. I then realized that I need some way to calibrate it and > verify it's accuracy. > > I didn't have any source of accurate time available. After searching the > internet and finding this mailing list I decided to try a GPS module. I > bought a $20 module from DX.com. It has a built in antenna, voltage > regulator, serial interface and most important, a 1 PPS output.The GPS is a > UBlox NEO-6M. After reading the specs on this module I see that they claim > a > 99% accuracy of <60ns for the time pulse signal. What does this mean? > What about the other 1%? How much variation can the time pulse have? If > it's > really 60ns it's much better than I need. > > I'm hoping that some of the time experts on this mailing list can give me > some idea what to expect from this GPS module. Also, if there are any > settings that I should change to get better timing performance. There are a > huge number of settings available when I run it's configuration program. I > have no idea what most of them do. > > I'm using one of the hardware timers on the PIC chip to measure the time > interval. The PIC is running with a 100ns (10MHz) instruction cycle. The > timer will provide 100ns resolution. I'm getting occasional variations of > about a microsecond. Because I'm using interrupt driven code to capture the > timer value there will be some unavoidable jitter in the timing. I was > expecting about 4 or 5 cycles (400ns - 500ns) but I'm getting more than > twice that. Is it safe to assume that these are due to problems in my > hardware or software? Is this from variations in the GPS PPS output? Maybe > I'm just not interpreting the data correctly. > > Below are links to some data plots: > > Four plots are shown. The first two are the Rate and Beat error that my > timer reports while monitoring the GPS PPS signal. Rate is normally the > average of two beats ( two time interval samples). If a clock is not in > beat > (the tick and tock take different amounts of time) the displayed rate would > jump back and forth. Averaging two beats eliminates this jump. I have > disabled this average in my code so that the rate is now showing each beat > and not the average of two. I turned it off because I expect that this > averaging could hide possible problems with my timer. Beat error is the > difference between the two beats. This shows the rate change for each pair > of beats. This is needed so that you can get the clock pendulum or balance > wheel adjusted properly. > > Raw Data <http://mesterhome.com/clock/data/RawData.png> > > > Average Data <http://mesterhome.com/clock/data/AveData.png> > > Average data has been filtered with a 100 sample running average. The plot > looks really good. The average is just hiding the instability. > > > I also noticed variations that appeared to be due to temperature changes. I > borrowed a temperature data logger from work and did some testing. The > temperature and rate graphs track perfectly. I can see my furnace cycling > and my programmable thermostat moving the temperature setting up and down! > That got me interested in trying a TCXO instead of the standard crystal > that > I was using. A $3.00 TCXO from EBay made a huge difference! Both of these > plots have the running average applied. You can't see the temperature > changes with the raw data. > > Crystal <http://mesterhome.com/clock/data/RTvTP.png> > > > TCXO <http://mesterhome.com/clock/data/RTvTPTC.png> > > In case anyone is interested, here is a link to a data file captured from > the clock timer. It's in CSV format. The first column labeled "Rate" is the > time for each beat of the clock. "Rate Avg" is a running average rate and > "Beat E" the beat error. > > Data file <http://mesterhome.com/clock/data/tcxo.csv> > > Finally, I think I might be turning into a time nut! For the clocks that I > work with this timer is already far better than I need. Millisecond > accuracy > Is good enough to test most mechanical clocks. Microsecond is great! I know > that a microprocessor based timer is capable of better performance. I then > had the problem of what I could use to measure the performance of my timer. > I Needed a better clock than my timer. Now I'm wondering if this cheap > Chinese GPS is good enough. I'm having fun tweaking the hardware and > software to see just how good I can get it to perform! > _______________________________________________ > 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. > _______________________________________________ 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.
