Brooke Clarke wrote: > Hi Ulrich: > > I think the answer is what other low cost options are available? I > would like to have a more modern TIC capability to add to the clock I'm > working on. But although there's been a lot of discussion about > different ways of making TIC measurements, it's not clear to me how to > do it on a budget. > > For example the TIC232 circuit by Richard H McCorkle is easy to > implement, but how good is it's noise floor. See: > http://www.piclist.com/techref/member/RHM-SSS-SC4/TIC232.htm > > Then there's the low cost frequency counting TIC that appeared in QEX > that we know trades performance for low cost so it's not a candidate. > > Any ideas on what circuits have a noise floor that's compatible with the > M12+T or it's newer equivalents and at the same time are in the low cost > category? > > Have Fun, > > Brooke Clarke > > w/Java http://www.PRC68.com > w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml > http://www.precisionclock.com > > Ulrich Bangert wrote: > > >> <>Tom, >> ..... >> >> What I wanted to explain is the Shera concept noise floor is a factor 20 >> above what a modern receiver can deliver (again inc. the sawtoth >> correction). And yes, you are right: There were different numbers when >> this concept was thought out! And exactly because different number were >> there when this concept was thougt out I am going to ask why people >> still built it today. >> > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > Brooke
I agree that most will tend to use an available circuit particularly if they are not too experienced/adventurous. The noise/resolution of the TIC232 will be a little worse than that of the Brooks Shera circuit. It would appear to use the internal counter timer which is clocked at 16MHz. Also this timer has no hardware for latching its count on the leading edge of an external signal so there must be some software component used to do this. This will almost inevitably add extra noise/uncertainty due to variations in the delay in reading the timer. The quoted resolution of 1.04ns for a 1 minute average is probably derived from a 62.5ns resolution for each individual measurement. One can only achieve the subnanosecond resolution required to avoid degrading the performance of an M12+ by using a clock frequency of 1GHz or more. Thus this method is probably too expensive and difficult to implement. Perhaps there would be some demand for a higher resolution replacement for the Brooks Shera system for those who have M12+ or equivalent performance timing receivers and high performance OCXOs or Rubidium standards who wish to achieve the best performance they can without breaking the bank. If so then perhaps we can collectively design such a system. Breaking the task down into more manageable parts will help ensure that the design is more quickly implemented As I see it the following methods can achieve the desired phase measurement resolution 1) Use a commercial TDC chip as the phase detector. Range 4millisec ( can be extended almost indefinitely by using a synchroniser and counter implemented in a gate array or its functional equivalent) Noise 65ps rms Cost ~ 100 euro Advantages someone has already designed and debugged the chip as long as the circuit layout recommendations are adhered to there should be no unforeseen problems. 2) Use an ADC to sample a sinewave formed by dividing down the OCXO frequency and filtering the resultant square wave Range half period of the sinewave frequency Noise (rms) < 0.0005 of the sinwave period (500ps with a 1MHz sinewave) Cost ~ $US20 ?? 3) Use dual simultaneous sampling ADCs to sample quadrature phased sinewaves derived by dividing down the OCXO frequency filtering the resulting square wave and using a quadrature hybrid to produce the quadrature phase sinwave pair. Extend range to as much as 1 second or more using dual synchroniser to sample a continuously running digital counter/timer. Range to several days or centuries if required, depending on counter lenght Noise (rms) < 0.0005 of the sinewave period (500ps with a 1MHz sinewave) Cost ~ $US40 ?? Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
