Stanley the faster counter also has the jitter, no change, as long as it is not tied to the input frequency. The 24 MHz is not unique, the 100 MHz is same technology just four times faster and thus gives me smaller steps on the D/A and since I use it on Rub. the full range of the 18 bit covers the full tuning range of the Rub. Bert In a message dated 6/27/2010 9:05:12 A.M. Eastern Daylight Time, [email protected] writes:
I have been thinking about a faster counter also but the Shera board was depending on the jitter in the 24 Mhz clock to average out the +- count. The faster clock would reduce the need for this but without the right amount of jitter we lose the benefit of this average. Stanley ----- Original Message ---- From: "[email protected]" <[email protected]> To: [email protected] Sent: Sat, June 26, 2010 8:14:02 PM Subject: Re: [time-nuts] yet another GPSDO design, or so Attilia What you want is basically a Shera Board. That design has been around for quite some time and has served me very well. I have a total of six running including two controlling Rubidium. There are in my opinion a couple of problems: not every 4066 works on the design the 18 bit D/A is very hard to find and now expensive and the single step of the D/A is intended for a 1.7 E-13 frequency step. I have build a input section that counts 100 MHz in stead of 24 MHZ making the unit create steps of 4.3 E-14 which works better on my Rubidium's and Datum FTS 1000. Also it eliminates the 4066's. Since I do not know how to write code that was my solution. I have also designed a later version Shera, with less IC's and a low cost dual D/A but I do not have the programming skill. If you contact me directly I will send you a copy of the QST Shera article, my design and the D/A data sheet.. I am sure you can replace the PIC with an Atmel device. In a message dated 6/26/2010 1:16:09 P.M. Eastern Daylight Time, [email protected] writes: Moin, I recently had a look at the data sheet of the LEA6-T GPS module from ublox, which now features a second time pulse output that is capable of delivering a 10MHz signal, synchronized to GPS. After thinking quite some time quite some time about building my own GPSDO and struggling with the question how to synchronize a 10MHz signal to a 1Hz signal that has some substantial phase noise, the new LEA6-T module seems like to make things a lot easier. Although the LEA6 specs do not say anything about how the timepulse output is generated or how it is synchronized to GPS, i assume that it will either have some jumps or phase/frequency noise due to oszillator and synchronization imperfections. But, it should be possible to use the LEA6-T together with some OCXO and a PLL setup to stabilize the OCXO to get a high quality frequency standard. Unfortunately, my knowledge in that field is rather limited, thus before starting to make wrong design decisions i'd like to ask for some advice here. My basic idea is to feed the 10MHz output of the LEA6-T and the 10MHz OCXO into a current output PFD, do some low-order filtering of the output signal. Feed that into an ADC which is read by a uC which in turn controls an DAC that sets over some amplifier stage the EFC input of the OCXO. As PFD i thought about using a ADF4002 from Analog, which is actually an PLL, but allows to bypass the input dividers, so that it can be used as pure current output PFD. I'm not yet sure what kind of output filter i want to use. I probably have to add at least one low noise opamp there, to isolate the PFD output/filter from the ADC. I'm also not sure what filter frequency i should use here. It will have to be below 10MHz for sure, probably in the lower kHz range, but how low is the question. The lower the easier gets the ADC stage and the less work has to be done in the uC, but using a low frequency filter either means using an active filter (noise) or high value R or L (again noise, especially the L might couple in 50Hz noise from the enviroment or show microphone effects). The ADC will be either a low-noise 16bit type or a 24bit type. This will largely depend on the sample rate to be used and the availabilty of the ADCs. Any good advices on what to use here? Should there be some form of signal conditioning done? If, what form of conditioning would you advise me to use? As a uC i thought about using a AT91SAM7 variant from Atmel. I know these beasts (and their bugs) pretty well by now and already have some code ready for those. I thought about clocking the uC with a 40MHz crystal that is synchronized to the 10MHz OCXO using a PLL. This would allow me to generate quite precise+accurate digital signals. Unfortunately, there doesnt seem to be VCXOs at 40MHz available so that means that i'd have to build one by hand. The loopfilter is going to end up in the uC as it is easier to build such low frequency filters digitally than in analog. I havent put much thought into how that filter should look like, as this can be easily changed later. The DAC will probably be a 16bit type (there does not seem any higher resolution DAC with sane specs and still reasonable availability). The amplifier for the DAC output will be a two stage amplifier. One stage that adds an (adjustable) offset and one stage that adds the (again adjustable) amplification. This approach is choosen as the needed EFC range will probably much lower than the full range. Hence the resolution of the DAC can be enhanced by producing only values within that range. The disadvantage here is that it requires calibration. A rough guestimate is that the whole thing will probably cost less than 500CHF (including PCB production, but excluding OCXO). Yes, i know, i could get a Rb frequency standard for that money on ebay. But where is the fun in that? ;-) Beside whether this setup makes sense, the two biggest questions i have are, what OCXO to use. Are the ISOTEMP 134-10 that are available on ebay "good enough" for such an application? Or shall i look for something better/different? And the other is, how do i amplify and distribute the 10MHz signal i get out of the OCXO to be used by other devices with minimal phase noise? Thanks for your help Attila Kinali -- If you want to walk fast, walk alone. If you want to walk far, walk together. -- African proverb _______________________________________________ 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. _______________________________________________ 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.
