[time-nuts] yet another GPSDO design, or so

Ed Palmer ed_palmer at sasktel.net
Sat Jun 26 18:38:29 UTC 2010

Hello Attila,

Another GPS board with a 10 MHz output is the Navsync CW-12 module 
(price ~US$85-90).  I measured the 1 PPS output and found a Standard 
Deviation of < 5 ns with a range of < 30 ns.  The 10 MHz output is kept 
on frequency by occasionally adjusting the period of the 10 MHz output 
by one cycle of the CPU clock (~8.3 ns = 120 MHz).  How often this 
happens depends on the exact clock frequency of the particular unit.  On 
mine it happens about 200 times per second.

I don't know if this board is better or cheaper than the LEA-6T, but you 
might want to look into it.


Attila Kinali wrote:
> 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

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