[time-nuts] is there a "best bet" advanced hobbyist buildable GPSDOdesign?

[Bruce Griffiths]

Scott
Scott Burris wrote:
> Bruce Griffiths wrote:
>   
>> As far as I know Dallas/Maxim appears to be the only source of suitable
>> affordable programmable delay chips for this particular application.
>> In principle one could use a tapped chain of gates in a CPLD, however
>> continuous calibration of the delay is required (a delay locked loop
>> controlling the gate propagation delay by adjusting its power supply
>> voltage to compensate for the effect of temperature variations is one
>> technique). However unless the Dallas chips become hard to obtain its
>> probably best to leave this as a backup option.
>>   
>>     
> What about sending the 1PPS signal through a number of HC family gates 
> and using a mux
> to select a tap -- is that better than using a CPLD? Hmm, probably the 
> delay varies too much from manufacturer to manufacturer
> to make this work reliably.  Probably temperature sensitive too.
>
>   
All of the above, however the major problems are that the individual
gate delay is too long and the designing a suitable multiplexer isnt easy.
I dont think that cascading 30 or more 1ns delay gates all in different
packages is going to work that well.
CPLDs have the advantage that to a first approximation all the gate
delays are identical.
However you have to force the configuration to interconnect them
appropriately software so as not to spoil the performance.
> What kind of delay characteristics are needed?  I see some other delay 
> lines with 100ps
> steps available, see:
>
>   
With an M12+T you need a minimum variable delay range of about 20ns or
so with a step size, accuracy and stability of better than 1ns
(resolution of sawtooth correction message) to avoid degrading performance.
Tom Clark suggested that the DS1020-15 with 150ps resolution is OK for
this receiver, however it needs to be calibrated over the delay range used.
Other receivers (particularly older ones) will need larger delay ranges.
The required delay range is around 0.5 to 1x  the receiver timing clock
period for the Motorola receivers.
Autocalibration techniques can be used to track the effects of
temperature and aging.
> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=408-1127-ND
>
> and put that through a mux?
>   
In principle this would work, the devil lies in the detail of the mux
design.
The mux delay needs to be independent of the selected tap.
The delay range provided by one of these devices is too small.
As a fallback option I have produced a draft schematic of a ramp style
delay generator with a resolution of 100ps and a range of 400ns or so.
The extended range allows calibration against a 10MHz OCXO derived clock
and should allow it to be used with most of the older Motorola Timing
receivers that provide sawtooth correction data.
Such calibrations can be interleaved between successive PPS pulses.
A 12 bit resolution DAC with a settling time (to 0.01%) of around
100millisec or so is required for this DAC.
It may be feasible to use a PWM DAC for this.
> Looks like some of the DS1020's are available through Maxim's e-commerce 
> site, but some have
> a 15 week lead time, depending on the particular model.
>   
>> What processor are you intending to use to decipher the sawtooth
>> correction messages from the GPS timing receiver?
>> You could use an inexpensive microprocessor dedicated to this simple task.
>> Another microprocessor can be used to discipline the OCXO.
>> Depending on your experience, this can be easier than using a single
>> microprocessor to do everything.
>>
>>   
>>     
> I have experience with PICs, Atmel AVRs, and various ARM flavors, and 
> limited experience
> with SOC processors in Xilinx FPGAs (Picoblaze and the like).  PICs and 
> AVRs are cheap, so no problem
> dedicating one to this task.
>
> Anyone have a list of GPS units which provide sawtooth correction data?  
> I have a few flavors of
> Motorola products and a Trimble Lassen IQ laying around.  They all 
> provide 1PPS signals, but I bet
> some don't have the necessary features.
>
>   
M12MT, M12+T, Trimble  Resolution-T all provide sawtooth correction data.
> Scott
>   



Bruce