[time-nuts] Thunderbolt? (re simple gpsdo.)

Don Latham djl at montana.com
Fri Dec 30 20:03:50 UTC 2011


May I suggest use of small controllers such as the Picaxe series? I find
them very useful around the "lab" for small tasks such as this. Easily
programmed, very reliable, and the software application can be as simple
or complex as needed, for example the suggested PID controller. No
soldering, no fuss, just change the program and get on with it. 8 bit
precision? not necessarily, capacitors are continuous... I use 1$ (yes!)
1st generation NES controllers (essentially 8 PB switches) for input,
usually, and there are little I/O assemblies available from such vendors
a sSparkfun. If you need them, 16-bit a/d and d/a's are there. There's
also a forum with, by now, lots of examples.
If things get more complex, the Arduino or the Propeller are available,
and, in the near future, a $35 Linux unit called Raspberry Pi.

I know, these things do not defy the laws of either physics or Murphy.
But for just messing around, they've replaced a whole cabinet of
flipflops,gates, amplifiers, and the like for me.
Happy New Year to all!
Don

Azelio Boriani
> Yes, it can be done based on a PPS only timing. You must design a PI
> (maybe
> PID) regulator: the EFC must stay steady when the phase difference
> between
> the two PPSes is zero (integral action). Then you must move the EFC
> (when
> there is a difference) proportionally with the difference itself and
> only a
> small part of the difference drives the integrator to
> increment/decrement
> the new steady EFC level. When the difference is again zero the
> proportinal
> part is gone and remains the small amount of correction for the integral
> part. There is a gain for the integral action (samll) and a gain for the
> porportional action (should be large, but to be evaluated).
>
> On Fri, Dec 30, 2011 at 6:26 PM, Chris Albertson
> <albertson.chris at gmail.com>wrote:
>
>> On Thu, Dec 29, 2011 at 8:24 PM, Mark Spencer <mspencer12345 at yahoo.ca>
>> wrote:
>> > One of the designs using the 10 kHz output from a Jupiter gps engine
>> and
>> a simple PLL to discipline an ocxo might be good starting point if
>> suitable
>> gps engines are still available.  There won't be much to tweak but the
>> performance could be quite good.
>>
>> I just finished reading about that one.  It requires no longer
>> available GPS reciever.   Maybe I should r-phrase the question:
>>
>> "What is the simplest design for a GPSDO that uses only the PPS signal
>> from a modern GPS?"
>>
>> Half of the reason for the question is academic.  Then if a simple
>> enough design presented itself it would be fun to try it.
>>
>> The simplest design I can think of now is based on a flip-flop.  The
>> PPS "sets' the FF and the next raising edge of the local oscillator
>> "resets" it.   (The local oscillator might need to be divided down or
>> a slower 1Mhz oscillator used so the FF remains "on" for a reasonable
>> time.)
>>
>> Next the FF gates a current source to a capacitor.   The voltage in
>> the cap is amplified and controls the local oscillator frequency via a
>> low pas filter.  Likely the "low pass filter" would be an active
>> device that we call an "integrator"
>>
>> You need to discharge the cap for the next cycle.  One could rig a
>> one-shoot timer to discharge the cap.  Actually there are about four
>> states that need to be cycled every second.
>>
>> Chris Albertson
>> Redondo Beach, California
>>
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-- 
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
R. Bacon
"If you don't know what it is, don't poke it."
Ghost in the Shell


Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com





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