[time-nuts] Phase modulation detection/NIST plan

J. Forster jfor at quikus.com
Sun Jul 8 23:29:10 UTC 2012


A risky assumption, and a cold start could be tricky.

Equatorial took many minutes to lock up, with a much higher data rate, and
it did it by slowly sweeping the local clock.

Aside: That's why military spread spectrum systems like good local clocks.
They lock up a whole lot faster that way.

-John

================



> Hi
>
> In this case the data format and it's contents are highly "computable". If
> you have a good local clock *and* an initial lock, the rest of what
> follows is predictable. That of course assumes we know the real format 
.
>
> Bob
>
> On Jul 8, 2012, at 6:58 PM, J. Forster wrote:
>
>> Hi Peter,
>>
>> That's be the hard way, but yes, if the message BPSK coded is computable
>> and of a known format. If the message contained more than time, like
>> solar
>> flux, it gets more complicated very rapidly.
>>
>> A similar thing was done with the Equatorial system 30+ years ago. In
>> that
>> case, each data bit was broken into something like 32 or 64 chips (I
>> don't
>> remember). There were two maximally distant, orthogonal chip patterns,
>> representing 1 and 0. The incoming BPSK message went through a 0 or 180
>> degree switch, then the IF stages. The switch was driven from a local
>> (known pattern) chip generator, so that if everything was synced up the
>> narrow band IF would put out the 0 or 1 that had been encoded. BTW, this
>> trick vastly improved the system S/N becaust it narrowed the receiver IF
>> bandwidth many times.
>>
>> If the chip pattern is not known (fixed) or computable (like a correct
>> TOD) things go to pot quickly.
>>
>> Rather than building such a kludge, it would be easier to use the locked
>> clock in a newly designed receiver and phase compare that to your local
>> standard directly.
>>
>> -John
>>
>> ==================
>>
>>
>>
>>
>>
>>
>>> Any possibility of using the decoded signal to un-do the modulation and
>>> feed the reconstituted signal to the older receiver?
>>>
>>>
>>>
>>> On 7/8/2012 12:56 PM, paul wrote:
>>>> Ei
>>>> Sorry if I have your name reversed. By taking this approach it
>>>> eliminates the ability to use wwvb as a frequency reference because it
>>>> destroys that traceability.
>>>> Thats what we are trying to preserve. Or at least re-establish for the
>>>> older phase measuring receivers.
>>>> Regards
>>>> Paul
>>>>
>>>> On 7/8/2012 12:10 PM, Tofurk Ei wrote:
>>>>> If the changeover you are talking about is this one:
>>>>> http://www.nist.gov/pml/newsletter/radio.cfm as a proof of concept a
>>>>> DVB-T
>>>>> dongle/upconverter combo could almost certainly handle PM easily to
>>>>> output
>>>>> whatever it encodes, when paired with gnuradio..
>>>>>
>>>>> The RTL2832U chip might also be able to handle some low band signals
>>>>> directly, using direct sampling. No upconverter.
>>>>>
>>>>> Regardless, then the data would be fed into gnuradio - the gnuradio
>>>>> developers GUI is called "gnuradio companion" It has a nifty way of
>>>>> doing
>>>>> this kind of thing, one builds a "flow graph" where the actual
>>>>> demodulation
>>>>> is simply laid out graphically and tested.
>>>>>
>>>>> When everything works to one's satisfaction the file is saved and it
>>>>> gets
>>>>> compiled - then it can run - its basically a python script.
>>>>>
>>>>> If the modulation scheme is public, I think you can be almost certain
>>>>> that
>>>>> gnuradio might be quite useful to rapidly design a tool to demodulate
>>>>> it.
>>>>> Perhaps very quickly.
>>>>>
>>>>> For the money, one really couldn't hope to beat the flexibility of
>>>>> this
>>>>> combination in any other manner. If I were interested in trying this
>>>>> I
>>>>> would join the gnuradio mailing list and ask there. Perhaps the
>>>>> answer is
>>>>> surprisingly simple.
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>>>>
>>>>
>>>>
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>>
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