[time-nuts] statistical distribution of initial frequency error in tcxos

[wb6bnq]

Hi Jim,

If you have a mother-ship and presuming that it is going to be within 
"RF" range of all little ones, how about sending a "PILOT" tone that 
comes from the mother-ship to phase lock all the little ones internal 
oscillators ???

If there is a master oscillator of any note (i.e., say a Rb unit) in the 
mother-ship, then a reasonably decent XO circuit may suffice or a TCXO.  
As for ranging with the signal using phase locking, well I would have to 
think about that for a bit.

This process was used back in the 1970's to lock the master frequency 
source of a few repeaters together so that they could simultaneously 
transmit on the same frequency with little to no interference.  In this 
case, if I recall correctly, a 1000 Hz pilot tone was used as the sync 
signal.

In your case you could, perhaps, transmit on some frequency that is out 
of the way and use whatever pilot tone you wish.  Depending upon the 
transmitter bandwidth maybe even transmit a 10 MHz sideband and do 
direct comparison to the local (TC)XO.  Many possibilities !!!

Bill....WB6BNQ


jimlux wrote:

> On 3/13/19 9:17 AM, Attila Kinali wrote:
>
>> Hoi Jim,
>>
>> On Wed, 13 Mar 2019 08:39:24 -0700
>> jimlux <jimlux at earthlink.net> wrote:
>>
>>> For instance, if I'm buying 10MHz oscillators with a spec of 5ppm,
>>> they'll all fall in a band +/- 50 Hz.  But how many are within 1 Hz?
>>> within 0.1Hz?
>>
>>
>> I have never measured TCXOs, but I've seen data of other devices
>> that are adjusted at the manufacturer. What you end up with is
>> something between a uniform distribution and truncated Gauss.
>> It is also very likely that most are withing 1ppm or even less,
>> depending on how much margin the manufacturer put into the spec.
>> Aging and environment will spread the values in a Gauss like fashion
>> (think of a (anomalous) diffusion process).
>>
>> Would it be rude to ask for more details on what you are planing to do?
>> It sounds interesting.
>>
>
> I'm looking at various radio issues with large swarms of tiny 
> spacecraft - specifically for making a radio telescope. Unlike 
> terrestrial interferometers where you have reference distribution 
> (local) or cesium clocks and masers (VLBI), we have to have some other 
> way to solve the problem.
>
> Fortunately, you can do a lot of post processing, so it's more a 
> matter of "knowledge" not "control" of the local frequency source on 
> the swarm nodes.
>
> One scheme to propagate references is to radiate your reference 
> oscillator so that all the other nodes can receive it, and each node 
> can then say "I'm node A, and relative to me, nodes B is 1% high, Node 
> C is 0.5% low, etc." and from that you can (maybe) solve for the 
> ensemble. One can also get some information about the relative 
> positions of nodes (by looking at the phase of the signal).
>
> (one does need at least one single "good" reference to tie them all 
> to.. in the case of most swarm schemes, there's a mother ship to relay 
> data back to Earth that can provide a broadcast reference, or if 
> you're close enough to Earth, you can radiate a high quality signal 
> from Earth).
>
> This whole scheme breaks horribly if too many signals are too close 
> together, and I suspect that this is the actual case.  So we need to 
> have a way to explicitly move the signals around.
>
> when you're building 1000 nodes, you would really like them to be
> a) all the same parts list and construction
> b) not require any customization
> c) and have the minimum number of parts on the parts list.
>
> (recognizing that I can probably do something like modulate the 
> reference broadcast with a single bit from some existing part and do 
> CDMA in some form - but it was a thought..)
>
> -- 
> It's funny - folks have spent a lot of time over the years making 
> oscillators that are "on frequency" - but with modern processing 
> techniques, a lot of times it's more about "good phase noise close in" 
> and "a way to measure and estimate the frequency" (the latter requires 
> decent ADEV if you're not making the measurement simultaneously) but 
> no real aging requirement.  (Which is good in space applications, 
> because radiation causes frequency shifts.)
>
> Take the NTIA 20Hz carrier frequency control requirement for SSB voice 
> radios in HF (to provide natural sounding voice without needing a 
> clarifier control). At 30MHz, that's <1ppm, and tough with a cheap 
> crystal. But if you have an integrated GPS receiver with 1pps, you 
> measure the current crystal frequency and adjust the DDS or PLL as 
> needed to compensate.
>
>
>
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