[time-nuts] PN sequence generation using GPS

[shalimr9 at gmail.com]

Or you try all the possible solutions all at once in parallel in a big FPGA and you have instant synch (at least in the time it takes to recognize you have it)

May be impractical for very long, complex sequences...

Didier KO4BB
 
Sent from my Verizon Wireless BlackBerry

-----Original Message-----
From: jimlux <jimlux at earthlink.net>
Sender: time-nuts-bounces at febo.com
Date: Wed, 16 Feb 2011 21:26:01 
To: <time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: Re: [time-nuts] PN sequence generation using GPS

On 2/16/11 5:04 PM, Joe Leikhim wrote:
> Thanks Bob;
>
> Does this mean that the 10 MHz clock needs to be somehow divided to an
> integer evenly divisible by 127 seconds?


>
> Also 8192 seems to be unfeasible as it would take 2.2 hours to
> initialize sync.
>
No.. your 1pps/sync loads the register with all ones... that's what the 
sync is.

If you want to synchronize without "help", what you do is slide the 
receiver steadily by the transmitter, slipping one chip at a time (or 
offsetting the frequency slightly).

The challenge is that you need to be "close enough" as you slide by for 
"long enough" to recognize that you've got sync.

Say you've got a 1023 long code (a 10 bit generator) running at 1 MHz. 
Say it takes 1 millisecond to tell if you are in sync. You can try all 
1023 possible relative phases in 1 second, roughly, and the average time 
will be a half second.

You can do this "try all phases" by running one clock 1ppm slow, 
relative to the other, or you can run them at the same speed, and drop a 
pulse every code period.  Depends on if you have a digital or analog 
implementation.. For a digital implementation, it's often convenient to 
detect when the generator register is all ones, which occurs only once a 
code period.


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