[time-nuts] ngTADD-1

Lester Veenstra lester at veenstras.com
Thu Apr 25 10:56:55 EDT 2013


Use a maximal length PRN sequence with a unique state (say all ones) tied to
1 pps)
Receive by correlation to recover time of unique state. 


Lester B Veenstra  MØYCM K1YCM W8YCM
lester at veenstras.com

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-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Hal Murray
Sent: Thursday, April 25, 2013 12:22 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] ngTADD-1


kevin at rosenberg.net said:
> For < 1ns jitter, do you have a recommendation of  modulation 
> (ASK/PSK?) and carrier frequency?

All the digital communication over fiber that I'm familiar with is simple
on-off baseband, no carrier.  Digital geeks use encoding rather than
modulation.

One complication with fibers is that the receiver has to work with a huge
dynamic range, so they all have an AGC.  I don't know what the time constant
on the AGC is.  The other complication is that the receiving end has to do
clock recovery.  That requires transitions so there is a limit on the max
number of consecutive 0s or 1s.  Modern protocols use 8B/10B or 64B/66B to
guarantee enough transitions.  (Wiki pages are good.)

The other goal of things like 8B/10B is an equal number of 0s and 1s so you
AC couple the signal path without introducing pattern sensitivity.  (The
4B/5B encoding used for FDDI had a couple of nasty cases.)

The AGC makes it hard to send a simple low duty cycle PPS pulse over fiber.

For something like 10 MHz, I'd send alternating 1s and 0s, at either 10
megabits or 20 megabits.  Somebody has to multiply by two.  I'd do it on the
easier end.

Suppose you want to send a PPS signal.  It's easy if you have the
corresponding 10 MHz clock.  I'd use Manchester encoding.  That is send
alternating 01 at 20 megabits for the non pulse time and 10 when the PPS
pulse is high.

-------------

Several years ago, there was discussion (or at least mention) of the radio
astronomy guys down in Chile doing clock synchronization over fibers.  They
had to correct for temperature and such.





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