[time-nuts] PN sequence generation using GPS

Bob Camp lists at rtty.us
Wed Feb 16 21:51:56 UTC 2011


Hi

According to the rules, you are not allowed to reset shift register other
than by feedback during a transmission. In order for a reset to be
"transparent" it would have to always happen at the same time. Usually
that's when the register is full or empty. 

A proper (MLS) PN sequence will be (2^m) -1 long. For the allowed ham
sequences that would be 2^7 -1 = 127, 2^13 -1 = 8191, or 2^19 -1 = 524287.
If they are not MLS then I'm off a bit. None of those divide into 10 MHz
very well at all, since they all are prime numbers.

If you use the shortest sequence, you would get to reset every 127 seconds
with a simple approach. You likely would do better to run a bit more math
and figure out the "right" sequence for the time you are at. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Joe Leikhim
Sent: Wednesday, February 16, 2011 4:14 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] PN sequence generation using GPS

For clarification;
I am investigating an experiment using GPS to create a FHSS or DSSS 
project similar to those of AMRAD and described in the ARRL Spread 
Spectrum Sourcebook. In those experiments, a specific shift register 
sequence was used (see below), the clock was free running and a reset 
tone was transmitted on irregular intervals (whenever synch was believed 
to be lost) over the radio circuit.

In my experiment I would like to derive the clock directly from a GPS at 
each radio and use the GPS to periodically reset the shift register 
without causing a glitch or disrupting the sequence. There is mention in 
the sourcebook of a relationship between clock speed, reset interval and 
shift register stage length. It is this area that I am confused.

My question is: If I use a 1PPS derived reset interval, a 10 MHz clock 
and the PN sequence below, will the reset interval intrude on the 
sequence? If so, what reset interval or fraction of clock speed will be 
least disruptive?

This is the example shift register PN sequence:

"There is some common notation for PN sequence identification. The 
sequences are often generated by a shift-register using feedback. The PN 
identification notation indicates which bits are modulo-2 added and fed 
back to the input of the shift-register.

As an example the [7,1] sequence is generated by modulo-2 adding 
register bits 1 and 7, inverting this and applying this to the input of 
the shift register."

Thanks
-- 
Joe Leikhim

Leikhim and Associates
Communications Consultants
Oviedo, Florida

www.Leikhim.com

JLeikhim at Leikhim.com

407-982-0446
WWW.LEIKHIM.COM


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