[time-nuts] PN sequence generation using GPS

[jimlux]

On 2/17/11 7:22 AM, Joe Leikhim wrote:
> I am trying to stay within the FCC Part 97 rules. The spreading or
> hopping will be of a narrowband (25 KHz BW) FM signal. I haven't decided
> on either the FHSS or DSSS approach. I had thought of a FH approach that
> exploited time of day to address a frequency look up table, but I think
> that is outside Part 97, although arguably it is a "net scheduler."
>
> 1) If I go with a FH approach running at around 10-20 hops/second will
> the 1 pps be sufficient?

yes...There have been a fair number of FM frequency hoppers over the 
years with 10-100 Hz hop rates.  At 10 Hz, you can be pretty sloppy in 
your hop timing (if you were off by 1 millisecond, it just looks like a 
1ms noise burst at 10Hz... sounds like ignition noise)

A common architecture was a Z80 or 6502 driving the PLL divisor bits in 
the MC145xxx PLL in an off the shelf FM land mobile or CB type radio.


>
> 2) Part 97 says the shift register cannot be reset by anything other
> than by itself during a transmission. Clearly many of the
> synchronization methods discussed in the ARRL SS Sourcebook utilize a
> synch method transmitted across the link or derived from a TV broadcast
> signal etc. I don't see the reason this rule exists?


Probably to allow a monitoring organization to not have to work too hard.

>
> 3) If I block the 1 PPS during PTT, and the receiving end asserts 1 PPS
> reset, will synch be lost? If not how effective will the freewheeling be
> during a 30 second exchange with 10 MHz GPS derived clock reduced to 20
> hops/second?

Practical experience says that you can use no external reference, and 
use the oscillator that you're using for the FM carrier.  Slow hopping 
doesn't require fantastically good timing.  You can actually use the 
squelch to resync on each hop.

When you hit the PTT, you either can start at a known frequency and 
restart the sequence (the receiver sits waiting at that home frequency) 
OR you can run the clock forward as a best guess, and then resync on 
each hop.




>
> 4) 20 hops per second is a 50 ms chip. Two radios 50 miles apart would
> be 270 us delayed. I don't think that should impair the analog FM. So
> don't envision needing any correlation adjustment. Comments?

Many, many slow hoppers of the 1980s worked exactly like that. Some used 
digitized voice (CVSD was popular) some were just analog FM.


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