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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