[time-nuts] PN sequence generation using GPS

[Bob Camp]

Hi

Packet turn around and DPLL burst times are a couple of things you can look
at as well as scanning. Since it's a full RX to TX, there may be things
happening that aren't just PLL related. The radios I've dug into were mainly
limited by the PLL. 

You need to be lucky / careful to get a radio that's reasonably fast. There
are (or at least were) many sets out there that are in the 10 to 20 ms
range.

I have seen some that took over 100 ms, but not a lot of models. Lots of
things start to go wrong when they turn around that slowly. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of jimlux
Sent: Thursday, February 17, 2011 12:53 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] PN sequence generation using GPS

On 2/17/11 9:30 AM, Bob Camp wrote:
> Hi
>
> System design is always about compromise.
>
> If you hop slowly, you "stomp" on each channel pretty hard. You are likely
> to get noticed when you do. The idea is to stomp so rarely and for so
short
> a time that you aren't noticed.
>
> If you hop fast, you need to adjust the sequence to match. If you have a
max
> distance of 50 miles / 270 us and a 270 us chip rate things are still
pretty
> easy. You only need to be able to adjust over a +/- 1 chip range. More or
> less like a fine tuning knob.
>
> If you are trying this with a conventional radio, there are some other
> issues. The PLL lock times are way longer than the 270 us you are looking
at
> for distance. Based on using radios as scanners, anything dimensioned in
> hops/second is likely sound bad.
>


My practical experience has been that the typical ham 2m or 440 rig (or 
a land mobile rig, which is essentially the same) has a PLL lock time in 
the 1-few millisecond range, and a hop length of 100 ms works pretty 
well.  The noise blanker helps deal with the "popping"..

I set up an experiment 20 or so years ago using a pair of FT-757GXs in 
the 40m band, driving the computer control port with frequency change 
commands, and it worked fairly well.  Didn't radiate (transmit into a 
dummy load, receive from leakage... the radios were sitting on the same 
table)

Any VHF/UHF rig already flips the PLL frequency when going from tx to Rx 
to accomodate the IF and/or repeater offset, so you can look at the 
"Rx/Tx turnaround times" for packet radio to get a feel for it.

If you use digitized voice (say, CVSD at 16 kbps, which is easy and 
doesn't sound too bad and doesn't have licensing issues) you can buffer 
it up and transmit at a higher rate (e.g. if you have a "dead time" of 
10 ms out of every 100ms, you transmit at 1.1 times the bit rate you 
digitized at)

One caution...
Spread Spectrum communications is something that is desirable by various 
"bad people" (because it's moderately secure and also low probability of 
intercept), and so you might attract attention of people who worry about 
things like that.  You might have a car with small hubcaps and lots of 
antennas sitting outside your abode one morning, with someone who "just 
has a few questions".




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