[NLRS] RF read water meters

[Doug Reed]

Dr. Gerald N. Johnson wrote:
> Could be. With a transmitted bandwidth of 405 kHz, the center frequency
> doesn't have to be held to within 0.1 ppm to be found.
.......
> Hopping by FCC regulations requires more than three frequencies, the
> test reports say for their bandwidth 25 are required and used.

My assumption is that they are not running spread spectrum but instead 
are working under the older Part 15 low power rules. The SS rules let 
you run up to 1 watt output. The low power rules had a limit below 1mw 
continuous with more power allowed if you used a burst mode like they 
do. That is probably how they ended up at 11.5mw average TX power.

I was wondering what data rate they are using. Is the 405KHz the BW 
rating of the TX signal or the RX BW of the receiver? A 400KHz TX BW 
implies something over 400 bits of info in the TX burst, that seems 
quite excessive. If it is the RX BW then it implies closer to 100 bits 
of TX data which is still a lot but much more reasonable.

ITI/GES was using a TX data rate that gave a TX BW of under 4KHz, but 
the crystal controlled TX-RX equipment had a RX BW of closer to 30-40KHz 
to accommodate frequency errors in the RX and TX crystals. The 434MHz 
SAW based products were allowed +-75KHz TX frequency errors and the RX 
IF was over 250KHz wide. The PLL based products were not as wide as the 
SAWs but not narrow enough to compete with the purely crystal based 
products. The extra RX BW helped reduce the RX range.

> Since the transmitters don't have receivers to trigger them, in the
> drive by situation they want them to transmit often so the odds of the
> mobile receiver being in range for at least one transmission is high.

That is what I hadn't considered. If they are doing drive-by data 
collection, that explains perfectly why they TX every 4 seconds. And one 
man can drive the area far faster than a man on foot can check it. And 
it eliminates data collection receivers which would cost money and 
require maintenance for something that only needs to be done once every 
few months....

Being active 2ms out of 4000ms they have a low risk of TX collision and 
as the RX gets closer to one or the other, the signal strength would 
help resolve the issue. In fact with a laptop in the car, GPS to locate 
position and GIS info showing the houses and meters, you could have the 
display turn from red to green when the signal had been received w/o 
error. It would certainly reduce the number of meters you had to 
physically check....

I've definitely enjoyed this discussion.

73, Doug Reed, N0NAS.