> > Manchester on the other hand represents 1s and 0s by
> > change of signal level. Each bit uses the same amount
> > of time but it cuts your total bandwidth in half. To
> > send a logic one, you hold the line low for half
> > a bit period then yank it high. For a logic zero, you hold
> > the line high for half a bit period and pull it low. Of
> > course, this isn't carved in stone, I've seen designs reverse this.
> >

The lowest cost RF system I have seen is a project I worked on
quite a few years ago for store electronic price tags. The
system could send and receive information from individual
price tags in the store (Typical grocery store 7000)
The data rate was slow and so was the RF. The processor clock ran
at 200 kHz the transmitter was the processor clock gated through
a pair of CMOS xor's controlled from the processor to
have complimentary outputs into a three or four turn coil of wire
as an antenna when it was transmitting.

The data rate was about 10 bits per second Manchester encoded. The
amazing thing was how well it worked, lots of electrical noise
around. The manufactured cost of the tag (processor 1 or 2 cmos
parts, case and lcd display was about $4.00 in the volume they
used.

--------------------------------------------------------

In an act of sheer brute force I was treated to a demonstration
a few years ago of a high speed processor bit bashing AM radio
through what was essentially a PWM and about a foot of wire
wrap wire.


w..

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