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'what is the difference? uart + rf of modem engine'
1998\12\02@220216 by bigchu

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Hi,

I am designing a RF project, I do not know what is the differenec betwteen
a  and  b.

  a .  microcontroller  or any UART + RF

  b.      microcontroller + modem chip (fx919) + RF

the project is car security system, communicate cars in the city. sometime
for car finding system.


 There are many wireless products  now( running at 9600 bps), what  is
the modem engine for.

Can I use microcontroller uart  +  RF , to  performce the job?.


Best regards,


James Chu

1998\12\04@100034 by Marc

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>    a .  microcontroller  or any UART + RF
>
>    b.      microcontroller + modem chip (fx919) + RF

You can transmit digital data directly with an FM RF transmitter. This is not ve
ry
effective (spectral wise) and use lots of bandwidth. Due to limited bandwidth at
the receiver (channel separator filter), the output will have smooth edges inste
ad
of the sharp ones you entered into the transmitter. Also, you often experience a
slight
shift to high or low voltage. In one word: it's not very noise immune.

With this type of xmitter you have to directly trade bandwith for bps. The bandw
idth
is dictated by the channel grid (for example 25khz). The bps is the max that you
can
correctly receive. With a simple Schmitt-Trigger, don't expect more than 2400bps
. With
advanced DSP algos you might get more :-)


A modem chip (don't know about the FX919!) can encode the digital bits spectrall
y
much better. This advantage can be used to achieve more bps throughput, or alloc
ate
less bandwidth (ie smaller grid) or just to be more noise resistant in a given
channel environment. Be sure to use a good modem, simple AFSK won't give you muc
h
advantage over direct FSK.

In general I found that modulation techniques in amateur / hobby radio are not
very sophisticated. Any old telephone modem is doing better work at encoding
data spectral-efficient. Probably most of the ICs are simple, too. Good
mod/demod requires DSP.

> the project is car security system, communicate cars in the city. sometime
> for car finding system.

At _best_, the above circuits can run a taxi signalling system in a small town.
For
car tracers that find your car after being stolen, you should check out
GSM modules. They can still ring you even while passing Moscow city limits.

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