Post by thread:what is the difference? uart + rf of modem engine

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

> 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.