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  1. #1
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    data over vhf

    Hai

    I have to send digital data (a GPS string ) from one point to another through VHF transmition. how to do that ?


    I have mic input of a VHF transceiver (144Mhz FM) at one end and a receiver at other side?

    DTMF or modem ? How ?

    suggessions please

    picstudent

    •   Alt27th April 2007, 13:26

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  2. #2
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    sending data over vhf

    used a celular phone by SMS service and codec NEMEA GPS codes

    or modulate audio tones mark and space and used a pair of handy radio


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  3. #3
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    vhf transmitter

    Quote Originally Posted by un-2000
    modulate audio tones mark and space and used a pair of handy radio

    This is the method I am looking for because the department has its own wireless nework.

    SMS option ruled out because not reliable at busy hoours or days .tried that method already.

    How to use mark space or DTMF? Some more details please?

    picstudent



    •   Alt8th May 2007, 10:03

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  4. #4
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    circuit+send digital data with am

    use DTMF chips to convert data to audio(and reverse) tones and send it .

    CHIPS: MT8888,MT8870,HT9200,....


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  5. #5
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    ht9200 celular

    You could also use FSK (F1D). Shift an audio tone (use standards of amateur radio) and demodulate / decode using PLL 567 / 4046 etc.

    Search for FSK and FSK decoder. A large number of amateur radio (including for 2 mtrs) FSK link schematics are available on the web.:D


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    •   Alt8th May 2007, 20:08

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  6. #6
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    f1d modulator mark and space

    Hi

    Thanks for the info

    Which will be good option for my purpose? DTMF or FSK.

    DTMF i have some experience in decoding ,but FSK I am new.

    What will be the cost of commercial FSK modulator and demodulator pair?

    So far cant locate any circuit.

    73s

    picstudent



  7. #7
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    digital data transmission using fm modulation

    If you really want to use VHF radios why not use a packet radio. Easy to interface (UART) RS232 interface.


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  8. #8
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    sending data over fm

    FSK will give you much higher baud rate (symbols) that DTMF. And due to high baud rate, you can use error detection and correction algorithms, which is necessary since the transmission will be half duplex(?).

    AFAIK, 1200 and 2400 Hz are the two frequencies used. You could use two oscillators and switch between the two for transmitting. And you could use one (or two) PLLs to decode on the rx side.

    http://www.exar.com/product.php?ProdNumber=XR2211
    http://www.edn.com/index.asp?layout=...cleid=CA170224
    http://tams-www.informatik.uni-hambu...modulator.html

    And there are a large number of RTTY/Piccolo other mod demod applications using PC soundcard.
    Using a PC will not cost you any thing (apart from a PC, which I guess, you have), and some analog circuit plus noise and interference suppression, provided you can use a PC (i.e. not a portable or low power usage).


    Packet Radio is a still better idea if you can use a PC in the project.

    Added after 30 minutes:

    Oh, I forgot, you could use D flipflop and monostable (4538) combination to decode FSK (one frequency).

    Set the monostable to pulse length of slightly less than 1/2400 and feed to data of a D flip flop. Clock the monostable and the D flip flop with audio (filtered and converted to square using schmidtts). Q =0 : 2400 present and Q=1 : 1200 is present. Though, this will give you erroneous results if the SNR is low or if the transmission breaks.


    There are very many ways to solve a problem, you need to see what suits your abilities / requirements and budget.


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  9. #9
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    fm data transmission

    As per my experience of data transmission over VHF DTMF gives quite good results, even with noise and fading, but its slow. DTMF can be generated on low cost micros with pwm, and decoded using a easily available MT8870.
    FSK transreceiver chips are costly and hard to find these days. I dont remember the part no. but I used them at 300 bps, but it is suspectible to noise. You can also check out OOK apart from FSK if the signal quality is good.


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  10. #10
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    send location gps via vhf

    Well, best performance, in a noisy environment, is achieved by synchronous CW (OOK), since it has the lowest band width and has much better tolerance to phase changes, fading than any other modulation. However, CW is a very low bit rate transmission.

    In real terms, FSK is probably the better of the lot. Amateur radio texts describe the complete details of the properties.

    It is not really necessary to use 'FSK' tranceiver chips. FSK can be easily generated by discrete oscillator switching or switching between two frequencies generated by a xtal oscillator. Obviously, synchronous switching is better. And using USTTY encoding, all most all kind of communication is possible.

    A simple PLL (4046) (actually two, with some decision making logic in hardware or software) can be used for demodulation.

    And this will cost less than DTMF chips. Using garden variety CMOS logic chips (4XXXx series), the units are easily repairable even in a college electronics lab.

    Of course, using a 16 bit microcontroller, you do not need DTMF chips at all for generating and demodulating DTMF. DTMF was designed for telephone networks, to signal various states / control. DTMF symbol rate is low and is limited to 16 symbols only.
    .


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  11. #11
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    16bit fm digital data

    Quote Originally Posted by asit
    As per my experience of data transmission over VHF DTMF gives quite good results, even with noise and fading, but its slow. DTMF can be generated on low cost micros with pwm, and decoded using a easily available MT8870.
    FSK transreceiver chips are costly and hard to find these days.
    $5 in one piece seems to you costly ?
    http://focus.ti.com/docs/prod/folder...T+ccweb_cc1021


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  12. #12
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    ht9200 radio interface

    if u need schematic of 2211-2206 based FSK modulator ,Demodulator . PM me


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