Shop OBEX P1 Docs P2 Docs Learn Events
FM transmission at ~150 MHz? — Parallax Forums

FM transmission at ~150 MHz?

missingNomissingNo Posts: 1
edited 2010-03-06 08:16 in Propeller 1
I've been searching around for how to implement this in a project I'm currently working on, and all I can find is a way of transmitting at 100 MHz give-or-take, however, I need a touch more than that.

In essence, I'm building a small-small scale paging network with a software implemented POCSAG encoder to transmit at 152.4800 MHz for rebanded and reprogrammed pagers. The device will have a mini API of sorts (potentially SOAP-based) and Ethernet connectivity (via the ENC28J60 by Microchip) to enable users on the LAN to send a message to a desired pager.

In essence, I need to take that encoded message and then transmit it as a stream on 152.48 MHz for the pagers to pick up.

What could I use in order to obtain my desired frequency? I've seen a few methods of getting up to 100 MHz, however I'm unfamiliar with radio design a touch.

Comments

  • kwinnkwinn Posts: 8,697
    edited 2010-03-05 04:03
    Not an expert on RF myself, but you might try producing a 50.827 MHz signal with the prop and using the third harmonic (square waves contain the odd harmonics). It would require a good filter to remove the 50.827MHz and the higher order harmonics. Not sure of the legality either so you should alsy check that out.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-03-05 04:24
    missingNo,

    You won't get a clean RF signal from the Prop due to PLL jitter. I've tried it in the 144-148 Mhz range and got a very poor quality signal with multiple spurious side lobes. Not good. The FCC will be all over your butt if you try broadcasting such a signal over the airwaves. Short answer: forget it.

    -Phil
  • pullmollpullmoll Posts: 817
    edited 2010-03-05 10:34
    It looks like you should be using an external, probably quartz based, oscillator and just do the FM from the Prop using the PWM object. I think you should find schematics of a simple enough oscillator with few parts on the net. Perhaps there's even a single chip solution. It's a been long time since I last dealt with HF smile.gif

    Juergen (DG9KBM)

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    He died at the console of hunger and thirst.
    Next day he was buried. Face down, nine edge first.
  • Dave HeinDave Hein Posts: 6,347
    edited 2010-03-05 13:51
    MissingNo,

    You could use the technique I posted at http://forums.parallax.com/showthread.php?p=883440·.· However, as Phil said the signal will have a lot of jitter, and it may not work with your pagers.· To get a clean signal you will need an external FM transmitter that you can modulate with a signal from the Prop.· If you intend to use this within a small area, such as a restaurant pager, you should be able to get by with a low power.

    Dave
  • kf4ixmkf4ixm Posts: 529
    edited 2010-03-05 14:34
    I agree with Phil, you'd be better off aquiring a premade transmitter for your desired frequency and using the prop for pocsag encoding.
  • muzzymuzzy Posts: 4
    edited 2010-03-05 23:22
    I used to design pocsag pagers for Philips. What products are you using - I might be able to offer a bit more help.

    Cheers,

    Mike
  • pmrobertpmrobert Posts: 677
    edited 2010-03-05 23:53
    You're playing with fire - the FCC has no sense of humor.
  • muzzymuzzy Posts: 4
    edited 2010-03-06 08:16
    >You're playing with fire - the FCC has no sense of humor.

    oh yes, I forgot that point [noparse]:o[/noparse])

    I'm assuming the poster is from the UK where pocsag was popular so the FCC doesn't apply, but you are right about it being illegal to do this. It will be illegal in any country.

    If you are generating a few millwatts with a range of a few feet, that will be fine. If you want to service users, forget it.
Sign In or Register to comment.