Fm Transmitter

Brand new to this, i wanna build a transmitter,

Operating frequency: 150 to 216 MHz

FM deviation: + or - 15 KHz

Operating Frequency: 150 to 216 MHz

RF Power Output: 50 mW

Whats the best way to go about this, PLL? capacitor/coil?

Any suggestion on stuff? and refernces to site? Dont know much, plan to
learn but at the moment want easy fast solution.



Thanks

 

Easy fast solution?

Sorry, there isn't any. Unless you actualy find out the necessary
background information you'll be working in a "monkey see, monkey do"
fashion, and won't be able to correct the inevitable problems which will
occur while you build and test this transmitter.

Further, have you checked the licencing situation with regard to this
transmitter. Will it be legal in your area?

You might try going to your local Radio Shack, or equivalent, and seeing if
they have any kits available with these specifications, but again, check the
licencing situation.

Assembling a kit won't really teach you anything, though.

John

 

A PLL (assuiming a crystal or other suitably stable reference) will just get
you a nice, stable carrier. What's your application? If it's just for
hobbyist uage, you probably don't need the complexity of a PLL.

You'll find that modifying the scads of designs on the web for 88-108MHz FM
transmitters to instead run 150-216MHz is easy. As for the +/-15kHz
deviation -- you'll see a lot of designs out there that _don't even use
something like a varactor diode_ to thereby allow a (readily) predictable
deviation with input voltage -- the simplest designs merely change the bias
of the transistor (that's serving as the 'negative resistance' for the
oscillator) and... poof!... the frequency of oscillation changes. (Arguably
it's due to reverse biased junctions in the transistor _acting_ like
varactors, it's just very difficult to predict the magnitude of these
changes.) I.e., if you really do need a specific deviation, look for a
design where it's clear what mechanism is being used to achieve the
deviation!

Finally, for 50mW of output power... that's enough power that -- while I
wouldn't be surprised to see someone having done it one transistor -- it's
easiest to generate your RF and then add an adjustable gain buffer amplifier
afterwards to obtain it. Unless you have a low power wattmeter lying
around, though, you'll have to infer how much power is getting out to your
antenna based on measuring, e.g., current and voltage through the amplifier.
Or build your own low power wattmeter.

Try some of these:
http://braincambre500.freeservers.com/
http://ourworld.compuserve.com/homepages/Bill_Bowden/homepage.htm#menu
http://www.commlinx.com.au/transmitters.htm
http://www.uoguelph.ca/~antoon/circ/circuits.htm
http://sound.westhost.com/project54.htm
http://tacashi.tripod.com/elctrncs/smplfmtr/smplfmtr.htm

---Joel Kolstad

 

These freqs are in the middle of the TV broadcase bands, and below 174
MHz, in the public service bands. In many countries, this power is
not allowed without a license. For narrow band FM, you would probably
be best off with a PLL design. Check on the newsgroup
alt.radio.pirate for more information.

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Radio Scrap withdrew those TXes from the market, apparently at more
than a gentle nudge from The Flunked Clown College.

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That's easy to do. A 50 ohm load with 1.5V RMS across it gives about
45 mW. So if you use one of those grain of wheat bulbs from radio
Shaft that are rated 1.5V at 25 mA, it will give an approximate
indication of the power output.

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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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Thank you.