DG said:
Brand new to this, i wanna build a transmitter,
Operating frequency: 150 to 216 MHz
FM deviation: + or - 15 KHz
RF Power Output: 50 mW
Whats the best way to go about this, PLL? capacitor/coil?
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.
Any suggestion on stuff? and refernces to site? Dont know much, plan to
learn but at the moment want easy fast solution.
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