New music into old radios – build your own AM transmitter
On another blog, a reader recommended a post in a blog by homo ludens electronicus – and what a blog it is! – see Power Inverter has more than a screw loose
Well, there’s plenty more there to highlight to Gadget Masters.
Take for example his detailing of a project to build a small AM transmitter.
I needed a small transmitter, which would allow me to transmit good, old music into my AM-only radios. So, one Saturday afternoon I got into gear, designed and built a very crude, terribly non-optimized little transmitter. It’s almost a joke expressed in electronics, full of poor design, so please don’t think that this is the best I can do! You must see it as a quick and dirty 5-hour effort, because that’s all the time the transmitter took to design, build, and test. Making this web page about it is taking much longer! I’m putting this thing on the web only because many people have asked me to do so, despite its crude design!
As you can see, the transmitter couldn’t be much simpler: A TTL quartz oscillator provides a 1MHz square wave which is used to directly drive a transistor in full switching mode. A tank circuit turns the square wave into an approximate sine wave, and the 50 Ohm output is taken from one eleventh of the tank capacitive reactance.
The modulation part is equally bare-bones-simple: Two input jacks accommodate stereo signals, which are simply added to form the mono signal needed to modulate the transmitter. A trimpot allows to adjust the modulation level. Setting it to the middle of its range will provide correct modulation depth with a typical line level signal, as provided by most CD players. The audio adder drives a power transistor, which modulates the supply voltage to the RF transistor. That’s pretty much all there is to it… Add a standard regulated 12V power supply, and an additional 6V regulator to bias the modulator and to power the TTL oscillator, and that’s the whole circuit.
He finishes with a small hint:
Before you consider copying this transmitter, make sure you don’t have a local AM station transmitting at or near 1.000MHz! If there is one, you would have to find a quartz oscillator for a different frequency in the broadcast band, and that might be a lot harder than finding the 1MHz oscillator! In that case, it might be better to consider using a totally different drive source.
Homo ludens electronicus can be read as ‘Man at electronics play’, I believe (ludens meaning ‘play’ in Latin).