Weird idea: PC-based AM carrier circuit radio

[Firepowerforfreedom]

OK, this is a really weird idea, but would it be possible to use a PC with some sort of AM transmission software to create a carrier circuit radio station?

[simon7000]

Yes. All that is needed is to change the .wav output file to a pulse width modulated version. The idea is that you toggle a single bit (or all of the bits) on a parallel output on and then off at a rate between 69.120 and 179.2 Mhz. That gives you your carrier frequency. This should actually be sending 7 bit packets serially. The 7 bits should be the 7MSB from the .wav file.

Of course AM is now almost extinct inside buildings as computers, the internet and even modern telephones all interfere with reception.

[Firepowerforfreedom]

Originally Posted by simon7000

Of course AM is now almost extinct inside buildings as computers, the internet and even modern telephones all interfere with reception.

Hmm, if that's the case, would it be possible to work in FM rather than AM?

[sofaspud]

Is that toggle rate correct for transmission in the 530-1600kHz AM band?

[simon7000]

Originally Posted by Firepowerforfreedom

Hmm, if that's the case, would it be possible to work in FM rather than AM?

Is that toggle rate correct for transmission in the 530-1600kHz AM band?

I have had folks do the AM coding. Never tried the FM version. Should be possible. Of course then you need an amplifier and antenna. Carrier current just doesn't do for FM.

7 Bits is enough for AM so 128 x carrier is bit rate.

[sofaspud]

d'oh! OK thanks. I'm following now.

[CopperTop]

A good reason to do something like this is to transmit 20s/30s/40s based internet stations to any vintage radios you have. I have done this in the past, and it gives you a reason to turn the things on now and again.

But not quite sure I follow the method you suggest Simon7000. It sounds like it might not meet FCC regulations!

Not too difficult to knock together a 'conventional' low powered AM transmitter anyway. My simple-headed approach would be to use a multiplier IC to do the actual amplitude modulation and a sine wave oscillator chip for the carrier - no inductors in sight. Boring but guaranteed to work...

[simon7000]

The FCC regulations are actually easy to meet at low power. With Pulse Width Modulation a simple RC or LC would do it, provided you actually get the ratiometric conversions right. As it is really all software in theory it is free.

But if you want to build a small transmitter you can do AM with 1 transistor and a few passive parts, for FM you have to add a diode.

I recently tried an AM transmitter inside an arena. With the output power a bit more than legal (well 10 to 20 db might be more than a bit) and a less than legal antenna length of 150 feet the reception distance almost made it to 50 feet. Digital signs, scoreboards, ethernet systems and just too many computer based systems to count. So that just did not work.

FM at 25mw covers the entire seating area!

[Firepowerforfreedom]

Originally Posted by simon7000

The FCC regulations are actually easy to meet at low power. With Pulse Width Modulation a simple RC or LC would do it, provided you actually get the ratiometric conversions right. As it is really all software in theory it is free.

But if you want to build a small transmitter you can do AM with 1 transistor and a few passive parts, for FM you have to add a diode.

I recently tried an AM transmitter inside an arena. With the output power a bit more than legal (well 10 to 20 db might be more than a bit) and a less than legal antenna length of 150 feet the reception distance almost made it to 50 feet. Digital signs, scoreboards, ethernet systems and just too many computer based systems to count. So that just did not work.

FM at 25mw covers the entire seating area!

I'm thinking of using it in a campus where you're never more than 25 ft away from a wall at a time.

[Firepowerforfreedom]

Also, is there any software that can apply the PWM live?