GPS serial data over an audio rf link

[Geoff172]

Hi, over the past year I tinkered with equipment to send video pictures from a model aircraft to the ground. The 2.4Ghz system that I have now is working well with very good range due in part to two patch antenna’s. The transmitter and receiver both have an audio channel with phono sockets which are not used. Can anybody suggest a circuit design which would convert serial data from my GPS module with a Serial Port Profile (Spp) output socket to send down this audio channel and be converted back on the ground?

Thanks Geoff

 

[Sceadwian]

Send the serial data as DTMF tones. You can send binary nibbles using DTMF Just use the digits 1-D as normal and use the asteric * and pound # sign for the E and F digits of a HEX number. Encoding and decoding of DTMF tones is a well covered topic.

 

[Nigel Goodwin]

Using DTMF tones would be incredibly slow - about as fast as semiphore flags

What you need is a crude modem, use one tone for '1' and another for '0', I would suggest using 300 baud and the tones that Bell used for them, as they are a standard. You can use a simple keyed oscillator for the transmitter (I've even seen 555's used) and a 567 PLL for the receiver.

 

[j.p.bill]

You can get carried away and let each bit in the word select a tone. Obviously, they are different for low and high states, and none of the tones are harmonically related. Mix then, send them, and selectively filter them out at the other end. Something like that has been working fine for about 40 years for data transmissions.