FSK decoding with a PIC

Kryten · Jan 29, 2009

Hi Im just staring a new project where im (hopefully) going to us a PIC of some sort to take a FSK signal and decode into serial data.

The FSK signal are 1200 baud with 0 @ 1200 Hz and 1 @ 2200 Hz.

The reason why is to make a small portable unit that takes APRS signals in to a laptop.

Im thinking of making it so that I can just hang the antenna directly in it .

Im hoping some one can help me with this..

AtomSoft · Jan 29, 2009

Google: FSK Decoder IC:
http://www.datasheetcatalog.org/datasheet/npc/CF8223A.pdf

Thats all i can find. Might be more but i know nothing on the subject. Sorry.

Nigel Goodwin · Jan 29, 2009

Well 30 years ago 6502's were reading and writing FSK to cassette tapes, with no extra hardware (other than a few resistors and capacitors), so a PIC is more than capable of the job.

Quick edit:

I've done a quick google, it used a double opamp and a transistor as well, here's the circuit

**broken link removed**

The cassette interface is at the bottom RIGHT corner.

Oops - quick edit - the diagram is of TANEX.

Kryten · Jan 30, 2009

Ok thanks buys.

But adding some trouble for me by saying that i want it to run off and with USB.
I was thinking about the possibilities of make some sort of small driver or other software that will be automatic downloaded to the computer, this for making the unit usable to other software like UIView.

Any thoughts if its possible?

Nigel Goodwin · Jan 30, 2009

Kryten said:
Ok thanks buys.

But adding some trouble for me by saying that i want it to run off and with USB.
I was thinking about the possibilities of make some sort of small driver or other software that will be automatic downloaded to the computer, this for making the unit usable to other software like UIView.

Any thoughts if its possible?

Certainly, just use an 18F series PIC with USB capability.

Although this idea sounds like a confused mixture of different centuries technology?.

Kryten · Jan 30, 2009

Well Nigel its not my fault that the radio amateurs started using FSK for APRS operations but anyways, it should be fun to make it happen, I might not have the time although i think that the most time consuming part of this project will be making the usb communication part.

As far as i know for decoding the FSK i will have to know the time for a "1" so that i can decode it correctly.

Nigel Goodwin · Jan 30, 2009

Kryten said:
Well Nigel its not my fault that the radio amateurs started using FSK for APRS operations but anyways, it should be fun to make it happen, I might not have the time although i think that the most time consuming part of this project will be making the usb communication part.

As far as i know for decoding the FSK i will have to know the time for a "1" so that i can decode it correctly.

Sorry, I didn't realise you were doing a radio project.

Presumably the 'time' will be a specific number of cycles - the old Microtan circuit I posted the link to uses an opamp to amplify the headphone output from a cassette recorder (so can do the same for a radio), uses a second opamp as a schmitt trigger to square it, then drives an NPN transistor to feed an I/O pin.

USB wise you will find it a LOT simpler to use a serial/USB converter, rather than try and do the USB directly.

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FSK decoding with a PIC

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