I have a device which outputs a serial signal using 1200 baud FSK.  I would like to connect this to a regular computer DE9 serial port to read the data stream.  It only needs to be one-way communication, there will only be signals coming in from the device.

I've never done conversion from one signal protocol to another like this.  Does anyone have a suggestion where I might start?  I am fully competent in component-level electronics fabrication and do a lot of experimenting with amateur radio circuits, that should give you an idea of my skill level.  (The stuff I build DOES work!)

Thanks,

Jim

What's the carrier frequency for the FSK?

Back in the day, we used to use the Motorola MC3356 RF/IF/FSK decoder IC to get from 110 MHz RF down to TTL data...pretty simple to get to 232 from there...

I'm tracing the circuit back from the transmitter section; transmitter is 403mhz.

I've stumbled upon a TTL level output on an unused header, and I think it parallels the data being sent by FSK.  I might be able to skip the FSK decoding altogether, at least for now.  If I can figure out the data packet format on the TTL line, then I can do what I need to do with the FSK output even if I can't see it directly.

Thanks for the lead on that Motorola chip, I'll go find a datasheet.

Jim

ETA:  There's a bunch of versions of the MC3356 listed; I assume you are referring to the MC3356DW, wideband FSK receiver?
www.ortodoxism.ro/datasheets/motorola/MC3356DW.pdf

Yep, that's the animal...

Good luck getting the internal LO running at a high enough frequency to receive 403 MHz!

Alas, this is ancient tech - There are probably dozens of newer chips that would make the job easier.

I often use the MX614DW from CML Micro....it is just a BELL202 modem....no sense in reinventing it...


I see you already have the component, are you just trying to receive digital data from the FSK signal?
If so, just get a MAX232 chip to convert your received data line to 232 levels and connect to your DB9's RX pin of your computers port.


oh, and there are plenty of bell202 and hart modems that will work with 1200/2200 FSK communication that hook direct to your 232 port or USB (installs as com port)  but for some reason they all want >$300 for them!


my favorite:
MicroFlex

http://www.microflx.com/MicroLink529_Specifications.htm

Quoted: I'm tracing the circuit back from the transmitter section; transmitter is 403mhz. I've stumbled upon a TTL level output on an unused header, and I think it parallels the data being sent by FSK.  I might be able to skip the FSK decoding altogether, at least for now.  If I can figure out the data packet format on the TTL line, then I can do what I need to do with the FSK output even if I can't see it directly. Thanks for the lead on that Motorola chip, I'll go find a datasheet. Jim ETA:  There's a bunch of versions of the MC3356 listed; I assume you are referring to the MC3356DW, wideband FSK receiver? www.ortodoxism.ro/datasheets/motorola/MC3356DW.pdf

Yeah - just get a 232 level converter and try it - it may need inverting though, but definitely worth a try!

The message packets (GPS data) should be fairly short, so I may just take some freeze frames on my oscilliscope and examine the data by hand.  I don't have a TTL--232 converter on hand (though, looking at them, I should buy a half dozen of those maxim max232 chips for future projects).

I *know* I have an old 2400 baud modem1200 baud Hayes Smartmodem sitting at home.  If I can confirm the FSK signal, I might be able to pull some data out with the modem.

Jim

p.s.--Thanks!