FSK Data Modulation and Demodulation

Mrbmw99 · Post by **Mrbmw99** » Thu Aug 19, 2004 1:07 pm

Hello! I am not quite sure that this is the correct forum for this thread, but since people on this board do know a LOT about radios I suppose I will throw it on here. For a while now, I have been interested in sending data over a two-way link. I also have recently been learning about how trunking data works, and for the most part I have a good understanding of it. Now, I would like to incorporate this knowledge into a similar project. I wish to simply send data over a two-way link. I will tap the desciminators and do all of that stuff, but what I need is a circuit to build. For testing, I have a PC and a microcontroller that communication at a selectable baud rate, which is 2400 for now. I have done a lot of research on the net for circuits to modulate and demodulate the data, but these are still a little confusing to me. What I am asking is does anyone here know of a fairly simple, yet fairly reliable FSK modulator and demodulator circuit that I can use? Maybe I don't even need to use FSK. Is there anything better? I also do not need anything fast, 2400 or 4800 baud is fine. Maybe there is a premade circuit that will do this? I basically only need the modem part, I will handle the controlling of the radio with a seperate circuit. I do not want to spend too much on this, but I am ready to spend what I think it's worth. I would really like to get this running, because this knowledge will help me a lot in the long run and I enjoy learning about this stuff! If you have any ideas, please post them here! Thanks! -Brett W

EDIT: By the way, I was thinking of a radio somewhere along the lines of a MaxTrac or Spectra or similar. It will only be maybe 10% duty cycle (if that), so these should work fine...

ka6sqg · Post by **ka6sqg** » Thu Aug 19, 2004 1:11 pm

At 2400 bps a modern PC can do the modulation and demodulation in software with a sound card, especially if this is just for an experiment.

Are you trying to emulate some existing protocol or design a new one from scratch?

If it was me, and I had to send that sort of data rate over the radio in real life, I'd probably just get a couple of 9600 bps packet TNCs and hook 'em up.

Mrbmw99 · Post by **Mrbmw99** » Thu Aug 19, 2004 7:55 pm

At 2400 bps a modern PC can do the modulation and demodulation in software with a sound card, especially if this is just for an experiment.

Are you trying to emulate some existing protocol or design a new one from scratch?

If it was me, and I had to send that sort of data rate over the radio in real life, I'd probably just get a couple of 9600 bps packet TNCs and hook 'em up.

Hmmm.. I know VB for the most part, and I have designed many programs, but none used the Sound Card... Any suggestions on where to start if I wanted to use this?

Basically, I want (in the end) to make my own protocol, but for the testing process it doesnt matter.

I have no real experiance with TNCs at all. Can you suggest any? Approx how much do these usually run? Can I mix different brand TNCs? Thanks!! -BW

Mike B · Post by **Mike B** » Thu Aug 19, 2004 9:16 pm

Make sure whatever radio service frequency you use allows the type of modulation that your modem will use. The bandwidth is also tied to the baud rate and may or may not be restricted, depending on the frequency you use.

Try and copy the trunking data circuits from an older trunking radio like a Syntor X. Some of the older trunking radios even had the trunking data interface on separate circuit boards that you might be able to take out. However, I am not sure what baud rate these older circuits ran at. Pick up an older trunking manual and check the service info to see if anything looks good.
http://www.open.org/~blenderm/syntorx/t ... tml#sxtman

doi · Post by **doi** » Thu Aug 19, 2004 10:35 pm

if you're skilled in homebrewing you might just as well try to build a modem using a cmx469 from CMLMICRO. i'm in the process of doing this myself and it works so far. it is quite straghtforward, no adjustments needed. i only tried 1200 baud but the modem can do up to 4800 baud. for the rest...

as in using 9600 baud several ham colleagues around here have tried it with g3ruh compatible modems and it eventually worked but it was a pain in the ***. by the way is there any other 9k6 standard that is easier to setup to get working than g3ruh? maybe some other kind of modulation that does not require the passband g3ruh does? some commercial stuff?

mr.syntrx · Post by **mr.syntrx** » Fri Aug 20, 2004 2:35 am

You can get 9600 baud gear from all sorts of places. Trio Datacom makes some particularly good equipment, such as their 5W UHF modems.

Mrbmw99 · Post by **Mrbmw99** » Fri Aug 20, 2004 4:55 pm

doi wrote:if you're skilled in homebrewing you might just as well try to build a modem using a cmx469 from CMLMICRO. i'm in the process of doing this myself and it works so far. it is quite straghtforward, no adjustments needed. i only tried 1200 baud but the modem can do up to 4800 baud. for the rest...

See, I am definately skilled in homebrewing. I have never used the cmx469 - do you have any websites or schematics to get me started? Maybe if I get this going good enough, I will have some PCB's made or something to make it a lot nicer. Thanks! -Brett W

doi · Post by **doi** » Fri Aug 20, 2004 11:08 pm

all i used was the schematic provided in the datasheet: http://www.cmlmicro.com/techregister/lo ... x469ad.pdf

this circuit is only a very simple (only on the outside) TTL to FSK converter. so it has no brains by itself. YOU will have to provide all the 'thinking' involved with protocols, checksums etc.

Mrbmw99 · Post by **Mrbmw99** » Sun Aug 22, 2004 9:22 am

Ok, I am waiting on a response from CML on how to buy these. What method did you use to buy them? I want to buy a few different pieces from them since they have a lot of useful products. Thanks! -BW

doi · Post by **doi** » Mon Aug 23, 2004 5:40 am

Mrbmw99 wrote:What method did you use to buy them?

well, I live in Romania so it was rather complicated to purchase the ic's. anyway I don't think this would apply to you - if you live in a more developed country it should be fairly straightforward, you should have a local sales rep. or something. if you contact them, as you already did, they will send you further info.

cfr131 · Post by **cfr131** » Wed May 24, 2006 8:41 am

doi wrote: as in using 9600 baud several ham colleagues around here have tried it with g3ruh compatible modems and it eventually worked but it was a pain in the ***.

I am jkust now traveling down this road. I am trying to mate a KPC9612+ high speed port to a GM300. I did as described elsewhere and injected the FSK signal at at the junction of R223, C228, R222 to bypass all the audio processing and directly modulate the VCO. But the TX eye pattern is very poor!. The signal at the input looks great, but the signal being recieved at the other end is very poor. To note the reciever at the other end is not the problem, it is the TXed signal from the GM 300. The eye pattern indicated that the GM300 is filtering out the low frequency content of the FSK signal.

Sounds like maybe others have seem this issue before any suggestions or specifics about further modifications?

regards,
_____
/

doi · Post by **doi** » Wed May 24, 2006 12:19 pm

what we have tried was (motorolawise) the gp300. some worked great the first time, others could not be made to work... don't really know why...