FSK Modems
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Posts: 46,084
Anyone have experience building their own (FSK) modems? I am trying to
expand a project of mine to include real-time data, and need a long distance
(~.5mi) unidirectional data stream. I have found an FM transmitter with
sufficient range, checked with the FCC (eductional research (read: science
fair project that I am expanding for the next level of
competition)--exempt!!), and hacked a walkman for a receiver. I have
attempted to use SEROUT from a BS1 to on/off modulate a 555 or two to
generate the data stream. The sticking point is demodulating the data. I
tried a 567 (too slow), and am now fiddling with a XR2211 to no avail. Any
other ideas? This is a data collection package for remotely-controlled
gliders , so size, weight, and cost are issues. Thanks very much,
Corsair000@a...
expand a project of mine to include real-time data, and need a long distance
(~.5mi) unidirectional data stream. I have found an FM transmitter with
sufficient range, checked with the FCC (eductional research (read: science
fair project that I am expanding for the next level of
competition)--exempt!!), and hacked a walkman for a receiver. I have
attempted to use SEROUT from a BS1 to on/off modulate a 555 or two to
generate the data stream. The sticking point is demodulating the data. I
tried a 567 (too slow), and am now fiddling with a XR2211 to no avail. Any
other ideas? This is a data collection package for remotely-controlled
gliders , so size, weight, and cost are issues. Thanks very much,
Corsair000@a...
Comments
Lots of interface circuits to use audio cassette recorders for data storage
were published before floppy drives got cheap. A search of periodicals in
the 70s and early 80s should turn up something. As I recall, Robert
Suding's interface was well accepted. It may have appeared in Byte or Dr.
Dobb's Journal. One of 'em also appears in Don Lancaster's CMOS Cookbook
(the Bit-Boffer).
Use an LM2917 to convert your frequency to voltage, then use a simple
comparator to get binary output. The 4046 PLL chip also does F->V
conversion, and it's very well described in Don's book!
Run your audio through a simple (L/C) low-pass filter and then rectify it.
You get a DC voltage that's a strong (inverse) function of frequency if
your audio frequencies lie on the cutoff slope. A comparator recovers data.
Use a comparator or clipper to convert audio to logic levels and trigger a
timer with it. The timer generates fixed-width pulses of width less than
your minimum audio cycle time. Integrate the pulses with a simple R/C
network. A comparator recovers your data. Or, you could use the pulse width
measurement capability in a microcontroller at your receiving site, to
deduce data directly from clipped audio...
If you switch to a BS2, and speed isn't a big issue, you could use the
DTMFOUT instruction for transmitting, then use a simple DTMF receiver chip
to recover your data. There's a list of DTMF RX chips here:
http://www.ee.washington.edu/eeca/text/DTMF.html.
You might also find this link interesting:
http://bart.us.es/~melus/gwlhw06.html but low-cost single-chip modems seem
real hard to find lately. This one might actually be available:
http://www.zilog.com/newprod/z02215.html.
>Anyone have experience building their own (FSK) modems? I am trying to
>expand a project of mine to include real-time data, and need a long distance
>(~.5mi) unidirectional data stream. I have found an FM transmitter with
>sufficient range, checked with the FCC (eductional research (read: science
>fair project that I am expanding for the next level of
>competition)--exempt!!), and hacked a walkman for a receiver. I have
>attempted to use SEROUT from a BS1 to on/off modulate a 555 or two to
>generate the data stream. The sticking point is demodulating the data. I
>tried a 567 (too slow), and am now fiddling with a XR2211 to no avail. Any
>other ideas? This is a data collection package for remotely-controlled
>gliders , so size, weight, and cost are issues.
Mike Hardwick, for Decade Engineering -- <http://www.decadenet.com>
Manufacturer of the famous BOB-II Serial Video Text Display Module!
>Anyone have experience building their own (FSK) modems? I am trying to
>expand a project of mine to include real-time data, and need a long distance
>(~.5mi) unidirectional data stream. I have found an FM transmitter with
>sufficient range, checked with the FCC (eductional research (read: science
>fair project that I am expanding for the next level of
>competition)--exempt!!), and hacked a walkman for a receiver. I have
>attempted to use SEROUT from a BS1 to on/off modulate a 555 or two to
>generate the data stream. The sticking point is demodulating the data. I
>tried a 567 (too slow), and am now fiddling with a XR2211 to no avail. Any
>other ideas? This is a data collection package for remotely-controlled
>gliders , so size, weight, and cost are issues. Thanks very much,
Grab a couple of 2400/4800/9600 baud external modems from the Salvation
Army or
some flea market/swap meet. Rip them apart, and have at it.
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>Corsair000@a...
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>eGroups.com Home: http://www.egroups.com/group/basicstamps/
>http://www.egroups.com - Simplifying group communications
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Scenix.com website and the source code to create a basic FSK modem with an SX
chip is available. Sxtech.com is also a great place to go for further sx
programming information. This is also the basis of ham packet radio and surfing
on that topic will provide much info on the various ways of wireless
transmission of data via modem. I am planning to try and send a GPS NMEA data
stream this way in a project to be started soon. Check the following link for
ham packet radio information and good luck!
http://www.tapr.org/index.html
Bill Mrozinski - DCS
Corsair000@a... wrote:
> Anyone have experience building their own (FSK) modems? I am trying to
> expand a project of mine to include real-time data, and need a long distance
> (~.5mi) unidirectional data stream. I have found an FM transmitter with
> sufficient range, checked with the FCC (eductional research (read: science
> fair project that I am expanding for the next level of
> competition)--exempt!!), and hacked a walkman for a receiver. I have
> attempted to use SEROUT from a BS1 to on/off modulate a 555 or two to
> generate the data stream. The sticking point is demodulating the data. I
> tried a 567 (too slow), and am now fiddling with a XR2211 to no avail. Any
> other ideas? This is a data collection package for remotely-controlled
> gliders , so size, weight, and cost are issues. Thanks very much,
>
> Corsair000@a...
>
>
> eGroups.com Home: http://www.egroups.com/group/basicstamps/
> http://www.egroups.com - Simplifying group communications
http://www.tfs.net/~petek/telemetry/index.html
Duncan
>
>Look here for a PIC based FSK modem design:
>
>http://www.tfs.net/~petek/telemetry/index.html
>
>Duncan
Thanks Ducan -
You may want to check these guys out:
http://www.americaii.com/
as a source for the TCM3105 chips, and perhaps pass it on to the author of
the
website. Findchips.com located than, and would indicate that they have 1000
in
stock. Certainly only direct inquiry would confim that.
Regards,
Bruce
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