RF Modem

Archiver · 2000-07-19 01:09

I would like to interface a modem to a stamp and UHF radio. I
need
only 1200 baud. Has anyone used a cermetek modem module or any
other type of modem?

Thanks

Larry Licht
llicht@u...

Archiver · 2000-07-19 15:43

> I would like to interface a modem to a stamp and UHF radio. I
> need
> only 1200 baud. Has anyone used a cermetek modem module or any
> other type of modem?

It is a very common misconception that a normal modem can be
connected to a radio and it will work. Nothing could be farther from
the truth.

Send data relaibly over RF is an *extremely* complicated
undertaking -so much so that there are companies that have entire
engineering departments working on products. In addition, most
radios will need to be modified to properly deal with a data stream -
and many -by virtue of their design for voice- do not modify well.

You didn't mention your requirement (range, reliability factor etc),
but you can probably get away with the modules that Parallax
carries (their RF modems -not the Cermetek Kit). These are quite
reliable over very short distances -but you should certainly
implement some kind of error-checking algorithm if you need to rely
on the data. You should note that this is not a reflection of the
quality of the product, but rather the nature of the task at hand.

Mark Hillier, VE6HVW
President, HVW Technologies Inc.
Canadian Distributors of Parallax Products and other Neat Stuff
Tel: +403-730-8603 Fax: +403-730-8903
http://www.hvwtech.com

Archiver · 2000-07-20 06:54

Hi,

CML makes ICs for radio data transmission at 1200/2400 baud, in formats from
MPT1327 FSK, MSK, free binary format, etc.
You can interface these modems to most radios, all you need is their
discriminator or audio output, microphone input, and PTT line, just like any
amateur radio TNC.
ALL radios, wether commercial or not, designed for voice communications, can
be used to send data. In the end, this data is FSK (frequency shift keying),
which is no more than a sine wave extremely coherent. Speech is a wave just
the same, just not sine or coherent. For example, normal 1200bps FSK
occupies a bandwith of about 2kHz, which is less than human speech, which
occupies around 3kHz (the basic components, high-fidelity like CD quality
uses much larger bandwidths).
This means that if you can send voice, you can send data. ALL radios also
have a microphone and audio pads in their PCBs where you can solder leads to
go to your modem and controller.

All the best,

Mike

>

Mensaje original

> De: Mark Hillier [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=gq1xaYXjnf0C-0Y5ovDbujoGBzsiCIg5BPeFHJ7TnX8XThvgxxQX3_gmPI6r7IV2cQ96ZW5L]Mark@H...[/url
> Enviado el: mi

Archiver · 2000-07-20 13:46

I'd have to agree with Mike here; in fact, doing it without a modem is what is
difficult, engineering intensive, and probably limited to very short distances
for reasonable reliability. Adding FSK modems to a cheap set of vhf/uhf
transmitters is complex but very do'able if you do your homework; its the FCC
you need to consider, walkie-talkie, CB,and FRS ( all the cheap transceiver
sources(g) ) have voice only restrictions I believe...Even the frequencies
allocated to data transmission have duty cycle restrictions and lots of other
complex rules that take all the fun out of it. However, 'under-the-radar' as a
goof, this is almost the perfect 'hack' project ( in the constructive sense of
the word! ) Investigate the amateur packet radio world for plenty of background
information. I say...go for it, sounds like fun!

Bill Mrozinski

Miguel Puchol wrote:

> Hi,
>
> CML makes ICs for radio data transmission at 1200/2400 baud, in formats from
> MPT1327 FSK, MSK, free binary format, etc.
> You can interface these modems to most radios, all you need is their
> discriminator or audio output, microphone input, and PTT line, just like any
> amateur radio TNC.
> ALL radios, wether commercial or not, designed for voice communications, can
> be used to send data. In the end, this data is FSK (frequency shift keying),
> which is no more than a sine wave extremely coherent. Speech is a wave just
> the same, just not sine or coherent. For example, normal 1200bps FSK
> occupies a bandwith of about 2kHz, which is less than human speech, which
> occupies around 3kHz (the basic components, high-fidelity like CD quality
> uses much larger bandwidths).
> This means that if you can send voice, you can send data. ALL radios also
> have a microphone and audio pads in their PCBs where you can solder leads to
> go to your modem and controller.
>
> All the best,
>
> Mike
>
> >

Mensaje original

> > De: Mark Hillier [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=-8N2tPhSXKzsRTcQ6SEPfm0B1kpCtImj3F4-PL38YxGaGqfo6x-JNke2pdZ3xT2CqAntyZV_fz2xLgc]Mark@H...[/url
> > Enviado el: mi

Archiver · 2000-07-20 16:45

Along these lines (FSK modem chips) Byon Garrabrant has a PIC project which
replaces the TNC in an APRS tracker. It's quite ingenious. The whole thing
including the .hex file is up at:

http://www.byonics.com/tinytrak/index.html

Another PIC based FSK modem is here:

http://www.tfs.net/~petek/projects.html

Couldn't find any strictly basic stamp examples. You could likely do it
with a stamp, but only at a relatively low baud rate...

Cheers, Duncan

RF Modem

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