Radio modems allow digital data communications
through radio waves. Conventional narrowband radio
techniques rely on a base "carrier" wave
that is altered in a systematic manner (modulated)
to embody a coded bit stream. Carrier waves can be
modified to incorporate digital data by varying
their amplitude, frequency or phase. The
radio-modem, although usually much slower than its
telephone counterpart, has the in-built capacity
of being a self-correcting data carrier. The
deterioration of the speed of transfer in bad
conditions can become annoying to the impatient
operator.Radio modems are available usually in two
different formats.Some modems are devices which
contains the modem partand radio transceiver as a
single device or module.The other kind of radio
modems are modem devices which aredesigned to to
interface to a normal "voice
communication" radio or similar device
(usually through headphone,microphone and PTT
connections).The standard way to convert the
serial data signal to a suitable format which can
be transmitted through the audio channel provided
by the radio. Radio amateur transceivers and other
transceivers designed for voice communications can
not be connected directly to terminals or
computers to perform digital data transmissions.
The binary signal exiting from the serial port of
a computer has a rectangular shape, has a strong
DC component and a great amount of harmonic
frequencies. Transceivers are generally planned
for voice (or telegraphy) transmission and admit
particularly frequency modulation (F3A), thus they
can not transmit DC (frequency leading towards
zero) nor frequencies that exceed human voice ran
ge (passband being almost 3 KHz wide). To overcome
these and other obstacles you don't have to
re-plan or rebuild a transceiver. You can instead
interpose between the PC and the transceiver a
particular modulator/demodulator which transforms
digital signals of the PCinto audio signals
between 300 and 3,000 KHz (passband of the
transmitter modulator); these signals are thus
sent to the transceiver as if coming from a
microphone. The device, normally called a modem.
It typically generates a continuous sinusoid
signal centered in the passband of the transmitter
input, whose parameters (amplitude, frequency,
phase) are varied by the serial digital signal
coming from the PC. Simplest approach is amplitude
modulation:You generate an audio tone when there
is 1 on line and notone when there is 0 on line.
This is the transmitter.The decoder is just a tone
decoder (just listens iftone exists).The
techniques of modulation are basically the
following:
- AM or amplitude modulation (used for very
slow speed) changes the transmitted signal
amplitude to each of the two logic statuses 0
and 1, the most extreme version of this turns
the carrier completely on and off
- FM or frequency modulation (for speeds up to
1,200 bytes/sec) pre-sets a given frequency
value in the generated sinusoid signal to each
of the two logic statuses 0 and 1
- PSK or phase modulation (for speeds up to
4,800 bytes/sec) pre-sets two different phases
of the carrier wave to the two logic statuses
0 and 1
- combined phase and amplitude modulation (for
higher speeds) combines the modification of
The most common modem modulation method for radio
communications use is some form of FSK (Frequency
Shift Keying). This type of modulationpasses
nicely through many kinds of radio radio based
"voice channels".Simple FSK modulation
through few kHw woice band channel can usuallygive
transfer speeds up to 1200 bps or 2400 bps. For
more speed some more advanced modulations are
needed.Those fster communication modulation
methods need to beoptimized for radio
communication (modulationsused in fast telephone
modems are optimized only fortelephone line and
usually perform very poorly on radio channel).Many
commercial radio modems provide a simple wireless
link with the ability to send any message protocol
at any data rate up to the maximum quoted. They
provide a simple means of passing RS232 data
streams with no message overhead in the form of
extra addressing or error-checking codes.Generally
with this kind of modem when there is enough
signal strenght and not much interference, data
supplied to the transmitter input is reproduced at
the distant receiver's output.When operating near
the limit of maximum range will the received
output be prone to data corruptions. The
transmitter is usually switched on by asserting
the transmitter control line. A short delay
(varies depending on device type used) is required
to allow the transmitter and receiver to establish
the link before any data can be sent.When the
transmitter is switched off at the end of a
message, a short burst of noise may occur before
the receiver's mute operates (this noise should be
ignored by the system connected to the
receiver).In radio communications the user may
need to provide error checking and/or addressing
to each mesage if the application requires
guaranteed data integrity.When using radio
communication some for of error checks,error
correction, and re-transmission methods are very
often needed tobe able to get reliable
communication over noisy radio channels.This noise
on radio channel causes transmission errors which
needsto be corrected by mode or handled in some
way by the communicationprotocol itself. The needs
vary depending on the transmission
distance,available signal power, radio band used
and radio modem type.The free space propagation
signal propagation rule for radio communications
is that a 6dB change in transmitter power is
required to double / halve the range. An imperial
'rule of thumb' for in-building operation requires
a 15dB change in path loss capability to change
the range by a factor of two (hhis is a very cruel
law).There are international and national bodies
that allocate frequency bands and issue
authorization to transmit signals. In some
countries, there are bands that are allocated for
public use without the need for any special
authorization. This is an important factor to
consider when selecting a radio modem, since
getting authorization to broadcast information is
often not an easy task. The bands that are
allocated for public use are of particular
interest. The 900 MHz band in the United States
and 2.4 GHz in most European countries are allowed
for spread spectrum communication without any
special authorization (but there are limitations
on the amount of power that one can use to
transmit signals). In Europe there are also
frequencies 418MHz, 433.92MHz and 868-870 Mhz for
low power short distance licence exempt
communications (there are ready made modules
available for this, check your country regulations
if their use is allowed in your country if you
live in Europe).
Soundcard to radio interfacing
Nowadays there exists radio modem software
for PC which uses PC soundcard as the signal
input and output device. Here you can find
information how to interface PC soundcard to
your radio.
- Sound
Card Interface with Tone Keyer - This is
an improved version of the audio interface
commonly used to connect a PC's soundcard to
a transceiver's receive and transmit audio
circuits for PSK31, SSTV, . The usual
version of this type of interface (including
the commercial "RigBlaster")
requires the use of a serial port to provide
PTT (push-to-talk) control for the radio's
transmitter. This version includes an audio
tone detector that keys the transmitter
whenever transmit audio is generated by the
application running on the PC.
- Sound
Card to Radio Interface Home Page
Telephone modem type equipment to radio
interfacing
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