a audio tone is shifted in frequency, e.g. from 1200 to 2200 Hz and transmitted with a standard FM-radio. Advantage: very simple connection by using the microphone and speaker connector of every radio. Disadvantage: the data rate is lower as the channel capability.
The frequency of the RF carrier is directly shifted. This requires a direct access to the modulator of the FM-radio transmitter. The receiver signal has to be tapped directly at the demodulator / FM-discriminator. Some radios with too narrow IF-filters need to be modified before use with FSK. Advantage: much higher transmission rate compared to AFSK. 9600 baud data rate is the maximum reasonable speed which can be used with a standard FM radio channel.
With Packet-Radio on short wave (HF), 300 baud FSK is common. The simplest way is to modulate a SSB-transmitter with a AFSK signal with 200 Hz shift. Which frequencies are actually used makes no difference if the difference is 200 Hz. Packet-Radio does not distinguish between 'Mark' and 'Space', i.e. it makes no difference if the frequencies are swapped.
The transmission speed with Packet-Radio is normally 1200 baud = 1200 bit/sec., this means approx. 150 characters per second. The mode is AFSK, that means one of two audio tones is applied to the microphone input of a common FM radio. Amateur radio uses normally the Bell 202 standard with tone frequencies 1200 and 2200 Hz. Just make a suited cable between radio and packet-radio-controller, which connects the signals 'mikrophone', 'loudspeaker', 'PTT' and ground.
1200 baud technique is widespread ad is used with most digipeaters. For data transmission with CB-radios (FM), 1200 baud AFSK is the optimum.
As data traffic constantly grows, more and more nodes move to faster transmission on the user access frequencies. New digipeaters skip the slow 1200 baud mode and use 9600 from beginning. In Germany (2000), 50% of the user access frequencies use 9600 baud, i.e. 80% of the total data uses the 8-fold speed.
The FSK modulation standard was defined by James Miller, G3RUH. It can be adapted for various radios and gives a maximum yield of speed for a given channel bandwidth.
The G3RUH-modem was developed primarly for amateur satellite communication, but is now used as the standard for terrestric radio links as well.
For 9600 baud, it is necessary to access directly the modulator and demodulator of the radio. In most cases, this requires to open the radio and make some modifications. The audio response must be constant from very low frequencies (10-20Hz) up to about 5 kHz. There must be no distortions in amplitude or phase within the total audio frequency spectrum.
For many FM-radios, modification instructions have been published, which explain in detail how to add the required connections to modulator and demodulator for proper 9600 baud operation.
New FM transceivers are sold '9600-baud-ready' with a 6 pin mini-DIN connector for direct connection to a FSK packet-radio-controller. Take care about the transmit / receive delay (TX-Delay) of the radio! Normally, it takes 100 to 300 milliseconds between transmitter keying and reception of the first valid data byte. This reduces the channel capacity by more than 50%. Good data radios have TX-Delay of 20 ms or less. The ideal transceiver is the TRX4S with less than 1 ms delay.
SYMEK has a large collection of modification instructions for most FM amateur radios. Ask us if you need instructions for your specific radio.
With this speed, the RF bandwidth is about 50 kHz, this is the double of the normal (20/25kHz) channel spacing. The IF filters of standard transceivers are too narrow to let such wide signasl pass without distortion. So, a user speed of 19200 baud did not succeed. For linking two digipeaters on a dedicated frequency, 19200 is often used (with special transceivers). 19200 baud can be decoded using the 1980's TNC2 concept, but the Z80 processor of these packet radio controllers is at its limits with 19200 baud data rate.
To connect the nodes of a radio net, high speed is necessary. The new satellites (launched in 2000) use 38400 baud or faster to downlink their picture data. SYMEK offers special equipment for reception and transmission of very high data rates and the matching high-speed packet-radio controllers (TNC3). TNC2 controllers with Z80 microprocessor cannot decode or transmit data so fast. The only radio with data capabilities of over 19200 baud up to 153kbaud is the SYMEK TRX4S.
With special radio transceivers thers speeds can be realised. In most cases, those very high speed links use frequencies above 1,2 GHz. Amateur radio is a playground for experimenters. Since 1998, fullduplex packet radio links with 1,220 kbit/s (1.2 Mbaud) are operative using TNC3S and inexpensive 10 GHz microwave radio links.