Frequency shifting a carrier between known fixed frequencies to convey digital data. FSK signals can be heard in the HF amateur bands around 14100 kHz, 21150 kHz and in numerous other places in the band.
There are numerous forms of FSK in use on the HF bands where the majority of this activity takes place. Applications include amateur radio, press agencies and meteorological broadcasts.
Normally, FSK modulation is achieved using just two frequencies to represent binary data. For example, a frequency of 2000 Hz may represent a binary 1 (mark) and 1000 Hz a binary 0 (space). This is illustrated above where the nominal carrier frequency of the RF signal is the centre frequency equidistant between the two FSK arms. The energy of the signal alternates between the mark and space frequencies to convey the digital message.
The spectrogram of an off-air FSK signal is shown below. This clearly illustrates how two frequencies can be used to convey binary information. It is even possible to read off the data bits by eye (if you assume a bit value for each tone).
Spectrogram of 2-level Frequency Shift
Keyed signal
Spectrogram obtained using software by Richard
S.Horne and AOR AR7030 Communications receiver.
More than two levels are also used by some advanced systems since more bits per tone can be sent. FSK signals may either be generated as an audio signal (baseband) then transmitted using a conventional SSB or FM transmitter, or a transmitter's carrier may be frequency keyed directly.
If the tones of the FSK transmissions are regarded as symbols, a 2-level FSK signal has 1 bit per symbol. By contrast a 4-level system has 2 bits per transmitted symbol. A multi-level signal is capable of much higher data rates but has the disadvantage that more bandwidth is required. If a fixed bandwidth is available, a multi-level FSK system will experience greater difficulty in resolving the symbols.
The rate at which symbols are sent is the baud rate; this need not be the same as the bit rate supported by the modem. Typical baud rates range from 45 baud up to 300 baud on HF with SSB modulation being used. Data rates of 1200 baud or more are commonly used on VHF narrowband FM as the modulation method.
Modern FSK modems have sophisticated data protocols supporting error correction through automatic repeat request (eg AMTOR) or packet structures allowing use within networks. There are several types of signal in use, some of which are listed in the sections below:
Continuous signals
