Automatic frequency control system and method for frequency-shift-key data transmission systems

A method of automatic frequency control for an FSK data transmission system, comprising:

• providing a transmitter;
• providing a receiver that includes an oscillator which provides a signal;
• transmitting, using said transmitter, an FSK signal that includes a first FSK signal that has a first frequency, a second FSK signal that has a second frequency which is different than said first frequency of said first FSK signal, and an FSK user data signal, wherein said first and second FSK signals are at predetermined locations in said FSK signal;
• receiving said FSK signal using said receiver, wherein said step of receiving includes mixing said FSK signal with said signal output by said oscillator to convert said FSK signal to a lower frequency spectrum;
• and
• using said first and second frequencies of said first and second FSK signals to adjust the frequency of said signal output by said oscillator to tune said receiver for said FSK user data signal that follows said first and second FSK signals.

An automatic frequency control system for an FSK data transmission system comprising:

• first means for transmitting an FSK signal that includes a first FSK signal, a second FSK signal that is different than said first FSK signal, and a third FSK signal for user data, wherein said first and second FSK signals are located in a preamble that precedes said third FSK signal;
• and
• second means for receiving said first FSK signal, said second FSK signal, and said third FSK signal and converting each of said first FSK signal, said second FSK signal and said third FSK signal to a different frequency, said means for receiving and converting includes a voltage-controlled-oscillator for outputting a signal having a frequency that is related to a control signal, a mixer for combining said signal output by said oscillator with said first FSK signal, said second FSK signal, and said third FSK signal to produce a fourth FSK signal, a fifth FSK signal, and a sixth FSK signal, respectively, with each having a different frequency than said first FSK signal, second FSK signal, and third FSK signal, means for providing a first signal that is representative of the frequency of said fourth FSK signal and a second signal that is representative of the frequency of said fifth FSK signal, means for sampling-and-holding said first signal representative of the frequency of said fourth FSK signal and said second signal representative of the frequency of said fifth FSK signal, and means for averaging said first signal that was sampled by said means for sampling-and-holding with said second signal that was sampled by said means for sampling-and-holding to produce said control signal that is used to adjust the frequency of said signal output by said oscillator, wherein adjustment of the frequency of said signal output by said oscillator tunes said second means for receiving for processing of said third FSK signal.