Pulse or digital communications – Transmitters – Angle modulation
Reexamination Certificate
1998-07-20
2001-08-14
Le, Amanda T. (Department: 2734)
Pulse or digital communications
Transmitters
Angle modulation
C375S306000, C332S100000, C331S167000, C331S172000, C455S107000
Reexamination Certificate
active
06275539
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a system and method for modulating a frequency signal through frequency-shift keying.
Frequency-shift keying (FSK), sometimes referred to as frequency-shift modulation or frequency-shift transmission, is a form of modulation in which the modulating wave shifts the output frequency between two predetermined values corresponding to digital values 0 and 1. FSK has been used to modulate unique codes or other information onto frequency signals for remotely controlling systems such as vehicle remote keyless entry systems, garage door openers, estate gate openers, home security systems, etc. The unique codes may be encrypted or rolled.
In the prior art, frequency-shift keying has be achieved by installing a varactor in series with the output of an oscillator circuit producing a frequency signal. A varactor is a semiconductor device, such as a diode, in which the capacitance of the device varies with the applied voltage. Thus, applying an analog voltage to the varactor varies the capacitance of the varactor thereby altering the total capacitance of the oscillator circuit and shifting the signal between two different frequency values.
Although this method of frequency-shift keying has been routinely employed, it has shortcomings. Foremost, varactors are relatively expensive and significantly increase the cost of the FSK circuitry. Accordingly, there is a need for a reliable and less expensive system and method for modulating a frequency signal through frequency-shift keying.
SUMMARY OF THE INVENTION
In a disclosed embodiment of this invention, a radio frequency transmitter circuit broadcasts a modulated signal carrying a unique code or packet of information. The inventive transmitter circuit includes an oscillator circuit for producing a signal having a first frequency and a coupling circuit disposed adjacent the oscillator circuit. When the coupling circuit is activated, a reflected impedance, derived from a change in inductance, is reflected from the coupling circuit onto the oscillator circuit. The reflected impedance alters or increases the inductance of the oscillator circuit thereby shifting the signal to a second frequency. The coupling circuit is sequentially activated and deactivated by a controller in a manner to modulate the signal with a unique code or packet of information. The modulated signal is broadcasted through an antenna by the radio frequency transmitter circuit. When captured by a conventional radio frequency receiver, the broadcasted signal is demodulated. If the code is valid, the receiver operates accordingly.
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Brooks & Kushman P.C.
Le Amanda T.
Lear Automotive Dearborn Inc.
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