Abstract:
A method is provided for aligning the center frequency of an infrared transmitter. The method comprises the steps of: (a) providing a voltage-controlled oscillator for driving the infrared transmitter, where the oscillator is adapted to receive a bias voltage from a microprocessor; (b) applying a bias voltage to the oscillator; (c) receiving an output signal from the infrared transmitter into an infrared receiver; (d) determining a frequency associated with the output signal; and (e) adjusting the bias voltage based on the frequency associated with the output signal, thereby aligning the center frequency of the infrared transmitter.

Description:
TECHNICAL FIELD 
     The present invention relates generally to an oscillator circuit associated with a transmitter device and, more particularly, to a method for electronically aligning the frequency of an oscillator circuit that may be used to drive an infrared transmitter. 
     BACKGROUND OF THE INVENTION 
     An infrared transmitter is designed to broadcast a modulated signal about a carrier frequency, and thus operates in conjunction with an oscillator circuit. It is well known to use a programmable phase lock loop (PLL) design to synthesize the frequency of the oscillator circuit. In this case, a microprocessor may be used in conjunction with the phase lock loop circuit to control the frequency of the oscillator circuit. Although PLL-controlled oscillators achieve good control over the center frequency, these types of circuits typically cost more than a conventional LCR-based oscillator. 
     In contrast, conventional LCR-based oscillators provide open loop control over the center frequency. Due to variations in the manufacturing process, the center frequency may vary between oscillators. Without a way to electrically control the center frequency of the oscillator, many well known mechanical alignment techniques are typically incorporated into the manufacturing process. In this way, the desired center frequency of the oscillator is accurately established. Unfortunately, manual alignment further increases the manufacturing costs associated with these types of oscillator circuits. The cost associated with mechanical alignment becomes more critical as the circuits are replicated in order to provide multiple channels within a single application. 
     Therefore, it is desirable to provide a method for electronically aligning the center frequency of an LCR-based oscillator circuit that may be used to drive an infrared transmitter. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method is provided for aligning the center frequency of an infrared transmitter. The method comprises the steps of: (a) providing a voltage-controlled oscillator for driving the infrared transmitter, where the oscillator is adapted to receive a bias voltage from a microprocessor; (b) applying a bias voltage to the oscillator; (c) receiving an output signal from the infrared transmitter into an infrared receiver; (d) determining a frequency associated with the output signal; and (e) adjusting the bias voltage based on the frequency associated with the output signal, thereby aligning the center frequency of the infrared transmitter. 
     For a more complete understanding of the invention, its objects and advantages, refer to the following specification and to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an alignment and temperature compensation system for an infrared transmitter in accordance with the present invention; 
     FIG. 2 is a schematic of a conventional LCR-based oscillator circuit; and 
     FIG. 3 is a schematic of an exemplary LCR-based oscillator circuit that is modified in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An alignment and temperature compensation system  10  embodying features of the present invention is depicted in FIG.  1 . The alignment and temperature compensation system  10  generally includes a product  12  (e.g., a circuit board is used in the transmitter portion of a wireless headphone system) and a tester device  16 . The product  12  further includes an adjustable voltage source  22 , a voltage-controlled oscillator circuit  24 , and at least one infrared transmitter device  14 . The oscillator circuit  24  is adapted to receive a bias voltage from the adjustable voltage source  22 , where the bias voltage determines a portion of the capacitance associated with the oscillator circuit  24 . In this way, adjustments to the bias voltage control the center frequency of the oscillator circuit  24 . The oscillator circuit  24  in turn drives the infrared transmitter device  14 . While the following description is provided with reference to an infrared transmitter, it is readily understood that the broader aspects of the present invention are applicable to other types of wireless transmitter devices. 
     The tester device  16  is provided for electronically aligning the center frequency associated with the infrared transmitter  14 . The tester device  16  generally includes an infrared receiver device  32 , a frequency detector component  34  and a frequency compensation component  36 . A communication link  38  enables the frequency compensation component  36  to effectuate any adjustments in the bias voltage generated by the adjustable voltage source  22 . As will be more fully explained below, a thermistor circuit  26  may optionally be connected to the voltage source  22  in order to improve the temperature stability of the oscillator circuit  24 . 
     In accordance with the present invention, a recently manufactured product enters a test station that houses the tester device  16 . Rather than mechanically aligning the oscillator circuit  24 , power is applied to the product  12 , whereby an output signal is generated from the infrared transmitter  14 . The infrared receiver device  32  is aligned with the product  12  such that it receives the output signal from the infrared transmitter  14 . The frequency detector component  34  determines the frequency associate with the output signal. In view of the desired center frequency, the frequency compensation component  36  calculates an adjustment value for the bias voltage and then communicates this adjustment value via the communication link  38  to the product  12 . 
     In response to the adjustment value, the product  12  is then able adjust the bias voltage generated by the adjustable voltage source  22 . The alignment process may be repeated until the desired center frequency for the product is achieved. It is envisioned that the frequency compensation component  36  may also calculate the offset between the frequency associated with output signal and the desired frequency, and then the product  12  would compute the appropriate adjustment value for the bias voltage. 
     For illustration purposes, a detailed schematic of a conventional LCR-based oscillator circuit  42  is shown in FIG.  2 . The oscillator  42  is adapted to receive a modulated input signal  44 . The output of the oscillator circuit  42  is then used to drive one or more infrared emitter diodes D 2 . An amplification circuit  44  may optionally be used to boost the drive signal to infrared emitter diode D 2 . Although the invention is not limited thereby, an exemplary three-stage amplifier circuit is shown in FIG.  2 . 
     The oscillator circuit is  42  further defined as a Hartley oscillator circuit. The principal components of the Hartley oscillator circuit include a first resistor R 5  and an inductor L 2  which is tapped through the use of a second resistor R 6  to form two inductors. The capacitor portion of the oscillator circuit is derived from a first capacitor C 2  in series with the capacitance value associated with a varactor diode D 1  which collectively are positioned in parallel with a second capacitor C 4 . The nominal capacitance provided by the varactor diode D 1  is fix biased by the values of two additional resistors, R 2  and R 4 . The remainder of the oscillator circuit  42  includes a first transistor Q 1 , another resistor R 7  and two additional capacitors C 6  and C 5 . In this embodiment, the inductor L 2  must be mechanically aligned to set the desired center frequency of the oscillator. Although a Hartley oscillator circuit is presently preferred, it is envisioned that other types of voltage-controlled oscillator circuits fall within the broader scope of the present invention. 
     In FIG. 3, a similar LCR-based oscillator circuit  50  is modified in order to facilitate electronic alignment of the center frequency in accordance with the present invention. The oscillator circuit has been adapted to receive a bias voltage from an adjustable voltage source, but otherwise the basic components and principles of operation are generally as described above. More specifically, resistor R 4  is connected via a filter circuit  52  to a microprocessor  54 . In operation, the microprocessor  54  controls the bias voltage applied to the varactor diode D 1 . By controlling the bias voltage, the microprocessor is able to control the capacitance value across the varactor diode D 1  which in turn determines the center frequency of the oscillator circuit. As result, the microprocessor provides software control over the center frequency of the oscillator and thus eliminates the need for mechanical alignment of the oscillator. The filter circuit  52  is used to smooth the pulse width modulated output signal from the microprocessor  54  into a clean DC bias voltage that can be input into the oscillator circuit  50 . It is envisioned that other types of filtering circuits may be used to achieve the appropriate signal form for inputting into the oscillator circuit  50 . 
     To improve temperature stability, a thermistor circuit  56  may be connected to the microprocessor  54 . The thermistor circuit  56  generates an input voltage signal proportional to the ambient temperature adjacent the oscillator circuit  50 . As will be apparent to one skilled in the art, an appropriate algorithm or look-up table may be incorporated into the microprocessor  54  and used to translate a temperature change into a frequency shift, whereby the bias voltage can be adjusted to achieve the desired center frequency. 
     While the above description constitutes the preferred embodiment of the invention, it will be appreciated that the invention is susceptible to modification, variation, and change without departing from the proper scope or fair meaning of the accompanying claims.