Abstract:
A trainable transmitter comprises a receiver, a signal generator, and a processor. The receiver receives a signal from a transmitter. The signal generator generates a signal having a frequency related to a frequency control signal supplied to a frequency control terminal of the signal generator. The processor is directly coupled to the frequency control terminal of the signal generator for supplying the frequency control signal and directly coupled to an output terminal of the signal generator for monitoring the frequency of the signal output from said signal generator.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to radio frequency (RF) trainable transmitters that are capable of learning the characteristics of a received RF signal, storing the characteristics in memory, and re-creating and transmitting the learned signal based upon the stored characteristics. 
     RF trainable transmitters have many applications. The primary application is to physically and permanently incorporate the trainable transmitter in a vehicle accessory, such as a visor, rearview mirror, or overhead console, in order to allow the trainable transmitter to be used to learn a garage door opening RF signal for subsequent transmission to the garage door opening mechanism mounted in a garage. As disclosed in U.S. Pat. No. 5,903,226, another application of RF trainable transmitters is to control household lights and appliances. 
     RF trainable transmitters are capable of learning the RF carrier frequency, modulation scheme, and data code of an existing portable remote RF transmitter associated with an existing receiving unit located in the vehicle owner&#39;s garage. Thus, when a vehicle owner purchases a new car having such a trainable transmitter, the vehicle owner may train the transmitter to the vehicle owner&#39;s existing clip-on remote RF transmitter without requiring any new installation in the vehicle or home. Subsequently, the old clip-on transmitter can be discarded or stored. 
     Because the trainable transmitter is an integral part of a vehicle accessory, the storage and access difficulties presented by existent clip-on remote transmitters are eliminated. Some examples of trainable transmitters are disclosed in U.S. Pat. Nos. 5,442,340; 5,479,155; 5,583,485; 5,614,885; 5,614,891; 5,619,190; 5,627,529; 5,646,701; 5,661,651; 5,661,804; 5,686,903; 5,699,054; 5,699,055; and 5,708,415, as well as in U.S. Pat. Nos. 5,903,22 and 5,854,593, all of which are commonly assigned to Prince Corporation. 
     A block diagram representing a typical RF trainable transmitter is shown in FIG.  1 . As described in more detail below, the RF trainable transmitter includes a signal generator  10  for generating the signals to be transmitted and for generating a reference signal used during the training process to identify the RF carrier frequency and to demodulate the received signal. Signal generator  10  operates under control of a microprocessor  16 , which selects the carrier frequency of the signal generated by signal generator  10  by applying a signal frequency control signal to input terminal b of signal generator  10 . Microprocessor  16  may also cause signal generator  10  to modulate the generated signal in accordance with a DATA signal applied to input terminal a of signal generator  10 . When transmitting a modulated signal, signal generator  10  outputs the modulated signal to a transmit amplifier  27  through output terminal d. The modulated signal is thus amplified and passed to an antenna  2  that transmits the RF signal as signal B to a remotely controlled apparatus  6 . 
     When the trainable transmitter is receiving a signal A from an original remote control transmitter  4  during the training mode, the received signal is fed from antenna  2  to an input of a mixer  8 . A reference signal output from terminal c of signal generator  10  is supplied to a second input of mixer  8 . Mixer  8  mixes the reference signal and the received signal A to generate a mixed output signal. The mixed output signal passes through a bandpass filter  12  and a processing circuit  14  to an input of a microprocessor  16  where it is further processed. 
     The RF trainable transmitter also includes user input switches  18  coupled to microprocessor  16  through a switch interface circuit  20 , to allow the user to initiate either training of a signal or transmission of a signal. Additionally, one or more light emitting diodes (LEDs)  22  or some other display or indicator circuit may be coupled to an output of microprocessor  16  to provide feedback information to the user. The RF trainable transmitter also includes a power supply circuit  24  that may be permanently or detachably coupled to the battery of a vehicle. 
     The RF trainable transmitter shown in FIG. 1 typically operates in either a training mode or a transmit mode. To cause the trainable transmitter to enter the training mode, a user presses one of switches  18 . Upon detecting that a switch  18  has been depressed for a predetermined time period, microprocessor  16  enters the training mode. During the training mode, the user activates original remote control transmitter  4  associated with a garage door opening mechanism (e.g., remotely controlled apparatus  6 ) to cause original remote control transmitter  4  to transmit the signal to be learned (A). While signal A is transmitted, microprocessor  16  first identifies the carrier frequency of signal A. 
     To identify the RF carrier frequency of the received signal, microprocessor  16  generates and supplies a frequency control signal (FREQ) to input terminal b of signal generator  10 . Signal generator  10  responds to the frequency control signal by generating an unmodulated RF reference signal having a frequency dictated by the frequency control signal received from microprocessor  16 . Antenna  2  supplies the RF reference signal to mixer  8 , which mixes the reference signal with the received signal A. Mixer  8  outputs a signal including the data code encoded in the received RF signal and having a carrier frequency that is equal to the difference between the carrier frequencies of the received RF signal and the RF reference signal. Narrow bandpass filter  12  is provided to pass a signal only when the carrier frequency of the signal from mixer  8  is 10.7 MHz. The output of bandpass filter  12  is passed through a processing circuit  14  back to microprocessor  16 . In this manner, microprocessor  16  can selectively vary the carrier frequency of the RF reference signal output from signal generator  10  until a signal is detected from processing circuit  14 . When a signal is detected from processing circuit  14 , microprocessor  16  will know that the carrier frequency of the received RF signal is 3 MHz different from the known carrier frequency of the RF reference signal. Once microprocessor  16  identifies and verifies the carrier frequency, it stores the value of the frequency control signal in its internal memory and digitizes and stores the data code that is demodulated by processing circuit  14 . 
     Subsequently, when a user wishes to cause the trainable transmitter to transmit a signal (B) to the garage door opening mechanism  6 , the user presses the associated switch  18  to instruct microprocessor  16  to begin transmitting the RF signal. Microprocessor  16  responds by reading the frequency data from its memory and providing a corresponding frequency control signal to signal generator  10 , while also reading from its memory the data code at the same rate at which it was recorded and supplying this data signal (DATA) to input terminal a of signal generator  10 . Signal generator  10  then generates a carrier signal having the selected frequency and modulates the amplitude of the signal with the data signal supplied from microprocessor  16 . This modulated RF signal (B) is output through antenna  2  to the remotely controlled garage door opening mechanism  6 . It should be noted that a plurality of switches  18  is provided to enable a plurality of signals to be learned and subsequently transmitted. 
     An early version of an RF trainable transmitter is disclosed in U.S. Pat. No. 5,614,885. In this version, signal generator  10  was generally constructed as shown in FIG.  2 . Specifically, signal generator  10  employed a voltage controlled oscillator (VCO)  110 , which generates a sinusoidal signal having a frequency dictated by the analog voltage level applied at its voltage control input terminal. To allow microprocessor  16  to control the voltage level applied to the voltage control input of VCO  110  using a digital value that may easily be stored in its memory, the output of VCO  110  is fed back through a divide-by-128 circuit  111  as well as a divide-by-N circuit  112  and is mixed by mixer  114  with a reference signal of fixed frequency as generated by a reference oscillator  113 . The value of N by which divide-by-N circuit  112  divides the frequency of the signal supplied thereto is provided from microprocessor  16 . The output of mixer  114  is supplied to a frequency discriminator circuit  115  that converts the received signal to a voltage signal that has a level corresponding to the frequency of the signal output from mixer  114 . Thus, by changing the value of N, microprocessor  16  can effectively adjust the voltage level input to VCO  110  and thereby select the frequency of the signal output from VCO  110 . 
     To modulate the signal output from VCO  110 , a switching transistor  116  is provided between the output of VCO  110  and antenna  2 . Switching transistor  116  is switched on and off in response to the data signal supplied from microprocessor  16 . In this manner, an amplitude-modulated (AM) signal may be generated and supplied to antenna  2  for transmission to the garage door opening mechanism  6 . 
     A problem with the implementation shown in FIG. 2 results from the fact that VCO  110  continuously generates signals during a transmit mode even during those periods when switch  116  is nonconductive. When VCO  110  continuously generates a signal, an AC signal is continuously transmitted through the wiring of the circuit, which tends to operate as a secondary antenna thereby transmitting RF signals when no signal is supposed to be transmitted. To better understand this phenomenon, the construction of VCO  110  is described in detail below. 
     FIG. 18 shows the general construction of VCO  110  as used in the circuits shown in FIGS. 2 to  4 . As shown, VCO  110  includes an oscillator  125  that generates a periodic signal having a frequency that varies in proportion to a voltage applied to terminal  126 . Terminal  126  is coupled to oscillator  125  via a resistor  127 . The output of oscillator  125  is applied to the base of a transistor  129 . As an oscillating output signal from oscillator  125  is applied to the base of transistor  129 , transistor  129  generates an oscillating current Ios (see FIG. 19) that in turn is passed through antenna  2  and the other components  130  of the trainable transmitter (see FIG.  19 ). The current draw of a signal generator  10  including such a VCO  110  is in the relatively high range of 110 to 115 mA. As a result of the relatively high oscillating frequency Ios passing through the wires of signal generator  10  and other portions of the trainable transmitter, a residual radiation is generated by VCO  110  during all periods in which it is operating. Consequently, a trainable transmitter constructed utilizing the signal generator shown in FIG.  2  and having a VCO  110  constructed as shown in FIG. 18 exhibits only 3 to 10 dB pulses, because VCO  110  continuously oscillates during such transmission periods. To overcome this problem, the implementation described in U.S. Pat. No. 5,479,155 and shown in FIG. 3 was adopted. 
     As shown in FIG. 3, signal generator  10 ′ similarly includes a VCO  110 , divide-by-128 circuit  111 , divide-by-N circuit  112 , reference oscillator  113 , and a mixer  114 . These components essentially operate in the same manner as described above. The difference in the two signal generators pertains to the manner in which the generated signal is modulated. To overcome the above-mentioned problem with the implementation shown in FIG. 2, a switching transistor  119  is provided that turns VCO  110  on and off in response to the data signal supplied by microprocessor  16 . In this manner, VCO  110  does not generate a signal during the times in which it is not supposed to. However, because the voltage control signal supplied to VCO  110  is dependent upon the feedback of the frequency generated by VCO  110 , a loop filter  117  and sample-and-hold circuit  118  are required to prevent the applied voltage from changing as VCO  110  is selectively turned on and off in a transmit mode. If the applied voltage were to change as VCO  110  is turned on and off, the frequency of VCO  110  would become sporadic. The provision of such a sample-and-hold circuit, however, creates other problems, since the capacitor used in the sample-and-hold circuit is relatively large and cannot be incorporated in an integrated circuit. Thus, to overcome that problem, the configuration described in U.S. Pat. No. 5,686,903 and shown in FIG. 4 was adopted. 
     The configuration shown in FIG. 4 for signal generator  10 ″ is similar to the prior configuration in that VCO  110  is selectively enabled and disabled in response to the data signal supplied from microprocessor  16 . Signal generator  10 ″ differs from the other signal generator implementations, however, in that a unique phase-locked loop circuit  121  is provided to receive the frequency control signal from microprocessor  16  and to generate the appropriate voltage level to apply to the voltage control input terminal of VCO  110 . Phase-locked loop circuit  121  performs this task by comparing the frequency generated by VCO  110  with a fixed reference frequency generated by reference oscillator  113 . To prevent phase-locked loop circuit  121  from responding erratically when VCO  110  is disabled, the data signal supplied to VCO  110  is also supplied to phase-locked loop circuit  121  so as to prevent the phase-locked loop circuit from changing the voltage level applied to VCO  110  during such periods that VCO  110  is disabled. A problem with the configuration shown in FIG. 4 is that phase-locked loop circuit  121  must be custom designed to be responsive to the data signal and therefore is more complicated and expensive to produce. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an aspect of the present invention to solve the above problems by providing a trainable transmitter that requires fewer parts and is therefore less expensive. An additional aspect of the present invention is to provide a trainable transmitter that has a well partitioned design using bipolar components for the RF circuitry and CMOS components for the microprocessor, thereby utilizing each technology where it is best suited. Yet another aspect of the present invention is to provide a trainable transmitter that operates at current levels of 40 mA or less. Still another aspect of the present invention is to provide a trainable transmitter in which the VCO continuously generates a signal during a transmit mode without causing any residual radiation of significant levels in the frequency bands of interest. 
     To achieve these and other aspects and advantages, the trainable transmitter of the present invention comprises a receiver for receiving a signal from an original transmitter, a signal generator for generating a signal having a frequency related to a frequency control signal supplied to a frequency control terminal of the signal generator, and a processor directly coupled to the frequency control terminal of the signal generator for supplying the frequency control signal and coupled to an output terminal of the signal generator for monitoring the frequency of the signal output from the signal generator. 
     The above aspects and advantages may alternatively or additionally be achieved by a trainable transmitter constructed in accordance with a different embodiment in which a transmitter for transmitting an RF signal to a receiver that is responsive to an amplitude-modulated RF signal having a predetermined data code and a carrier frequency within a predetermined frequency band to which the receiver is tuned. The transmitter comprises a signal generator for generating an RF carrier signal having a carrier frequency that is outside the predetermined frequency band of the receiver and a frequency-dividing circuit coupled to an output of the signal generator. When enabled, the frequency-dividing circuit divides the frequency of the RF carrier signal to output a signal having a carrier frequency falling within the predetermined frequency band of the receiver. When disabled, the frequency-dividing circuit passes the RF carrier signal received from the signal generator without dividing its frequency. The transmitter further comprises a control circuit for generating a modulation signal representing the predetermined data code and supplying the modulation signal to the frequency-dividing circuit to selectively enable and disable the frequency-dividing circuit in response to the modulation signal, such that the frequency-dividing circuit generates a modulated RF signal. The transmitter also comprises an antenna coupled to receive the modulated RF signal output from the frequency-dividing circuit and to transmit the modulated RF signal to the receiver. 
     The above aspects and advantages may alternatively or additionally be achieved by a trainable transmitter constructed in accordance with yet another embodiment in which the trainable transmitter comprises a receiver for receiving a signal from a transmitter, a signal generator including a differential VCO for generating a signal having a frequency related to a frequency control signal supplied to a frequency control terminal of the signal generator, and a control circuit coupled to the receiver and to the frequency control terminal of the signal generator for supplying the frequency control signal so as to control the frequency of the signal generated by the differential VCO. 
     The above aspects and advantages may further be achieved by a trainable transmitter constructed in accordance with another embodiment of the present invention whereby the trainable transmitter comprises a receiver for receiving a signal from a transmitter, a signal generator for generating an unmodulated signal having a frequency related to a frequency control signal supplied to a frequency control terminal of the signal generator, and a control circuit coupled to the receiver and to the frequency control terminal of the signal generator for supplying the frequency control signal. The control circuit operates in a training mode and a transmission mode. In the training mode, the control circuit controls the signal generator and monitors a connection to the receiver so as to learn characteristics of the received signal, including its carrier frequency. During the transmission mode, the control circuit controls the signal generator to generate an unmodulated signal having the learned carrier frequency while modulating the generated signal after it is output from the signal generator, such that the trainable transmitter transmits a modulated signal during the transmission mode having a signal pulse variation greater than 10 dB. 
     As described further below, the trainable transmitter of the present invention may be implemented using any one of five different embodiments. 
    
    
     These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is an electrical diagram in block form illustrating a trainable RF transmitter; 
     FIG. 2 is an electrical circuit diagram in block and schematic form illustrating a first conventional signal generator for use in the trainable transmitter shown in FIG. 1; 
     FIG. 3 is an electrical circuit diagram in block and schematic form illustrating a second conventional signal generator for use in the trainable transmitter shown in FIG. 1; 
     FIG. 4 is an electrical circuit diagram in block and schematic form illustrating a second conventional signal generator for use in the trainable transmitter shown in FIG. 1; 
     FIG. 5 is a perspective view of a trainable transmitter of the present invention; 
     FIG. 6 is a fragmentary perspective view of a vehicle interior having an overhead console for housing the trainable transmitter of the present invention; 
     FIG. 7 is a perspective view of a visor incorporating the trainable transmitter of the present invention; 
     FIG. 8 is a perspective view of a mirror assembly incorporating the trainable transmitter of the present invention; 
     FIG. 9 is an electrical circuit diagram in block form illustrating a trainable transmitter constructed in accordance with a first embodiment of the present invention; 
     FIGS. 10A and 10B are signal diagrams showing a signal as generated by the signal generator according to the first embodiment of the present invention, and the same signal as detected by the receiver of a remotely controlled device; 
     FIG. 11 is an electrical circuit diagram in block form illustrating a trainable transmitter constructed in accordance with a second embodiment of the present invention; 
     FIG. 12 is a timing diagram illustrating frequency measurement principles utilized in the present invention; 
     FIG. 13 is an electrical circuit diagram in block form illustrating a trainable transmitter constructed in accordance with a third embodiment of the present invention; 
     FIG. 14 is an electrical circuit diagram in block form illustrating a trainable transmitter constructed in accordance with a fourth embodiment of the present invention; 
     FIG. 15 is an electrical circuit diagram in schematic form illustrating a differential VCO constructed in accordance with the present invention; 
     FIGS. 16A to  16 C are diagrams showing different currents flowing through the differential VCO shown in FIG. 15; 
     FIG. 17 is an electrical circuit diagram in block form illustrating a trainable transmitter constructed in accordance with a fifth embodiment of the present invention; 
     FIG. 18 is an electrical circuit diagram illustrating a conventional VCO used in the conventional signal generators shown in FIGS. 2 to  4 ; and 
     FIG. 19 is a diagram illustrating the current flowing through transistor  128  of the conventional VCO shown in FIG.  18 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 shows a trainable transmitter  143  of the present invention. Trainable transmitter  143  includes three pushbutton switches  144 ,  146 , and  147 ; an LED  148 ; and an electrical circuit board and associated circuits (FIG. 9,  11 ,  13 ,  14 , or  17 ) that may be mounted in a housing  145 . As explained in greater detail below, switches  144 ,  146 , and  147  may each be associated with a separate garage door or other device to be controlled. Trainable transmitter housing  145  is preferably of appropriate dimensions for mounting within a vehicle accessory, such as an overhead console  150  as shown in FIG.  6 . In the configuration shown in FIG. 6, trainable transmitter  143  includes electrical conductors coupled to the vehicle&#39;s electrical system for receiving power from the vehicle&#39;s battery. Overhead console  150  includes other accessories, such as map reading lamps  152  controlled by switches  154 . It may also include an electronic compass and display and/or trip computer (not shown). 
     Trainable transmitter  143  may alternatively be permanently incorporated in a vehicle accessory, such as a visor  151  (FIG. 7) or a rearview mirror assembly  153  (FIG.  8 ). Although trainable transmitter  143  has been shown as incorporated in a visor and mirror assembly and removably located in an overhead console compartment, trainable transmitter  143  could be permanently or removably located in the vehicle&#39;s instrument panel or any other suitable location within the vehicle&#39;s interior. 
     First Embodiment 
     The electrical components of a trainable transmitter constructed in accordance with a first embodiment of the present invention are shown in FIG. 9. A trainable transmitter according to the first embodiment includes many of the elements included in the trainable transmitter discussed above with reference to FIG.  1 . Specifically, the trainable transmitter according to the first embodiment includes a signal generator  200 , an antenna  202 , a transmit amplifier  206 , a mixer  208 , a bandpass filter  212 , a processing circuit  214 , a microprocessor  216 , a plurality of switches  218 , a switch interface circuit  220 , an LED  148 , and a power supply circuit  224  for coupling to a battery  226  of the vehicle in which the trainable transmitter may be installed. As described below, the trainable transmitter according to this first embodiment of the present invention uniquely differs from the trainable transmitters discussed above with reference to FIGS. 1 to  4  in the specific construction of the signal generator. 
     As shown in FIG. 9, signal generator  200  includes a VCO  230 , which preferably generates carrier signals having a carrier frequency in the range of 440 MHz to 880 MHz. The specific frequency of the carrier signal generated by VCO  230  is selected by microprocessor  216 , which generates a frequency control signal that is input to a conventional phase-locked loop circuit  232  in signal generator  200 . Phase-locked loop circuit  232  may be a conventional circuit that is capable of receiving a digital control signal identifying a specified frequency so as to compare the phases of signals output from VCO  230  and a reference oscillator  234 , and output an analog voltage signal that has a voltage level that varies based upon the phase comparison. The output of phase-locked loop circuit  232  is filtered by a low-pass filter  236  and passed through a buffer  238  to the frequency control input terminal  231  of VCO  230 . VCO  230  responds to the voltage level of the analog voltage signal applied to input terminal  231  by varying the carrier frequency of the signal it generates. 
     Like the signal generating circuit shown in FIG. 2, signal generator  200  is constructed such that VCO  230  continuously generates a carrier signal during both the training and transmission modes. By constructing signal generator  200  to operate in this continuous manner, phase-locked loop circuit  232  need not be customized so as to be selectively enabled and disabled during the transmission mode by the amplitude shift key (ASK) data output from microprocessor  216 , which is used to modulate the generated carrier signal. Because phase-locked loop circuit  232  may be a conventional off-the-shelf circuit, the cost of producing the trainable transmitter shown in FIG. 9 may be significantly reduced from the prior version that utilizes the signal generator  10 ″ shown in FIG.  4 . Furthermore, the signal generator shown in FIG. 4 requires current levels in the range of 110 to 115 mA, while standard phase-locked loop circuits are available, however, that are optimized for low current applications that have significantly lower current level requirements. Such standard phase-locked loop circuits operate with currents as low as 20 mA and even as low as 2 mA, such as the 0.8 or 1.06 MHz phase-locked loop circuit, part No. LMX2316 available from National Semiconductor. 
     To further reduce any adverse effects of any residual radiation generated by signal generator  200  during those periods in the signal transmission mode between transmitted pulses, VCO  230  is constructed to generate RF carrier signals having carrier frequencies outside the frequency band to which the intended receivers of the remotely controlled equipment are tuned. Specifically, VCO  230  generates signals in a first frequency band of 440 MHz to 880 MHz, whereas garage door opener receivers are narrowly tuned to frequencies in a second band of 220 MHz to 440 MHz. Thus, any residual radiation that is generated by signal generator  200  is in a frequency range outside the frequency bands of the intended receivers. Therefore, the residual radiation will not interfere with the reception by those receivers of a signal transmitted within the frequency bands to which they are tuned. 
     In order for the signal generator  200  to generate a modulated RF signal to which a receiver having a frequency reception band in the typical 220 MHz to 440 MHz range will respond, signal generator  200  includes a divide-by-2 circuit  240  that is coupled between the output of VCO  230  and transmit amplifier  206  and mixer  208 . When divide-by-2 circuit  240  is enabled and VCO  230  generates a carrier signal having a frequency in the range of 440 MHz to 880 MHz, signal generator  200  will output a signal having a carrier frequency in the range of 220 MHz to 440 MHz. 
     During a transmission mode, the carrier signal generated by VCO  230  is modulated by applying the data code signal output from microprocessor  216  to an enable/disable input port  242  of divide-by-2 circuit  240 . In this manner, the divide-by-2 circuit is selectively enabled and disabled in response to the data signal supplied from microprocessor  216 . The modulated signal output from divide-by-2 circuit  240  is a frequency-modulated signal similar to that shown in FIG.  10 A. Because the receiving bandwidth of most receivers in garage door openers and other remotely operated devices are relatively narrow and fall within the 220 MHz to 440 MHz frequency range, the frequency-modulated signal generated by signal generator  200  would appear to the receiver circuitry as the signal shown in FIG. 10B, whereby the frequency component that is twice that of the tuned frequency is effectively filtered from the signal. Thus, the receiver will see a signal that is effectively amplitude modulated with the data code to which it is to respond and which has a carrier frequency within the frequency band to which the receiver is tuned. 
     If it is desired to only transmit an amplitude-modulated signal from antenna  202 , the data signal from microprocessor  216  may additionally be applied to an enable/disable terminal of transmit amplifier  206 , such that the transmit amplifier is disabled during those periods in which the divide-by-2 circuit  240  is disabled, and would otherwise transmit a signal at a frequency twice that to which the receiver is tuned. 
     The first embodiment may also be constructed using a tunable antenna, such as that disclosed in U.S. Pat. No. 5,699,054. Because such a tunable antenna can be tuned to a relatively narrow bandwidth, the antenna can be tuned to further suppress the transmission of the generated signal when it has a frequency twice that to which the receiver is tuned. 
     While the first embodiment is described above as utilizing a VCO  230  that generates signals having frequencies twice that of which an intended receiver may respond, any VCO may be utilized that generates signals having frequencies that are any multiple of the intended transmission frequency so long as a frequency divider circuit is utilized that divides the frequency of the signal generated by the VCO by that multiple. 
     Because the signal generator of the first embodiment is constructed to respond to the same frequency control signals and data signals as supplied by a microprocessor of the prior trainable transmitters, microprocessor  216  may be programmed to function in the same manner as those of the prior trainable transmitters described in the U.S. patents identified above. 
     Second Embodiment 
     FIG. 11 shows a trainable transmitter constructed in accordance with a second embodiment of the present invention. The trainable transmitter of the second embodiment is similar to that of the first embodiment except for the construction of signal generator  300  and the programming and configuration of microprocessor  316 . As described below, signal generator  300  does not include any type of phase-locked loop circuit at all, but rather the frequency synthesis is performed by microprocessor  316 . 
     To select and adjust the frequency of the signal generated by VCO  330 , microprocessor  316  and a digital-to-analog converter  336  provide an adjusting analog voltage to the VCO. This may be done by storing a voltage on a capacitor of digital-to-analog converter  336  and then allowing the microprocessor to adjust the stored voltage up and down by small selectable increments. The analog signal output from digital-to-analog converter  336  is applied to the frequency control terminal  332  of VCO  330 . VCO  330  is preferably configured to generate signals having carrier frequencies anywhere within the 220 MHz to 440 MHz frequency band. 
     Because the same analog voltage for the frequency control signal will not necessarily always result in a signal generated by VCO  330  having the same frequency due to variations in operating temperature, it is desirable to have microprocessor  316  monitor the frequency of the signal generated by VCO  330  so as to make adjustments to the frequency control signal and thereby adjust the frequency of the generated signal when necessary. To enable microprocessor  316  to monitor the frequency of the signal output from VCO  330 , a feedback signal is passed through a prescaler circuit  338  to an input port  318  of microprocessor  316 . Prescaler  338  may be a frequency-dividing circuit as described in more detail below. 
     There are basically two ways for microprocessor  316  to measure the frequency of the signal received at its input terminal  318 . The first method is to measure the time period of a cycle of the signal applied to terminal  318 . To increase the accuracy of such a measurement, a number of such measurements may be taken and then averaged. 
     A second and more preferred technique for measuring frequency is to count the number of cycles in a predetermined time period, hereinafter referred to as “the gate time.” The frequency is then determined by dividing the number of counts by the gate time. Because the number of counts is an integer, the accuracy of the frequency measurement is inversely proportional to the gate time (GATE). Because it is advantageous to first divide the frequency of the signal generated by VCO  330  using prescaler circuit  338 , microprocessor  316  must multiply the frequency of the signal applied to terminal  318  by the value (PRESCALE) at which prescaler circuit  338  divides the frequency of the signal output from VCO  330 . Thus, the accuracy of the frequency measurement is equal to 1/(GATE PRESCALE). While it would appear that to obtain the most accurate measurement one would wish to increase the gate time as long as possible, longer gate times decrease the responsiveness of microprocessor  316 . Therefore, tolerances are established for the accuracy of the measurement, as needed for the trainable transmitter to effectively assimilate and reproduce a learned signal. 
     During the training mode whereby the trainable transmitter is receiving a signal and signal generator  300  is required to generate a reference signal to apply to mixer  208 , the frequency tolerance of the system ±500 kHz. To maintain the output signal of VCO  330  within ±500 kHz of the desired frequency, the frequency of the output signal should be measured within ±50 kHz or 100 kHz. Knowing that the frequency measurement accuracy is equal to 1/(GATE PRESCALE), the gate time for obtaining measurement within 100 kHz is 320 μsec when the prescaler is a divideby-32 circuit (i.e., PRESCALE=32). Thus, microprocessor  316  is programmed to count the number of cycles of the signal applied to input terminal  318  occurring within a 320 μsec period in order to determine the frequency during a training mode. Microprocessor  316  may monitor the frequency by continuously taking measurements of the frequency and thereby adjust the digital value of the frequency control signal to adjust the analog voltage applied to the frequency control terminal  332  of VCO  330 , which in turn adjusts the frequency of the signal output from signal generator  300 . 
     According to the embodiment shown in FIG. 11, a modulated signal is obtained by applying the data code to an enable/disable terminal  334  of VCO  330 . The data code may, for example, have a modulation frequency of 25 kHz. As a result of the modulation, which occurs during a transmit mode, microprocessor  316  cannot simply count the number of cycles occurring in a predetermined gate time of, for example, 320 μsec. For a 25 kHz data signal that is at a logic high state 50 percent of the time, the VCO may be turned continuously on for as little as a 20 μsec period. A 20 μsec gate time only provides a 1.5 MHz accuracy. Therefore, given the embodiment illustrated in FIG. 11, a different frequency measurement technique must be used to measure and monitor frequency during a signal transmission mode. Because microprocessor  316  will know from the data signal when VCO  330  will be transmitting and when it will not, microprocessor  316  may limit its measurements to those periods of time in which VCO  330  is transmitting. Thus, for example, microprocessor  316  may limit its measurement to the 20 μsec gate times during which VCO  330  may be transmitting. 
     To increase the accuracy of its frequency measurement, microprocessor  316  may accumulate the counted cycles for a plurality of samples taken over a plurality of such gate times. A problem arises, however, due to the accuracy of the measurement technique that any inaccuracies of measurement occurring during any one 20 μsec sample will also accumulate. For example, as shown in FIG. 12A, when the number of cycles occurring within a gate time are not exactly equal to an integer value, the resulting error is multiplied by the number of samples accumulated for the measurement. A solution to this problem is to slightly vary the gate time for each sample in a small but consistent way. Thus, as shown in FIG. 12B, the number of cycles counted during each gate time will vary thereby eliminating the accumulation of any errors in the measurement occurring during any one gate time sample. In practice, the gate times are staggered by one instruction cycle of the microprocessor. The stagger is equal to 4 divided by the CPU oscillator frequency. 
     By staggering the gate times as discussed above, frequencies may be measured within the frequency tolerances for the device, except in situations in which the frequency of the signal output from VCO  330  has a harmonic relationship to the amount of stagger used. For example, if a 10 MHz signal is applied to terminal  318  and the CPU is running at 10 MHz, the sampling points will line up with the measured frequency thereby causing an accumulation of error of each sample. FIG. 12C illustrates the nature of the problem. The 10 MHz signal has a cycle time of 100 μsec. A microprocessor operating at 10 MHz has an instruction cycle, one instruction per 400 μsec. Thus, each gate is staggered by 400 μsec. Assuming then that the microprocessor measures 22 counts during the first gate, it would then measure 18 counts during the second gate, 14 counts during the third gate, and 10 counts during the fourth gate. Thus, the accumulated counts for these three gates would be 64. If, however, the signal received at input terminal  318  is just under 10 MHz, one less cycle would be counted in each of the three gate periods thereby resulting in an accumulated count of 60. Such a change in count values may not accurately reflect the actual difference in the frequencies applied at input terminal  318 . Because there are certain frequencies within the 220 MHz to 440 MHz band that are forbidden for transmission of signals and because there are certain frequencies that are very likely candidate frequencies for garage door opener signals, a solution to the synchronization problem discussed above is to select a microprocessor having a frequency that is harmonically related to a frequency in one of the bands that are forbidden or otherwise unlikely frequencies for a garage door opener transmitter. Thus, microprocessor  316  is preferably selected to have an operating frequency of 17.100 MHz. 
     Third Embodiment 
     FIG. 13 shows a trainable transmitter constructed in accordance with a third embodiment of the present invention. The third embodiment combines aspects of the first and second embodiments. Specifically, microprocessor  416  is used to directly monitor and control the frequency of VCO  230  in a manner similar to the second embodiment. Signal generating circuit  400 , however, includes a VCO  230  that operates in the 440 MHz to 880 MHz band, as well as a divide-by-2 circuit  240  that selectively divides the frequency of the signal output by VCO  230  in response to the data signal applied to an enable/disable terminal  242  of circuit  240 . By combining the aspects of the first and second embodiments, the problems with the second embodiment concerning frequency measurement during a signal transmission mode may be avoided. This is because VCO  230  is intended to continuously transmit at the selected frequency during the signal transmission mode, with the modulation being performed by selectively enabling and disabling divide-by-2 circuit  240  rather than VCO  230 . Thus, microprocessor  416  may measure the frequency of the signal output from VCO  230  over gate times of the same duration both during the training and signal transmission modes. 
     Fourth Embodiment 
     FIG. 14 shows a trainable transmitter constructed in accordance with a fourth embodiment of the present invention. The trainable transmitter shown in FIG. 14 is similar to the second embodiment shown in FIG. 11, with the exception that VCO  330  is replaced with a differential VCO  430  that is constructed as shown in FIG. 15 as described further below. Additionally, the trainable transmitter of the fourth embodiment does not turn differential VCO  430  on and off as does the trainable transmitter of the second embodiment. Instead, the amplitude-shift-key data from microprocessor  316  is used to selectively enable and disable a last stage of transmit amplifier  206  and a first automatic gain control stage  406  of the transmit amplifier. Thus, according to the fourth embodiment, the signal generated by differential VCO  430  is modulated by keeping differential VCO  430  continuously oscillating, while more effectively modulating the signal using the first and last stages of the transmit amplifier. 
     As shown in FIG. 15, VCO  430  is configured as a differential VCO that includes an oscillator  432  that is similar to oscillator  125  shown in the conventional VCO  110  (FIG.  18 ), with the exception that a central tap in the inductor is grounded in oscillator  432 . Consequently, scillator  432  outputs two oscillating signals of opposite phase having a frequency corresponding to the voltage applied at terminal  431 . Oscillator  432  is coupled to terminal  431  via a resistor  434 . The two opposite phase signals generated by oscillator  432  are passed through coupling capacitors  442  and  440  to the bases of two differential transistors  436  and  438 , respectively. The drains of transistors  436  and  438  are commonly coupled to ground through a resistor  448 , while the sources of each of transistors  436  and  438  are respectively coupled to resistors  444  and  446 . The opposite ends of resistors  444  and  446  are commonly coupled to a positive voltage source. 
     With the arrangement shown in FIG. 15, differential oscillator  430  draws a constant current Ios as illustrated in FIG. 16A, while still generating oscillating current output signals Iout and Iout, which correspond to the oscillating current I 1  and I 2 , respectively, as illustrated in FIGS. 16B and 16C. Because currents  11  and  12  are sinusoidal and of opposite phase, the surn of currents I 1  and I 2  always remains constant and hence current Ios is always constant. Because current Ios remains constant, no residual radiation is generated by the wires through which Ios flows. 
     Because differential VCO  430  has such a low residual radiation, a trainable transmitter such as that shown in FIG. 14 may be constructed whereby the VCO is allowed to continuously oscillate during a transmit mode while the modulation is performed at the first and last stages of the transmit amplifier. A trainable transmitter so constructed can produce pulses in excess of 50 dB during the transmit mode. This represents a significant improvement over the 3 to 10 dB pulses produced by the trainable transmitter described above in FIGS. 1 and 2. Additionally, differential VCO  430  draws significantly lower levels of current thereby reducing any drain on the vehicle&#39;s battery. Another advantage to having VCO  430  continuously generate a signal during the transmit mode is that microprocessor  316  can more readily measure the frequency without resorting to the sampling techniques described above with respect to the second embodiment shown in FIG.  11 . 
     Fifth Embodiment 
     FIG. 17 shows a fifth embodiment of the trainable transmitter of the present invention. The trainable transmitter according to the fifth embodiment is similar to the first embodiment except that VCO  230  of the first embodiment is replaced with a differential VCO  430  and divide-by-two circuit  240  is eliminated from the fifth embodiment. According to the fifth embodiment, VCO  430  is configured to generate signals having wavelengths within the range to which associated receivers will respond. Due to the low residual radiation produced by VCO  430 , VCO  430  is controlled to continuously generate a signal during a transmit mode, while the generated signal is modulated at the first and last stages  206  and  406  of the transmit amplifier. In this regard, the fifth embodiment is very similar to the fourth embodiment. The fifth embodiment differs, however, in that a standard phase-locked loop circuit  232  is employed to monitor and vary the frequency of the signal generated by VCO  430  in a manner similar to that described above with respect to the first embodiment of the present invention. 
     Although the above embodiments have been described for trainable transmitters generally used for learning signals received from garage door opener transmitters and subsequently transmitting the learned signals, it will be appreciated that the trainable transmitters may also be programmed and used for receipt of other signals, such as remote keyless entry (RKE) signals. Further, the trainable transmitters may be connected to a vehicle bus for communicating with other vehicle accessories in response to such received signals. Moreover, other accessories may then instruct the trainable transmitter to transmit a particular signal. Additionally, the trainable transmitter of the present invention may be used to learn and retransmit codes in accordance with a rolling code algorithm as described in U.S. Pat. No. 5,661,804. Further, the trainable transmitter of the present invention may be used to receive signals from various vehicle parameter sensors, such as tire pressure sensors as disclosed in U.S. Pat. No. 5,661,651. 
     The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the Doctrine of Equivalents.