Abstract:
A processor controlled FM receiver implements a technique for reducing adjacent channel interference without requiring additional components. Initially, the technique determines whether a desired channel has interference. If so, signal information on adjacent channels is collected. The desired channel is then shifted away from one of the adjacent channels if the collected signal information indicates that only one of the adjacent channels is appreciably interfering with the desired channel.

Description:
TECHNICAL FIELD 
     The present invention relates to detecting and reducing adjacent channel interference in a radio receiver, and more specifically, to determining the presence of an interfering upper adjacent channel or lower adjacent channel and shifting the frequency of a mixing signal to reduce adjacent channel interference. 
     BACKGROUND OF THE INVENTION 
     Commercial AM (amplitude modulation) and FM (frequency modulation) broadcast bands include a plurality of evenly spaced channels. Broadcast stations are allocated a channel for broadcasts within an assigned frequency range. The power spectrum of a transmission depends on the energy content of a radiated signal at each frequency. While most energy in a transmission can be limited to an assigned channel, some radiated energy will be at frequencies outside the assigned channel. This radiated energy can manifest itself as noise in an adjacent channel. The noise can include ultrasonic noise (USN) and wide band amplitude modulation (WBAM). 
     Assignment of broadcast channels to transmitters has been determined according to geographic location and other factors to minimize interference (noise) between transmission of adjacent channels. However, in highly populated areas there is frequently a limited number of available channels. In this situation, radio receivers must often cope with strong signals on adjacent channels which create signal components in the desired channel. These signal components interfere with the reception of the desired signal. Interference has traditionally been considered objectionable when the total power in an adjacent channel signal is about 30 dB greater than the total power in the desired channel signal. 
     Prior art receivers have detected the presence of objectionable adjacent channel signals by various methods. These methods have included separately tuning each channel and measuring its signal strength, detecting beat components caused by adjacent channels and detecting the difference in signal levels of a narrow band portion of the desired channel and the full band of the desired channel. In these receivers, when adjacent channels were not objectionable, a wide band intermediate frequency (IF) filter was used to maximize desired signal quality. When an adjacent channel was objectionable, a narrow band IF filter was switched into the signal path to eliminate adjacent channel interference. However, introduction of the narrow band IF filter into the signal path introduced distortion into the desired signal and affected its quality. 
     Another prior art approach used an adjacent channel detector with a tri-band filter that filtered the IF signal to derive a lower adjacent channel signal, a desired channel signal and an upper adjacent channel signal. The signal levels of the three channels were then compared. If only one of the adjacent channels had a signal level greater than that of the desired channel, then adjacent channel interference reduction was initiated. This was accomplished by changing the frequency of a mixing signal coupled to an IF mixer so as to move away from the interfering adjacent channel signal. This effectively moved the interfering adjacent channel signal out of the IF pass band. The addition of variable bandwidth active IF filters, switchable IF filters or tri-band filters (to adjust the bandwidth of the desired channel based on the level of interference of adjacent channels) added additional cost to the receiver. Detection circuitry associated with the tri-band filter approach further increased part count and cost of the receiver. 
     SUMMARY OF THE INVENTION 
     The present invention provides a technique for reducing adjacent channel interference in a processor controlled FM receiver. Initially, the technique determines whether the desired channel has interference. If so, signal information on adjacent channels is collected. The desired channel is then shifted away from one of the adjacent channels when the collected signal information indicates that only one of the adjacent channels is appreciably interfering with the desired channel. An advantage of the present invention is that it allows for a reduction in adjacent channel interference, in receivers that include alternate frequency (AF) switching capability, without requiring additional components. 
     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 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a typical prior art superheterodyne receiver; 
     FIG. 2 is a block diagram of a single conversion superheterodyne receiver according to an embodiment of the present invention; 
     FIG. 3 is a block diagram that illustrates the tuner of FIG. 2, according to an embodiment of the present invention, in greater detail; 
     FIG. 4 is a block diagram illustrating the tuner of FIG. 3, according to an embodiment of the present invention, in more detail; and 
     FIGS. 5A-B are flowcharts illustrating a technique for reducing adjacent channel interference, according to an embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A majority of commercial FM receivers are single conversion superheterodyne receivers. These receivers are capable of tuning across the FM broadcast band (approximately 88 to 108 MHz). In these FM receivers, the intermediate frequency (IF) has traditionally been 10.7 MHz with an allocated bandwidth of 200 kHz for each station. Today, many commercial FM receivers are capable of performing alternate frequency (AF) switching under processor control. 
     As will be appreciated by one skilled in the art, an AF switch has traditionally been utilized when the quality of a desired signal is deficient. The channel to switch to is determined by examining radio data system (RDS) information, which is normally transmitted by each broadcasting station. For example, a broadcasting station&#39;s transmission typically includes RDS information about sister stations. A sister station is one which is simultaneously broadcasting the same information, e.g., national public radio (NPR). When the quality of the signal on the desired channel is deficient, the receiver (under processor control) switches to a sister station and obtains signal information on the sister station. If the quality of the sister station is better, the receiver switches to the sister station in a manner that is not detectable by a listener. 
     The present invention is particularly advantageous when used in conjunction with a receiver that is capable of monitoring signal conditions (e.g., ultrasonic noise) and switching to an adjacent channel. For practical reasons, the receiver must be capable of shifting to an adjacent channel, determining the signal strength of the adjacent channel and returning to the desired channel all in a time that is imperceptible to a listener. A processor controlled receiver implementing the present invention must typically accomplish this in about 7 mS or less. In the preferred embodiment, the present invention is implemented in a receiver system which has the ability to determine the level of an adjacent channel signal in such a time frame. Additionally, the receiver system can determine the level of a desired signal as well as noise levels (e.g., USN) of the desired signal. 
     High noise levels (e.g., USN) on the desired channel indicate interference. In response to undesirable noise levels, a programmed processor performs an AF switch to check signal levels of adjacent channels. When one of the adjacent channels is at a level (with respect to the desired channel) that indicates interference with the desired channel, the receiver is tuned away from the interfering channel. If the noise level on the desired channel improves, the receiver is left in its current state. If the noise level did not improve, the receiver is returned to its original setting. 
     FIG. 1 shows a simplified block diagram of a single conversion superheterodyne receiver  100  according to the prior art. Antenna  102  receives radio frequency (RF) signals and supplies those signals to a RF amplifier  104 . RF amplifier  104  provides pre-amplification for the received signal and couples those signals to an FM mixer  106 . FM mixer  106  receives a local oscillator signal from a local oscillator  114 . By varying the frequency of the local oscillator signal, the receiver can be tuned across the FM broadcast band. The FM mixer  106  beats the received signal with the local oscillator signal and provides the mixed signal to IF amplifier  108 . IF amplifier  108 , which includes an IF filter at its input, amplifies the mixed signal and provides an amplified mixed signal to the detector  110 . 
     Detector  110  may include an automatic gain control (AGC) circuit which is coupled to IF amplifier  108  and provides a gain signal to control the gain of IF amplifier  108 . Detector  110  detects the audio signal and provides the audio signal to an audio frequency (AF) amplifier  112 . The audio frequency amplifier amplifies the audio signal and provides an amplified audio signal to a speaker  116 . 
     FIG. 2 depicts a single conversion superheterodyne receiver  200  according to an embodiment of the present invention. While only a single conversion receiver is discussed herein, the techniques according to the present invention are equally applicable to other receivers, e.g., double conversion, triple conversion, etc. Antenna  202  couples a received signal to an RF amplifier  204 . RF amplifier  204  amplifies the received signal and provides the signal to a FM mixer  206 . FM mixer  206  beats the received signal with an oscillator signal provided by tuner  214  and provides a mixed signal. 
     The frequency of the oscillator signal is adjusted responsive to an I 2 C signal provided by a processor  216 . In this context, the term processor may include a general purpose processor, a microcontroller (i.e., an execution unit with memory, etc., integrated within a single integrated circuit) or a digital signal processor. As above, varying the frequency of the oscillator signal allows the receiver to be tuned across the FM broadcast band. FM mixer  206  then provides the mixed signal to a first FM IF amplifier  208 A which amplifies the mixed signal. FM IF amplifier  208 A includes an IF filter at its input. FM IF amplifier  208 A is coupled to a second IF amplifier  208 B. FM IF amplifier  208 B also includes an IF filter at its input. FM IF amplifier  208 B amplifies the mixed signal and provides the mixed signal to a FM detector  210 . Alternatively, both FM IF amplifiers  208 A and  208 B could be contained within a single functional unit. FM detector  210  receives the amplified mixed signal and detects the audio signal. The audio signal is provided to an audio frequency (AF) amplifier  212 . The audio frequency amplifier  212  amplifies the audio signal and provides it to a speaker (not shown). 
     FIG. 3 further illustrates tuner  214  of FIG.  2 . Tuner system  312  includes programmable frequency divider  304 , phase detector  306 , charge pump  308  and programmable divider  310 . Programmable frequency divider  310  receives input from oscillator  318 . Programmable frequency divider  304  and programmable divider  310  are programmed by a processor  216  across an I 2 C bus. A beat signal that is ultimately coupled to FM mixer  206  is dependent upon the value programmed into programmable registers (not shown in FIG. 3) of programmable divider  310  and programmable frequency divider  304 . Programmable frequency divider  304  receives an oscillator signal from crystal oscillator  302 . 
     The oscillator signal is divided, as appropriate, and is applied to the phase detector  306 . Phase detector  306  also receives an input signal from programmable divider  310 . Phase detector  306  provides an output signal to charge pump  308 . Charge pump  308  is coupled to a loop filter  314 . Loop filter  314  is coupled to a tuning network  316 . Loop filter  314  helps to establish the proper transient response and filtering for transient and steady-state operation. Tuning network  316  is coupled to a voltage controlled oscillator (VCO)  318 . VCO  318  produces an output signal whose frequency deviation about a center frequency is proportional to its input voltage. VCO  318  is coupled to programmable divider  310 . 
     Programmable divider  310  divides the output of the VCO  318  and provides it to phase detector  306 . The output of VCO  318  is also provided to a divider  320 . The output (mixing signal) of divider  320  is coupled to FM mixer  206 . As will be appreciated by one skilled in the art, utilizing tuning system  312 , the receiver can be tuned across the FM broadcast band at the direction of processor  216 . This programmable feature of many commercial receivers can be advantageously used to minimize the impact of adjacent channel interference. 
     FIG. 4 illustrates tuning system  312  in more detail. Processor  216  addresses and controls tuning system  312  utilizing an I 2 C bus. Processor  216  is coupled to a main control block  404 . Main control block  404  receives commands from processor  216  and sets appropriate registers ( 304 A and  310 A) of program frequency divider  304  and program divider  310  as directed. Programmable divider  310  can be implemented as a pulse swallow type counter. Utilizing a programmable frequency divider  304  allows for the use of a number of different crystals. Buffer  408  buffers the output of crystal  406  into programmable frequency divider  304 . Alternatively, programmable frequency divider  304  could include a buffer and be directly coupled to crystal  406 . The output of VCO  318  is coupled to the input of programmable divider  310 . 
     Phase detector  306  generally includes both a phase and a frequency detector. As previously disclosed, phase detector  306  receives inputs from both programmable frequency divider  304  and programmable divider  310 , and in response to the inputs provides an output to charge pump  308 . In a typical application, the output of charge pump  308  is provided to a loop filter  314  as previously discussed in conjunction with FIG.  3 . 
     FIGS. 5A-B illustrate an adjacent channel interference reduction routine  500 , according to an embodiment of the invention. Upon entering routine  500 , control transfers to step  502 . In step  502 , if a desired signal level is greater than a value of ten and a USN level of the desired signal is greater than a value of five, control transfers to step  506 . In this context, a desired signal level of ten refers to the value of a digital word. The value of the digital word is determined by reading the voltage level of the desired signal with an A/D converter (internal to processor  216 ). Alternatively, the A/D converter can be external to processor  216 . The voltage reading is then converted to a digital word with a value from zero to sixty-three. In the preferred embodiment, the internal A/D converter is a 6-bit A/D converter with a resolution of 47 mV. 
     Similarly, a USN level of five refers to the value of a digital word. In the preferred embodiment, a detected audio signal, of the desired channel, is coupled through a 70 kHz high pass filter. The USN level, of the desired channel, is then determined by taking a voltage reading of the filtered audio signal using a 3-bit A/D converter. The voltage reading is then converted to a digital word with a value from zero to seven. In the preferred embodiment, the resolution the 3-bit A/D converter is 340 mV. As utilized herein, a particular level refers to the value of a digital word. 
     In step  502 , if the level of the desired signal is not greater than ten or the USN level is not greater than five, control transfers to step  504 . In step  504 , routine  500  causes the receive to remain tuned to the current frequency. From step  504 , control passes to step  536  where the routine  500  returns to the calling program. 
     In step  506 , routine  500  performs an alternate frequency (AF) switch to check the signal levels of the upper and lower adjacent channels. If the level of the lower adjacent channel is greater than the desired channel level plus five, control transfers to step  516 . If the level of the lower adjacent channel is less than the desired channel level plus five, control transfers to step  510 . In step  510 , if the level of the upper adjacent channel signal is greater than the desired channel level plus five, control transfers to step  514 . If the level of higher adjacent channel signal is less than five plus the desired channel level, control transfers to step  512 . In step  512 , routine  500  then directs that the VCO not change frequency (because there is no adjacent channel that is presently interfering). From step  512 , control then transfers to step  536  where routine  500  is exited. 
     In step  514 , since the upper adjacent channel signal is interfering with the desired channel signal the VCO is detuned. This is accomplished by programming programmable divider  310  to a lower frequency (changing the divide by N#). This, in essence, detunes the VCO away from the interfering channel. In the preferred embodiment, the VCO is detuned in about 50 kHz increments (station is tuned by about 25 kHz). From step  514 , control then transfers to step  530 . 
     In step  530 , routine  500  next determines whether the USN level is equal to or better than the standard tuning level. If the USN is less than it previously was, control transfers to step  534 . In step  534 , routine  500  then directs the VCO to remain tuned to the current frequency. From step  534 , routine  500  then proceeds to step  536 . When the USN noise level has increased, in step  530 , control transfers from step  530  to step  532 . In step  532 , routine  500  causes the receiver to return to its original frequency. From step  532 , control passes to step  536  where routine  500  is exited. 
     From step  508 , when the level of the lower adjacent channel is greater than the level of the desired channel plus five, control transfers to step  516 . In step  516 , routine  500  determines whether the level of the upper adjacent channel is greater than five plus the level of the desired channel. If so, control transfers to step  520 . If not, control transfers from step  516  to step  518 . In step  518 , routine  500  causes the VCO to be detuned toward the upper adjacent channel. From step  518 , control transfers to step  530 . As previously discussed, in conjunction with step  530 , routine  500  determines whether the USN level is equal to or better than it previously had been. 
     If detuning the VCO toward the upper adjacent channel has decreased the USN level, control passes from step  530  to step  534 . Otherwise, control transfers from step  530  to step  532 . Step  534  directs the VCO to remain at the current frequency. Step  532  causes the VCO to return to its original frequency. From step  532  or step  534 , control transfers to step  536  and routine  500  is exited. 
     In step  516 , if the level of the upper adjacent channel is greater than the desired channel level plus five, control transfers to step  520 . In step  520 , routine  500  then compares signal magnitudes of the upper adjacent and lower adjacent channels. If the signal level of the channels does not vary by more than a value of five, then control transfers from step  520  to step  522 . In step  522 , routine  500  directs the VCO to remain in its current state. From step  522 , control transfers to step  536  where routine  500  is exited. From step  520 , if the upper and lower adjacent channel signal levels differ in signal strength by more than five, control transfers to step  524 . 
     In step  524 , routine  500  determines whether the upper adjacent channel signal strength is less than the lower adjacent channel signal strength. If the upper adjacent channel signal strength is less than the lower adjacent channel signal strength, control transfers to step  528 . If the upper adjacent channel signal strength is greater than or equal to the lower adjacent channel strength, control transfers to step  526 . In step  526 , the VCO is detuned toward the lower adjacent channel. From step  526 , control then transfers to step  530 . In step  528 , routine  500  directs that the VCO be adjusted toward the upper adjacent channel. From step  528 , control then transfers to step  530 . 
     Utilizing the above-described technique, a receiver can typically improve the signal-to-noise and distortion (SINAD) level on the desired channel. The above-described receiver is especially advantageous in mobile applications, e.g., a receiver in an automobile. In the automotive environment, a receiver may frequently change geographic listening areas. In this situation, having a processor controlled receiver that can optimize listening conditions is desireable. 
     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.