Abstract:
Station scan circuitry for a radio-frequency receiver and corresponding methods are disclosed that efficiently determine the presence of a station on available channels. The station scan circuitry includes circuitry that determines if the signal power on a given channel exceeds a threshold value. Additional circuitry compares the channel signal strength and the adjacent channel signal to determine if a ratio of the two exceeds a threshold level. If both the signal power and the signal strength ratio are sufficient, the station scan circuitry indicates that a station has been found. To make the signal strength comparison, the station scan circuitry includes circuitry for determining a post-filter signal strength and a pre-filter signal strength for the received signal.

Description:
This application is related to the following U.S. patent applications that have been filed concurrently herewith and that are hereby incorporated by reference in their entirety: Ser. No. 09/265,663 filed on Mar. 10, 1999, entitled “Method and Apparatus for Demodulation of Radio Data Signals” by Eric J. King and Brian D. Green; Ser. No. 09/265,659 filed on Mar. 10, 1999, entitled “Method and Apparatus for Discriminating Multipath and Pulse Noise Distortions in Radio Receivers” by James M. Nohrden, Brian D. Green and Brian P. Lum Shue Chan; Ser. No. 09/265,752 filed on Mar. 10, 1999, entitled “Digital Stereo Recovery Circuitry and Method For Radio Receivers” by Brian D. Green; Ser. No. 09/414,209 filed on Oct. 7, 1999, which claims the benefit of provisional application Ser. No. 60/123,634 filed on Mar. 10, 1999, entitled “Quadrature Sampling Architecture and Method For Analog-To-Digital Converters” by Brian P. Lum Shue Chan, Brian D. Green and Donald A. Kerth; and Ser. No. 09/265,758 filed on Mar. 10, 1999, which has issued as U.S. Pat. No. 6,225,928 on May 1, 2001, entitled “Complex Bandpass Modulator and Method for Analog-to-Digital Converters” by Brian D. Green. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to circuits for station scan functionality in a radio receiver. More specifically, the present invention relates to techniques for conducting a station scan in a digital receiver. 
     2. Description of the Related Art 
     In a given geographic area, numerous stations may be broadcasting radio frequency (RF) signals on different channels. These RF signals may be AM or FM signals and may include desired program information. A radio receiver present within this geographic area, either fixed or mobile, will attempt to receive and recover the program information being broadcast by the stations. Often it is desirable for the radio receiver to have the ability to scan for the presence of stations in the geographic area. For example, when a person is traveling in an automobile through an unfamiliar geographic area, that person will likely not know the channels on which stations are broadcasting program information. Such station scan to capabilities, however, often require a radio receiver to include additional circuitry that may add undesirable costs and complexity to the radio receiver. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, station scan circuitry efficiently determines the presence of a station on available channels. Channels are analyzed to determine if the signal power on a given channel exceeds a threshold value. A comparison is also made between the desired channel signal strength and the adjacent channel signal strength. If the signal power and the ratio of signal strengths exceed selected threshold levels, the station scan circuitry indicates that a station has been found. To make the signal strength ratio comparison, the station scan circuitry makes a comparison of the post-filter signal strength and the pre-filter signal strength for the received signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an embodiment for an intermediate frequency (IF) AM/FM radio receiver 
     FIG. 2 is a block diagram of an embodiment for the digital receiver within the IF AM/FM radio receiver 
     FIG. 3 is a block diagram of an embodiment for station scan circuitry according to the present invention. 
     FIG. 4 is a flow diagram for a station scan algorithm according to the present invention. 
     FIG. 5 is a graphical representation of a relative signal strengths for an example received signal. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a block diagram is depicted for an embodiment of an intermediate frequency (IF) AM/FM radio receiver  150 . A frequency converter circuitry  102  converts a radio frequency (RF) signal  110  received from the antenna  108  to an IF frequency  112 . The frequency converter circuitry  102  utilizes a mixing signal  114  from a frequency synthesizer  104  to perform this conversion from the RF frequency range to the IF frequency range. Control circuitry  106  may apply a control signal  117  to frequency synthesizer  104  to choose the mixing signal  114  depending upon the station or channel that is desired to be received by the IF receiver  150 . The digital receiver circuitry  100  processes the IF signal  112  and produces desired output signals, for example, audio output signals  118  and data output signals  120 , which may be radio data signal (RDS) information. These output signals may be provided to interface circuitry  122  and output to external devices through interface signals  124 . The control circuitry  106  may communicate with the digital receiver circuitry  100  through signals  116  and may communicate with the interface circuitry  122  through signals  121 . In addition, control circuitry  106  may communicate with external devices through the interface circuitry  122 . 
     FIG. 2 is a block diagram of an embodiment for the digital receiver  100 . The IF input signal  112  is amplified by a variable gain amplifier (VGA)  202 . The output of the variable gain amplifier (VGA)  202  may be filtered with anti-aliasing filters if desired. Sample-and-hold (S/H) circuitry  204  samples the resulting signal and produces a real or in-phase (I) output signal and an imaginary or quadrature (Q) output signal. The Q signal is related to the I signal by being 90 degrees out of phase with respect to the I signal. The analog-to-digital converter (ADC) circuitry  206  processes the I and Q signals to form an I digital signal  220  and a Q digital signal  222 . The ADC circuitry  206  may be, for example, two fifth order delta-sigma ADCs that operate to convert the I and Q signals to one-bit digital I and Q data streams  220  and  222 . The digital output signals  220  and  222  of the ADC circuitry  206  are passed through digital decimation filters  208  to complete channelization of the signals and to produce decimated I data signal  224  and Q data signal  226 . The decimation filters  208  may also remove quantization noise caused by ADC  206  and provide some anti-aliasing filtering. 
     Demodulation of the decimated I and Q data signals may be performed by AM/FM demodulator  210 . The demodulator  210  may include, for example, a CORDIC (COordinated Rotation DIgital Computer) processor that processes the digital I and Q data streams  224  and  226  and outputs both angle and magnitude data for the I and Q digital data signals. For FM demodulation, the demodulator  210  may also perform discrete-time differentiation on the angle value outputs. The demodulated signal  211  may be further processed by signal conditioning circuitry  214 , which may also receive signal  225  from the decimation filter circuitry  208 . The signal conditioning circuitry  214  may provide any desired signal processing, including for example detecting weak signal conditions, multi-path distortions and impulse noise and making appropriate modifications to the signals to compensate for these signal problems. 
     The stereo decoder  216  processes the demodulated signal  211  to decode the left and right channel information from the multiplexed stereo signal and to provide the desired audio output signals  118 . The signal conditioning circuitry  214  provides signal  215  to the stereo decoder  216  to control the output of the stereo decoder depending upon the processing performed by the signal conditioning circuitry  214 . The stereo decoder  216  may also provide additional signal processing as desired. The demodulated signal  211  may also be processed by a data decoder  200  to recover data from the FM multiplex signal using, for example, a synchronous digital demodulator. The output of the data decoder  200  provides the desired data output signals  120 , which may be, for example, RDS clock and data signal information. 
     FIG. 3 is a block diagram of an embodiment for station scan circuitry  300 . The decimation filter circuitry  208  may include a first filter (F 1 )  302 , which may for example be a finite response filter (FIR), cascaded with a second filter (F 2 )  304 , which may for example be an infinite response filter (IIR). The filter (F 1 )  302  may receive the digital I and Q data signals  220  and  222  and produce filtered I and Q signals  310  and  312 . In turn, filter (F 2 )  304  may receive the filtered I and Q data signals  310  and  312  and produce the decimated I and Q data signals  224  and  226 . The I and Q signals  310  and  312  from filter (F 1 )  302  may be the signal  225  provided from the decimation filters  208  to the signal conditioning circuitry  214 . The I and Q signals  224  and  226  from filter (F 2 )  304  are provided to the AM/FM demodulator  210 , which as depicted is a CORDIC demodulator. The CORDIC AM/FM demodulator  210  outputs a demodulated signal  211 , which includes a phase value (φ)  317 , a magnitude value (mag)  314 , and a multiplexed signal ( 315 ) that is the result of differentiating the phase value. It is noted that CORDIC AM/FM demodulators have been used in prior devices and are well-known. It is also noted that for the purpose of the present invention, as discussed below, a demodulator or a CORDIC demodulator is not required, as long as an indication of the power level associated with the desired channel signal is provided. 
     The station scan circuitry  300  is within the signal conditioning circuitry  214 . Magnitude determination circuitry (MAG)  301 , which may be a CORDIC processor, converts the I and Q data signals  310  and  312  to a magnitude value (adj 13  mag)  316 . The magnitude value (adj 13  mag)  316  and the magnitude value (mag)  314  are provided to the signal strength determination circuitry  303 , which is within the station scan circuitry  300 . 
     As discussed in more detail with respect to FIG. 5 below, the magnitude value (adj_mag)  316  represents the magnitude of the signal strength where the adjacent channel signals  522  and  532  have only been slightly filtered out. The magnitude value (mag)  314  represents the magnitude of the signal strength where the adjacent channel signals  522  and  532  have been mostly filtered out. The signal strength determination block  303  determines whether a station has been found and outputs an appropriate station found signal  320 . This station found signal  320  may be utilized by other circuitry within the signal processing circuitry  214  or may be provided as an output to other circuitry within the digital receiver  100  or within the AM/FM radio receiver  150 . For example, station found signal  320  may be within signals  116  that are communicated between the digital receiver  100  and the control circuitry  106 . It is noted that magnitude value (mag)  314  and the magnitude value (adj_mag)  316  represent indications of the power level for the desired channel signal and the adjacent channel signals, respectively. These power level indications may be obtained by other techniques as desired for a particular implementation or application. 
     Referring now to FIG. 5, a graphical representation is depicted showing, as an example, relative signal strengths for the IF signal  112 . The y-axis  504  represents magnitude, and the x-axis  502  represents frequency in MHz. The signal strength at the desired channel (e.g., 100.1 MHz)  516  is represented by signal  518 . Dotted line  510  is included as a reference to provide an indication of the relative signal strength level for the desired channel signal  518 . It is noted that the desired channel  516  is dependent upon the current channel setting for the radio receiver  150 . The two channels immediately next to the desired channel  516  are adjacent channel (e.g., 99.9 MHz)  517  and adjacent channel (e.g., 100.3 MHz)  520 . The signal strengths for adjacent channels  517  and  520  are represented by signals  532  and  522  respectively. The channels that lie two channel widths from the desired channel  516  are deemed the alternate channels, for example alternate channel  524 . The signal strength for alternative channel  524  is represented by signal  526 . In the example depicted, the adjacent channel signal  532  has a smaller signal strength than the desired channel signal  518 . The alternate channel signal  526  has a larger signal strength than the desired channel signal  518 . And the adjacent channel signal  522  has a much larger signal strength than the desired channel signal  518 . 
     The filter (F 1 ) circuitry  302  and the filter (F 2 ) circuitry  304 , which may include FIR and/or IIR filters cascaded together, act to suppress undesired channels. The dotted lines  512  and  514  represent the corners for the filter circuitry  302  and  304 , such that the desired channel signal  518  is isolated from the other signals in the IF signal  112 . The line  506  represents the signal rejection provided by only the filter (F 1 ) circuitry  302 . In contrast, the line  508  represents the signal rejection provided by the filter (F 1 ) circuitry  302  in combination with the effect of the filter (F 2 ) circuitry  304 . The I and Q data signals  310  and  312  would, therefore, be filtered only by the signal rejection associated with line  506 . The decimated I and Q data signals  224  and  226  output by the decimation filter circuitry  208  would be filtered by the signal rejection associated with line  508 . The rejection level  528  between the desired channel  516  and the adjacent channel  520  is the adjacent channel selectivity for the digital receiver. The rejection level  530  between the desired channel  516  and the alternative channel  524  is the alternative channel selectivity. 
     Referring back to FIG. 3, the signal (adj_mag)  316  represents the magnitude of the IF input signal  112  with filtering provided only by the filter (F 1 )  302 . In FIG. 5, this correlates to the signal  112  having been filtered by the signal rejection associated with line  506 . The signal (mag)  314  represents the magnitude of the IF input signal  112  with filtering provided by both the (F 1 ) filter circuitry  302  and the filter (F 2 ) circuitry  304 . In FIG. 5, this correlates to the signal  112  having been filtered by the signal rejection associated with line  508 . Because the signal (adj_mag)  316  will be influenced more by the adjacent channel signals  522  and  532  than the signal (mag)  314 , the signal (adj_mag)  316  is associated with the magnitude of the adjacent channel signals  522  and  532 . Because the signal (mag)  314  will be influenced less by the adjacent channel signal  522  than the signal (adj_mag)  316 , the signal (mag)  314  is associated with the magnitude of the desired channel signal  518 . 
     To determine whether a station has been located, the signal strength determination circuitry  303  may initially determine if the signal level for the magnitude signal (mag)  314  at the current desired channel  516  exceeds a power threshold. This power threshold value may be set such that the signal power level for the magnitude signal (mag)  314  at the desired channel  516  is higher than any reasonably strong leakage signal from alternate channels. Alternatively, if spurious noise leakage is typically worse than alternate channel leakage, then the power threshold value may be selected or programmed based upon a reasonable amount of expected spurious noise. Typically, the power threshold value selected will depend upon a trade-off between the false detection of stations and no detection of stations. In addition, the power threshold value may be programmable so that it is loaded into the digital receiver  100  by external control circuitry  106 . 
     Once a station is found that exceeds the power threshold value, the signal strength determination circuitry  303  may utilize a comparison of the relative signal strengths or power levels for the desired channel magnitude signal (mag)  314  and adjacent channel magnitude signal (adj_mag)  316  to determine whether a signal of adequate strength has been received for a particular station. If these two magnitudes are close together, the signal strength determination block may conclude that the strength of the desired channel signal  518  is relatively large compared to the strength of the adjacent channel signals  522  and  532 , such that the desired channel signal  518  dominates both the pre-filter and post-filter magnitude determinations  316  and  314 . In contrast, if these two magnitudes differ by a selected amount, which may be a set or a programmable amount loaded internally or through the external control circuitry  106 , the signal strength determination block may conclude that the strength of the desired channel signal  518  is relatively small compared to the strength of the adjacent channel signals  522  and  532 , such that the adjacent channel signals  522  and  532  significantly influence the post-filter magnitude determination  316 . For this latter case, it is noted that the significant difference will correlate to the adjacent channel magnitude signal (adj_mag)  316  being significantly larger than the desired channel magnitude signal (mag)  314 . 
     For example, as depicted in FIG. 5, the adjacent channel signal  522  is large compared to the desired channel signal  518 . Thus, the adjacent channel magnitude signal (adj_mag)  316  would be significantly larger than the desired channel magnitude signal (mag)  314 . In contrast, if the desired channel becomes the channel  520  and the desired channel signal is now signal  522 , the adjacent channel magnitude signal (adj_mag)  316  and the desired channel magnitude signal (mag)  314  will be relatively close together. 
     FIG. 4 is a flow diagram of an embodiment for a station scan algorithm  400  that may be implemented by signal strength determination block  303  to determine if a station has been found. An initial channel is selected in the start block  402 . In block  404 , the power level for the desired channel, such as the magnitude signal (mag)  314 , is determined from the desired channel signal. Decision block  406  then checks to see whether the power level exceeds the power threshold value. If not, the radio receiver  150  moves onto the next channel in block  414 . Once the next channel has been selected, control passes back to block  404  for a determination of the power level for the new channel. Once the power value for a channel exceeds the power threshold value, control passes to block  408 . It is again noted that although a CORDIC demodulator is depicted in FIG. 3 for providing the magnitude signal (mag)  314  as an indication of the power level for the desired channel, an indication of the power level of the desired channel could be determined from the output of the filter  304  using other techniques, as desired. 
     In block  408 , the post-filter and pre-filter signal magnitude values  314  and  316  are determined. As discussed above, the desired channel magnitude value (mag)  314  is determined from the I and Q data signals  310  and  312 , and the adjacent channel magnitude value (adj_mag)  316  is determined from the I and Q data signals  224  and  226 . In decision block  610 , these magnitude values are compared to see if their ratio falls within an acceptable range. For example, the ratio may be compared to the adjacent channel rejection specification for the digital receiver  100 , which may be for example 30 dB. If the ratio falls outside this range (e.g., &gt;30 dB), indicating that the adjacent channel magnitude value (adj_mag)  314  is large with respect to the desired channel magnitude value (mag)  316 , control passes to block  414 . A new channel is again selected in block  414 , and control passes back to block  404 . Once a ratio is found that falls within the selected range, control passes to block  412  where an appropriate indication is provided by station scan circuitry  300  through station found signal  320 . The station scan algorithm ends with block  414 . 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.