Frequency offset correction in receivers having non-ideal synthesizer channel spacing

Abstract

In a receiver of a transmission system in which the data transmission rate is not an integer multiple of the spacing between transmission channels, a single oscillator is used to generate both the system clock used to process the data signal as well as the mixing signal used to downconvert the received RF signal to an intermediate frequency (IF). The frequency error in the IF signal that results from mixing the RF signal at a less-than-ideal mixing frequency is compensated by selecting an appropriate mixing signal frequency applied when downconverting the IF signal to baseband. In a transmitter, the mixing signal frequency used to upconvert the outgoing baseband signal to IF is selected to pre-compensate for the frequency error resulting from upconverting the IF signal to RF using a less-than-ideal mixing frequency. In either case, the receiver/transmitter can be implemented without having to provide a dedicated reference oscillator for converting signals between RF and IF.

1 . In a receiver, a method comprising: 
 downconverting a received signal from an input frequency to an intermediate frequency (IF) by mixing the received signal using a first mixing signal whose frequency differs from an ideal mixing frequency for the received signal such that the IF signal has a frequency error; and    downconverting the IF signal to baseband by mixing the IF signal using a second mixing signal whose frequency is selected based on the frequency error of the IF signal to generate a baseband signal having a frequency error less than the frequency error of the IF signal.    
     
     
         2 . The invention of  claim 1 , wherein: 
 the input frequency is a radio frequency;    the downconversion from RF to IF is implemented in an analog domain; and    the downconversion from IF to baseband is implemented in a digital domain, wherein the IF signal is A/D converted prior to being downconverted to baseband.    
     
     
         3 . The invention of  claim 1 , further comprising: 
 selecting a channel in the received signal;    selecting the frequency of the first mixing signal based on the selected channel; and    selecting the frequency of the second mixing signal based on the frequency error of the IF signal.    
     
     
         4 . The invention of  claim 3 , further comprising: 
 selecting a new channel in the received signal;    selecting a new frequency for the first mixing signal based on the new channel, wherein the new first mixing signal frequency corresponds to a new frequency error in the IF signal; and    selecting a new frequency for the second mixing signal based on the new frequency error of the IF signal.    
     
     
         5 . The invention of  claim 1 , wherein the first mixing signal is generated from an oscillator that is also used to generate a clock signal used to decode data in the received signal.  
     
     
         6 . The invention of  claim 5 , wherein the frequency of the clock signal and a phase detector frequency of a synthesizer that generates the first mixing signal have a non-integer relationship.  
     
     
         7 . The invention of  claim 1 , wherein a data transmission rate for the received signal and a phase detector frequency of a synthesizer that generates the first mixing signal have a non-integer relationship.  
     
     
         8 . A receiver comprising: 
 a first downconverter adapted to downconvert a received signal from an input frequency to an intermediate frequency (IF) by mixing the received signal using a first mixing signal whose frequency differs from an ideal mixing frequency for the received signal such that the IF signal has a frequency error; and    a second downconverter adapted to downconvert the IF signal to baseband by mixing the IF signal using a second mixing signal whose frequency is selected based on the frequency error of the IF signal to generate a baseband signal having a frequency error less than the frequency error of the IF signal.    
     
     
         9 . The invention of  claim 8 , wherein: 
 the input frequency is a radio frequency;    the downconversion from RF to IF is implemented in an analog domain; and    the downconversion from IF to baseband is implemented in a digital domain, wherein the IF signal is A/D converted prior to being downconverted to baseband.    
     
     
         10 . The invention of  claim 8 , wherein: 
 the frequency of the first mixing signal is selected based on a selected channel in the received signal; and    the frequency of the second mixing signal is selected based on the frequency error of the IF signal.    
     
     
         11 . The invention of  claim 10 , wherein: 
 a new frequency for the first mixing signal is selected based on the selection of a new channel in the received signal, wherein the new first mixing signal frequency corresponds to a new frequency error in the IF signal; and    a new frequency is selected for the second mixing signal based on the new frequency error of the IF signal.    
     
     
         12 . The invention of  claim 8 , wherein the first mixing signal is generated from an oscillator that is also used to generate a clock signal used to decode data in the received signal.  
     
     
         13 . The invention of  claim 12 , wherein the frequency of the clock signal and a phase detector frequency of a synthesizer that generates the first mixing signal have a non-integer relationship.  
     
     
         14 . The invention of  claim 8 , wherein a data transmission rate for the received signal and a phase detector frequency of a synthesizer that generates the first mixing signal have a non-integer relationship.  
     
     
         15 . The invention of  claim 8 , further comprising: 
 an oscillator adapted to generate a system clock signal;    a first mixing signal generator adapted to generate the first mixing signal from the system clock signal;    a second mixing signal generator adapted to generate the second mixing signal; and    digital processing adapted to process the baseband signal based on the system clock signal.    
     
     
         16 . The invention of  claim 15 , wherein: 
 the first downconverter is an analog mixer adapted to convert the received signal into an analog IF signal;    the first mixing signal generator is a PLL-based synthesizer;    the receiver further comprises an analog-to-digital converter adapted to convert the analog IF signal into a digital IF signal;    the second downconverter is a digital downconverter adapted to convert the digital IF signal into the baseband signal; and    the second mixing signal generator is a numerically controlled oscillator (NCO).    
     
     
         17 . The invention of  claim 15 , wherein the frequency of the system clock signal and a phase detector frequency of a synthesizer that generates the first mixing signal have a non-integer relationship.