Direct-conversion receiver system and method, especially a GPS receiver system with high pass filtering

Abstract

The invention relates to a direct conversion receiver and a method in a direct conversion receiver for processing received radio signals that are modulated and centered at a carrier frequency, the modulation extending a sideband above and below the carrier frequency. The method comprises the steps of mixing a local oscillator frequency signal with said received radio signals for generating baseband frequency signals; filtering out generated disturbing direct current (DC) components of said baseband signals centered at the zero frequency; setting said local oscillator frequency signal equal to or about the carrier frequency plus an offset frequency, said offset frequency being equal to the difference between the carrier frequency and a null frequency, said null frequency centered at a notch of said sideband; and centering said notch at the zero frequency of said baseband signals through mixing. The invention relates particularly to suppression of DC offsets generated in the receivers.

1 . A method comprising: 
 producing a baseband signal by mixing a modulated first signal having a carrier frequency and at least one sideband, the at least one sideband having at least one spectral notch frequency, and a second signal having a frequency about equal to the at least one spectral notch frequency; and    filtering the baseband signal.    
   
   
       2 . The method of  claim 1 , wherein the at least one spectral notch frequency comprises a multiple of a chip rate of a modulation signal of the first signal.  
   
   
       3 . The method of  claim 1 , wherein the filtering comprises high pass filtering.  
   
   
       4 . The method of  claim 3 , wherein the filtering further comprises bandpass filtering.  
   
   
       5 . The method of  claim 1 , further comprising adjusting the frequency of the second signal by processing the baseband signal.  
   
   
       6 . The method of  claim 1  further comprising splitting the first signal into two channels, and mixing each of the resulting first signals respectively in the channels with respective second signals having about a 90-degree phase shift with respect to each other.  
   
   
       7 . An apparatus comprising: 
 a first mixer configured to mix a modulated first signal having a carrier frequency and at least one sideband produce a baseband signal, the at least one sideband having at least one spectral notch frequency, and configured to produce a second signal having a frequency about equal to the at least one spectral notch frequency; and    a filter configured to filter the baseband signal.    
   
   
       8 . The apparatus of  claim 7 , wherein the at least one spectral notch frequency comprises a multiple of a chip rate of a modulation signal of the first signal.  
   
   
       9 . The method of  claim 7 , wherein the filter comprises a high pass filter.  
   
   
       10 . The apparatus of  claim 9 , wherein the filter further comprises a bandpass filter.  
   
   
       11 . The apparatus of  claim 7 , further comprising a processor configured to adjust the frequency of the second signal.  
   
   
       12 . The apparatus of  claim 7 , further comprising a splitter configured to split the first signal into first and second channels, the first mixer disposed in the first channel, a second mixer disposed in the second channel, and a local oscillator configured to provide respective second signals to the first mixer and the second mixer with a 90 degree phase shift between the second signals.  
   
   
       13 . The apparatus of  claim 1  wherein the apparatus comprises a GPS receiver system.  
   
   
       14 . A method for processing comprising: 
 mixing a modulated first signal having a carrier frequency with a second signal having a frequency that is different from the carrier frequency to generate an in-phase baseband signal;    mixing the modulated first signal having a carrier frequency with a third signal that is different from the carrier frequency to generate a quadrature baseband signal; and    regulating a local oscillator to select an offset frequency that is a difference between the carrier frequency and a spectral notch frequency centered at an amplitude notch of a sideband of the carrier frequency and generate the second signal and the third signal.    
   
   
       15 . The method of  claim 14  further comprising centering the notch at a zero frequency of the baseband signal.  
   
   
       16 . The method of  claim 14  further comprising suppressing the in-phase signal centered at the zero frequency and suppressing the quadrature signal centered at the zero frequency.  
   
   
       17 . A signal processing device comprising: 
 means for producing a baseband signal by mixing a modulated first signal having a carrier frequency with a second signal having a frequency different from the carrier frequency; and    means for filtering the baseband signal.    
   
   
       18 . The signal processing device of  claim 17  further comprising: 
 means for splitting the modulated first signal into first and second channels;    means for mixing a signal in the first channel with a first output of a local oscillator;    means for mixing a signal in the second channel with a second output of the local oscillator that is phase shifted ninety-degrees from the first output;    means for regulating the local oscillator to select an offset frequency that is a difference between the carrier frequency and a spectral notch frequency centered at an amplitude notch of a sideband of the carrier frequency.    
   
   
       19 . The signal processing device of  claim 17  further comprising means for filtering in-phase signals and quadrature signals that are greater than a set corner frequency.  
   
   
       20 . The signal processing device of  claim 17  further comprising means for regulating a local oscillator for selecting an offset frequency that is a difference between the carrier frequency and a spectral notch frequency centered at an amplitude notch of a sideband of the carrier frequency.