Automatic gain control

Abstract

The invention relates to gain control for an amplifier for received radio signals by means of an automatic gain control signal (c). A down converter ( 103 ) produces signal components (I; Q) that are amplified in a respective amplifier ( 104, 105 ). Then, the resulting signal components (I P-LP ; Q P-LP ) are digitised in A/D-converters ( 110; 111 ) for further processing in a digital signal processor ( 112 ) and are also fed to a respective signal level detector ( 113; 114 ) whose output signals ([I]; [Q]) are combined ( 115 ) into a combined signal (V Σ ). This signal (V Σ ) is forwarded to a gain control unit ( 116 ), which produces the automatic gain control signal (c). The automatic gain control signal (c) controls the gain of the amplifiers ( 104; 105 ) such that the resulting signal components (I P-LP ; Q P-LP ) are maintained at signal level ([I]; [Q]) less than a predetermined limit level being adapted to the A/D-converters ( 110; 111 ).

1 . A method for controlling gain of an amplifier for received radio signals in a radio receiver, the radio receiver having a controllable gain responsive to an automatic gain control signal (c), the method comprising: 
 amplifying received radio frequency signals (RF),    level detecting at least one down converted signal (I P-LP ; Q P-LP ) resulting from amplified radio frequency signals (RF P ), and    generating the automatic gain control signal (c) in response to at least one down converted signal level ([I]; [Q]) such that the controllable gain maintains the at least one down converted signal level ([I]; [Q]) at less than a predetermined limit level, characterised by    varying a time constant (τ) of the automatic gain control signal (c) in response to a time derivative parameter (V Σ ) of the at least one down converted signal (I P-LP ; Q P-LP ) such that the automatic gain control signal (c) is adapted to a power variation rate of the received radio frequency signals (RF).    
     
     
         2 . A method according to  claim 1 , characterised by 
 allocating a relatively small value to the time constant (τ) if the time derivative parameter (V Σ ) has a comparatively high value, and    allocating a relatively large value to the time constant (τ) if the time derivative parameter (V Σ ) has a comparatively low value.    
     
     
         3 . A method according to  claim 2 , characterised by generating a pulse signal (R) if the time derivative parameter (V Σ ) exceeds a threshold value (V T ), the pulse signal (R) at least being indicative of a message start in the received radio frequency signals (RF).  
     
     
         4 . A method according to  claim 3 , characterised by during reception of a message the pulse signal (R) being indicative of an erroneous setting of the controllable gain.  
     
     
         5 . A method according to any one of the claims  3  or  4 , characterised by setting the threshold value (V T ) on basis of a power level (P RF ) of the received radio frequency signals (RF).  
     
     
         6 . A method according to any one of the claims  3 - 5 , characterised by digitally processing the pulse signal (R) to produce a time constant value (τ set-D ), the time constant value (τ set-D ) forming a basis for the time constant (τ) of the automatic gain control signal (c).  
     
     
         7 . A method according to  claim 6 , characterised by updating the time constant value (τ set-D ) after a specific time period.  
     
     
         8 . A method according any one of the claims  1 - 7 , characterised by 
 combining two signal components of the at least one down converted signal (I P-LP ; Q P-LP ) to form a combined signal (V Σ ), and    producing the time derivative parameter (V Σ ) by estimating a time derivative of the combined signal (V Σ ).    
     
     
         9 . A method according to  claim 8 , characterised by estimating time derivative of the combined signal (V Σ ) by high pass filtering the combined signal (V Σ ).  
     
     
         10 . A method according to any one of the claims  8  or  9 , characterised by deriving the two signal components (I P-LP ; Q P-LP ) by means of quadrature demodulation of the received radio frequency signals (RF).  
     
     
         11 . A method according to any one of the claims  6 - 10 , characterised by controlling the generation of the automatic gain control signal (c) on basis of the time constant (τ), a desired value (V des ) and an average value of the combined signal (V Σ ).  
     
     
         12 . A computer program directly loadable into the internal memory of a digital computer, comprising software for performing the steps of any of the claims  1 - 11  when said program is run on a computer.  
     
     
         13 . A computer readable medium, having a program recorded thereon, where the program is to make a computer perform the steps of any of the claims  1 - 11 .  
     
     
         14 . An arrangement for controlling gain of an amplifier for received radio signals in a radio receiver, comprising 
 at least one amplifier ( 102 ) for amplifying received radio frequency signals (RF),    a gain control loop ( 112 - 120 ) for controlling gain of at least one amplifier ( 104 ;  105 ) in response to an automatic gain control signal (c), the gain control loop ( 112 - 120 ) including 
 at least one signal level detector ( 113 ,  114 ) for receiving at least one down converted signal (I P-LP ; Q P-LP ) resulting from amplified radio frequency signals (RF P ) and producing in response thereto at least one down converted signal level ([I]; [Q]), and  
 a gain control signal generator ( 116 ) for receiving the at least one down converted signal level ([I], [Q]; V Σ ) and producing in response thereto the automatic gain control signal (c) such that the gain control loop ( 112 - 120 ) maintains the at least one down converted signal level ([I]; [Q]) at less than a predetermined limit level,  
 characterised in that the gain control signal generator ( 116 ) is adapted for varying a time constant (τ) of the automatic gain control signal (c) in response to a time derivative parameter (V Σ ) of the at least one down converted signal (I P-LP ; Q P-LP ) to compensate for a power variation rate of the received radio frequency signals (RF).  
   
     
     
         15 . An arrangement according to  claim 14 , characterised in that the gain control signal generator ( 116 ) is adapted for 
 allocating a relatively small value to the time constant (τ) if the time derivative parameter (V Σ ) has a comparatively high value, and    allocating a relatively large value to the time constant (τ) if the time derivative parameter (V Σ ) has a comparatively low value.    
     
     
         16  An arrangement according to  claim 15 , characterised in that the gain control loop ( 112 - 120 ) includes a ramp detector ( 118 ) for generating a pulse signal (R) if the time derivative parameter (V Σ ) exceeds a threshold value (V T ).  
     
     
         17 . An arrangement according to  claim 16 , characterised in that the gain control loop ( 112 - 120 ) includes a digital signal processor ( 112 ) for receiving the pulse signal (R) and producing in response thereto a time constant value (τ set-D ), the time constant value (τ set-D ) forming a basis for the time constant (τ) of the automatic gain control signal (c).  
     
     
         18 . An arrangement according to any one of the claims  16  or  17 , characterised in that the digital signal processor ( 112 ) is adapted to set the threshold value (V T ) on basis of a power level (P RF ) of the received radio frequency signals (RF).  
     
     
         19 . An arrangement according to any one of the claims  14 - 18 , characterised in that the gain control loop ( 112 - 120 ) includes 
 at least two signal level detectors ( 113 ,  114 ), which each produces a respective down converted signal level ([I]; [Q]),    a combiner for receiving the down converted signal levels ([I]; [Q]) and producing in response thereto a combined signal (V Σ ), and    a derivation unit ( 117 ) for receiving the combined signal (V Σ ) and producing in response thereto a time derivative parameter (V Σ ).    
     
     
         20 . An arrangement according to  claim 19 , characterised in that the derivation unit ( 117 ) includes a high pass filter.  
     
     
         21 . An arrangement according to any one of the claims  19  or  20 , characterised in that the radio receiver comprises a quadrature demodulator ( 103 ) and a frequency oscillator ( 121 ) for deriving two signal components (I P-LP ; Q P-LP ) by means of quadrature demodulation of the amplified received radio frequency signals (RF P ).  
     
     
         22 . An arrangement according to any one of the claims  17 - 21 , characterised in that 
 the gain control loop ( 112 - 120 ) includes means ( 119 ;  120 ) for receiving the time constant value (τ set-D ) and producing the time constant (τ) in response thereto,    the digital signal processor ( 112 ) is adapted for producing a desired value (V des ) indicative of an appropriate combined signal level, and    the gain control signal generator ( 116 ) is adapted for receiving the time constant (τ), the desired value (V des ) and a time limited average value of the combined signal (V Σ ) and producing in response thereto the automatic gain control signal (c).