Methods and systems for calibrating an analog filter

Abstract

Devices and methods capable of addressing filter responses are disclosed. For example, a method for compensating a first low-pass filter and a second low-pass filter is disclosed. The method includes injecting a reference tone f R and a cutoff tone f C into the first low-pass filter, and measuring respective filter responses of the reference tone f R and the cutoff tone f C while changing capacitor codes that control a cutoff frequency of the first low-pass filter until a first capacitor code I CODE is determined that most accurately causes the first low-pass filter to utilize a desired cutoff frequency f 0 , performing a similar operation for the second low-pass filter until a second capacitor code Q CODE is determined, and calibrating for mismatch between the first low-pass filter and the second low-pass filter.

What is claimed is: 
     
       1. A method for compensating for non-idealities in a filter circuit that includes programmable filter circuitry including a first low-pass filter and a second low-pass filter both having a common desired cutoff frequency f 0 , the method comprising:
 for a first desired bandwidth BW 0  corresponding to the common desired cutoff frequency f 0 , injecting a reference tone IR and a cutoff tone fC into the first low-pass filter, and measuring respective filter responses of the reference tone fR and the cutoff tone fC while changing capacitor codes that control a cutoff frequency f 0 -I of the first low-pass filter until a first capacitor code ICODE is determined that causes the first low-pass filter to match the desired cutoff frequency f 0  as close as possible given an available resolution of the capacitor codes; 
 for the first desired bandwidth BW 0 , injecting the reference tone fR and the cutoff tone fC into the second low-pass filter, and measuring respective filter responses of the reference tone fR and the cutoff tone fC while changing capacitor codes that control a cutoff frequency f 0 -Q of the second low-pass filter until a second capacitor code QCODE is determined that causes the second low-pass filter to match the desired cutoff frequency f 0  as close as possible given the available resolution of the capacitor codes; and 
 further calibrating for mismatch between the first low-pass filter and the second low-pass filter for one or more additional bandwidths greater than the first desired bandwidth BW 0 . 
 
     
     
       2. The method of  claim 1 , wherein the one or more additional bandwidths include a second desired bandwidth BW 1 , where BW 1 =N×BW 0 , where N is a positive integer greater than 1. 
     
     
       3. The method of  claim 2 , wherein calibrating for mismatch between the first low-pass filter and the second low-pass filter includes:
 for a respective second cutoff frequency f 1 , where f 1 =(N×f 0 )+Δf, where Δf is a cutoff frequency offset for the second desired bandwidth BW 1 :
 determining a capacitor code offsets ΔI OFFSET  and ΔQ OFFSET ; 
 adding the capacitor code offset ΔI OFFSET  to the first capacitor code I CODE  to produce a first compensated capacitor code I C-CODE ; and 
 
 adding the capacitor code offset ΔQ OFFSET  to the second capacitor code Q CODE  to produce a second compensated capacitor code Q C-CODE , wherein the second cutoff frequency f 1 =(N×f 0 )+Δf, where Δf is a cutoff frequency offset for the second desired bandwidth BW 1 . 
 
     
     
       4. The method of  claim 3 , wherein
 BW 0 =20 MHz, BW 1 =40 MHz, f 0 =8.75 MHz, f 1 =18.75 MHz, and Δf=1.25 MHz; or wherein 
 BW 0 =20 MHz, BW 1 =80 MHz, f 0 =8.75 MHz, f 1 =38.75 MHz, and Δf=3.75 MHz. 
 
     
     
       5. The method of  claim 3 , wherein calibrating for mismatch between the first low-pass filter and the second low-pass filter further includes:
 determining a fractional capacitor code CI FRAC  corresponding to the first desired bandwidth BW 0 , the fractional capacitor code CI FRAC  being a value that lies between two consecutive capacitor codes [I CODE , I CODE+1 ], and that ideally corresponds to both a zero phase difference and a zero power difference between the first low-pass filter and the second low-pass filter; and 
 using the fractional capacitor code CI FRAC  to determine the capacitor code offsets ΔI OFFSET  and ΔQ OFFSET . 
 
     
     
       6. The method of  claim 5 , wherein determining the fractional capacitor code CI FRAC  includes:
 interpolating a line using a plurality of points with each point having a first dimension being a combined I-Q capacitor code [I CODE , Q CODE ], and a second dimension being a respective measured phase offset between the first low-pass filter and the second low-pass filter using a respective combined I-Q capacitor code; and 
 selecting a combined I-Q capacitor code value that corresponds to a substantially zero phase difference between the first low-pass filter and the second low-pass filter. 
 
     
     
       7. The method of  claim 5 , wherein using the fractional capacitor code C FRAC  to determine the capacitor code offset ΔI OFFSET  and ΔQ OFFSET  includes:
 rounding the fractional capacitor code CI FRAC  to a nearest integer to produce the capacitor code offset ΔI OFFSET  and ΔQ OFFSET ; 
 adding the capacitor code offset ΔI OFFSET  to the first capacitor code I CODE  to produce the first compensated capacitor code I C-CODE ; and 
 adding the capacitor code offset ΔQ OFFSET  to the second capacitor code Q CODE  to produce the second compensated capacitor code Q C-CODE . 
 
     
     
       8. The method of  claim 7 , wherein using the fractional capacitor code CI FRAC  to determine the capacitor code offsets ΔI OFFSET  and ΔQ OFFSET  includes:
 rounding to the nearest integer a scaled value=[(1+αΔfc)*ΔC FRAC ] to produce the capacitor code offsets ΔI OFFSET  and ΔQ OFFSET , where ΔC FRAC  is a difference between the first capacitor code CI FRAC  and the second capacitor code Q CODE , a is a scaling factor derived from empirical data, and Δfc is a capacitor code difference corresponding to the cutoff frequency offset Δf; 
 adding the capacitor code offset ΔI OFFSET  to the first capacitor code I CODE  to produce the first compensated capacitor code I C-CODE ; and 
 adding the capacitor code offset ΔQ OFFSET  to the second capacitor code Q CODE  to produce the second compensated capacitor code Q C-CODE . 
 
     
     
       9. The method of  claim 8 , further comprising:
 applying the first compensated capacitor code I C-CODE  to the first low-pass filter; and 
 applying the second compensated capacitor code Q C-CODE  to the second low-pass filter. 
 
     
     
       10. A wirelessly operating device that operates according to the method of  claim 1 . 
     
     
       11. A device for compensating for non-idealities in a filter circuit that includes programmable filter circuitry including a first low-pass filter and a second low-pass filter both having a common desired cutoff frequency f 0  corresponding to a first desired bandwidth BW 0 , the device comprising:
 code search circuitry that controls the first low-pass filter and the second low-pass filter; 
 tone generation circuitry that injects a reference tone f R  and a cutoff tone f C  into both the first low-pass filter and the second low-pass filter, 
 measurement circuitry that: (1) measures respective filter responses of the reference tone f R  and the cutoff tone f C  while the code search circuitry changes capacitor codes that control a cutoff frequency f 0-1  of the first low-pass filter until a first capacitor code I CODE  is determined that causes the first low-pass filter to match the desired cutoff frequency f 0  as close as possible given an available resolution of the capacitor codes; and (2) measures respective filter responses of the reference tone f R  and the cutoff tone f C  while the code search circuitry changes capacitor codes that control a cutoff frequency f 0-Q  of the second low-pass filter until a second capacitor code Q CODE  is determined that causes the second low-pass filter to match the desired cutoff frequency f 0  as close as possible given the available resolution of the capacitor codes; and 
 calibration circuitry configured to calibrate for mismatch between the first low-pass filter and the second low-pass filter for one or more additional bandwidths greater than a first desired bandwidth BW 0  of the desired cutoff frequency f 0 . 
 
     
     
       12. The device of  claim 11 , wherein each of the one or more additional bandwidths include a second desired bandwidth BW 1 , where BW 1 =N×BW 0 , where N is a positive integer greater than 1. 
     
     
       13. The device of  claim 12 , wherein the calibration circuitry is further configured to:
 for a respective second cutoff frequency f 1  for the second bandwidth BW 1 , determine a capacitor code offsets ΔI OFFSET  and ΔQ OFFSET ; 
 add the capacitor code offset ΔI OFFSET  to the first capacitor code I CODE  to produce a first compensated capacitor code I C-CODE ; and 
 add the capacitor code offset ΔQ OFFSET  to the second capacitor code Q CODE  to produce a second compensated capacitor code Q C-CODE ; 
 wherein the second cutoff frequency f 1 =(N×f 0 )+Δf, where Of is a cutoff frequency offset for the second desired bandwidth BW 1 . 
 
     
     
       14. The device of  claim 13 , wherein the calibration circuitry is further configured to calibrate for mismatch between the first low-pass filter and the second low-pass filter by:
 determining a fractional capacitor code CI FRAC  corresponding to the first desired bandwidth BW 0 , the fractional capacitor code CI FRAC  being a value that lies between two consecutive capacitor codes [I CODE , I CODE+1 ], and that ideally corresponds to both a zero phase difference and a zero power difference between the first low-pass filter and the second low-pass filter; and 
 using the fractional capacitor code CI FRAC  to determine the capacitor code offset ΔI OFFSET  and ΔQ OFFSET . 
 
     
     
       15. The device of  claim 14 , wherein the calibration circuitry is further configured to determining the fractional capacitor code CI FRAC  by:
 interpolating a line using a plurality of points with each point having a first dimension being a combined I-Q capacitor code [I CODE , Q CODE ], and a second dimension being a respective measured phase offset between the first low-pass filter and the second low-pass filter using a respective combined I-Q capacitor code; and 
 selecting a combined I-Q capacitor code value that corresponds to a substantially zero phase difference between the first low-pass filter and the second low-pass filter. 
 
     
     
       16. The device of  claim 15 , wherein the calibration circuitry is further configured to use the fractional capacitor code C FRAC  to determine the capacitor code offset Δ OFFSET  by:
 rounding the fractional capacitor code C FRAC  to a nearest integer to produce the capacitor code offset Δ OFFSET ; 
 adding the capacitor code offset Δ OFFSET  to the first capacitor code I CODE  to produce the first compensated capacitor code I C-CODE ; and 
 adding the capacitor code offset Δ OFFSET  to the second capacitor code Q CODE  to produce the second compensated capacitor code Q C-CODE . 
 
     
     
       17. The device of  claim 15 , wherein using the fractional capacitor code CI FRAC  to determine the capacitor code offsets ΔI OFFSET  and ΔQ OFFSET  includes:
 rounding to the nearest integer [(1+αΔfc)*ΔC FRAC ] to produce the capacitor code offsets ΔI OFFSET  and ΔQ OFFSET , where ΔC FRAC  is a difference between the first capacitor code CI FRAC  and the second capacitor code Q CODE , a is a scaling factor derived from empirical data, and Δfc is a capacitor code difference corresponding to the cutoff frequency offset Δf; 
 adding the capacitor code offset ΔQ OFFSET  to the first capacitor code I CODE  to produce the first compensated capacitor code I C-CODE ; and 
 adding the capacitor code offset ΔQ OFFSET  to the second capacitor code Q CODE  to produce the second compensated capacitor code Q C-CODE . 
 
     
     
       18. The device of  claim 11 , wherein the device is configured to:
 applies the first compensated capacitor code I C-CODE  to the first low-pass filter; and 
 applies the second compensated capacitor code Q C-CODE  to the second low-pass filter. 
 
     
     
       19. A wirelessly operating device that incorporates the device of  claim 11 .