CC1020-RTR1 datasheet(35/91 Pages) TI

35 / 91 page

CC1020

SWRS046

Page 35 of 92

12.6. Image Rejection Calibration

For perfect image rejection, the phase and

gain of the “I” and “Q” parts of the analog

RX chain must be perfectly matched. To

improve the image rejection, the “I” and

“Q” phase and gain difference can be fine-

tuned by adjusting the PHASE_COMP and

GAIN_COMP registers.

This allows

compensation for process variations and

other nonidealities. The calibration is done

by injecting a signal at the image

frequency, and adjusting the phase and

gain difference for minimum RSSI value.

During image rejection calibration, an

unmodulated carrier should be applied at

the image frequency (614.4 kHz below the

desired channel), No signal should be

present in the desired channel. The signal

level should be 50 – 60 dB above the

sensitivity in the desired channel, but the

optimum level will vary from application to

application. Too large input level gives

poor results due to limited linearity in the

analog IF chain, while too low input level

gives poor results due to the receiver

noise floor.

For

best

RSSI

accuracy,

use

AGC_AVG(1:0]

during

image

rejection

calibration

(RSSI

value

averaged over 16 filter output samples).

The RSSI register update rate then equals

the receiver channel bandwidth (set in

FILTER register) divided by 8, as the filter

output rate is twice the receiver channel

bandwidth. This gives the minimum

waiting time between RSSI register reads

(0.5

used

below).

Chipcon

recommends

the

following

image

calibration procedure:

Define 3 variables: XP = 0, XG = 0 and DX = 64.

Go to step 3.

Set DX = DX/2.

Write XG to GAIN_COMP register.

If XP+2·DX < 127 then

write XP+2·DX to PHASE_COMP register

else

write 127 to PHASE_COMP register.

Wait at least 3 ms. Measure signal strength Y4

as filtered average of 8 reads from RSSI register

with 0.5 ms of delay between each RSSI read.

Write XP+DX to PHASE_COMP register.

Wait at least 3 ms. Measure signal strength Y3

as filtered average of 8 reads from RSSI register

with 0.5 ms of delay between each RSSI read.

Write XP to PHASE_COMP register.

Wait at least 3 ms. Measure signal strength Y2

as filtered average of 8 reads from RSSI register

with 0.5 ms of delay between each RSSI read.

10. Write XP-DX to PHASE_COMP register.

11. Wait at least 3 ms. Measure signal strength Y1

as filtered average of 8 reads from RSSI register

with 0.5 ms of delay between each RSSI read.

12. Write XP-2·DX to PHASE_COMP register.

13. Wait at least 3 ms. Measure signal strength Y0

as filtered average of 8 reads from RSSI register

with 0.5 ms of delay between each RSSI read.

14. Set AP = 2·(Y0-Y2+Y4) – (Y1+Y3).

15. If AP > 0 then

set DP = ROUND( 7·DX·(2·(Y0-Y4)+Y1-

Y3) / (10·AP) )

else

if Y0+Y1 > Y3+Y4 then

set DP = DX

else

set DP = -DX.

16. If DP > DX then

set DP = DX

else

if DP < -DX then set DP = -DX.

17. Set XP = XP+DP.

18. Write XP to PHASE_COMP register.

19. If XG+2·DX < 127 then

write XG+2·DX to GAIN_COMP register

else

write 127 to GAIN_COMP register.

20. Wait at least 3 ms. Measure signal strength Y4

as filtered average of 8 reads from RSSI register

with 0.5 ms of delay between each RSSI read.

21. Write XG+DX to GAIN_COMP register.

22. Wait at least 3 ms. Measure signal strength Y3

as filtered average of 8 reads from RSSI register

with 0.5 ms of delay between each RSSI read.

23. Write XG to GAIN_COMP register.

24. Wait at least 3 ms. Measure signal strength Y2

as filtered average of 8 reads from RSSI register

with 0.5 ms of delay between each RSSI read.

25. Write XG-DX to GAIN_COMP register.

26. Wait at least 3 ms. Measure signal strength Y1

as filtered average of 8 reads from RSSI register

with 0.5 ms of delay between each RSSI read.

27. Write XG-2·DX to GAIN_COMP register.

28. Wait at least 3 ms. Measure signal strength Y0

as filtered average of 8 reads from RSSI register

with 0.5 ms of delay between each RSSI read.

29. Set AG = 2·(Y0-Y2+Y4) – (Y1+Y3).

30. If AG > 0 then

set DG = ROUND( 7·DX·(2·(Y0-Y4)+Y1-

Y3) / (10·AG) )

else

if Y0+Y1 > Y3+Y4 then

set DG = DX

else

set DG = -DX.

31. If DG > DX then

set DG = DX

else

if DG < -DX then set DG = -DX.

32. Set XG = XG+DG.

33. If DX > 1 then go to step 2.

34.

Write XP to PHASE_COMP register and

XG to GAIN_COMP register.

If repeated calibration gives varying

results, try to change the input level or

increase the number of RSSI reads N. A

good starting point is N=8. As accuracy is

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