Single-Chip Low Power RF Transceiver for Narrowband Systems ...

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
com pensation 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
sen sitivity in the desired channel, but the
optim um 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] = 11
during image
rejection calibration (RSSI value is
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 ms is used below). Chipcon
recommends the following image
calibration
procedure:
1.
Define 3 variables: XP = 0, XG = 0 and DX = 64.
Go to step 3.
2. Set DX = DX/2.
3.
Write XG to GAIN_COMP register.
4. If XP+2·DX < 127 then
write XP+2·DX to PHASE_COMP register
else
write 127 to PHASE_COMP register.
5. 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.
6. Write XP+DX to PHASE_COMP register.
7. 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.
8.
Write XP to PHASE_COMP register.
9. 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.
CC1021
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-
Y 3) / (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
SWRS045B Page 35 of 89