CC1021_16 Datasheet, PDF (30/91 Pages) Texas Instruments – Single Chip Low Power RF Transceiver for Narrowband Systems

Not Recommended for New Designs NRND
CC1021
SWRS045D – JANUARY 2006 – REVISED JANUARY 2015
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Equation 21 can be used to calculate the power, P, in dBm from the RSSI readout values in Figure 5-11
and Figure 5-12.
P = 1.5 × [RSSI – RSSI_ref] + P_ref
(21)
Where:
P is the output power in dBm for the current RSSI readout value.
RSSI_ref is the RSSI readout value taken from Figure 5-11 or Figure 5-12 for an input power level of
P_ref.
NOTE
The RSSI readings in decimal value changes for different channel filter bandwidths.
The analog filter has a finite dynamic range and is the reason why the RSSI reading is saturated at lower
channel filter bandwidths. Higher channel filter bandwidths are typically used for high frequency deviation
and data rates. The analog filter bandwidth is about 160 kHz and is bypassed for high frequency deviation
and data rates and is the reason why the RSSI reading is not saturated for 153.6 kHz and 307.2 kHz
channel filter bandwidths in Figure 5-11 and Figure 5-12.
Figure 5-11. Typical RSSI Value vs Input Power for Different
Channel Filter Bandwidths, 433 MHz
Figure 5-12. Typical RSSI Value vs Input Power for Different
Channel Filter Bandwidths, 868 MHz
5.9.6 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 to 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.
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