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Cc112x cc120x fb2pll

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CC112x / CC120x

Feedback to PLL (FB2PLL)

Feedback to PLL (FB2PLL) extends the RX filter BW (RX_BW) without increasing the noise bandwidth.

Setting FREQOFF_CFG.FOC_LIMIT = 0 the programmed RX filter BW is extended by +/-RX_BW/4. As an example, if RX filter BW is programmed to 50 kHz and FREQOFF_CFG.FOC_LIMIT = 0, the noise bandwidth is still 50 kHz (i.e. the bandwidth which sets the noise floor), but the effective RX filter BW is 75 kHz.

Setting FREQOFF_CFG.FOC_LIMIT = 1 the programmed RX filter BW is extended by +/-RX_BW/8. As an example, if RX filter BW is programmed to 50 kHz and FREQOFF_CFG.FOC_LIMIT = 1, the noise bandwidth is still 50 kHz (i.e. the bandwidth which sets the noise floor), but the effective RX filter BW is 62.5 kHz.

The following measurements show that for the same effective RX filter BW, enabling FB2PLL improves sensitivity and close-in selectivity. The measurement results are the average of 5 CC1200 devices.

Case 1: 1.2 kbps, +/-3 kHz, 2-GFSK. OBW (99%) = 6.9 kHz

Programmed RX_BW [kHz] 14.37 10.96 9.69
FREQOFF_CFG.FOC_LIMIT FB2PLL not enabled 1 0
Effective BW [kHz] 14.37 13.7 14.54
Sensitivity (1% BER) [dBm] -121.4 -121.9 -122.1
Interferer offset [kHz] Selectivity [dB] Selectivity [dB] Selectivity [dB]
-37.5 54.8 56.2 55.6
-25 55.4 55.6 55.8
-12.5 53.8 55.2 55.2
12.5 54.6 55.0 55.2
25 55.2 56.0 56.0
37.5 55.2 56.0 56.0

Case 2: 4.8 kbps, +/-2.4 kHz, 2-GFSK. OBW (99%) = 8 kHz

Programmed RX_BW [kHz] 12.63 9.69
FREQOFF_CFG.FOC_LIMIT FB2PLL not enabled 0
Effective BW [kHz] 12.63 12.11
Sensitivity (1% BER) [dBm] -118.8 -119.5
Interferer offset [kHz] Selectivity [dB] Selectivity [dB]
-37.5 52.6 53.4
-25 52.6 54.0
-12.5 52.0 53.0
12.5 51.6 53.0
25 52.6 53.8
37.5 52.4 53.4


Sensitivity Measurements

The table below gives sensitivity figures (1% BER) for different RF system parameters (e.g. RF frequency, data rate, deviation, RX filter BW, modulation format,....).

Part number RF frequency [MHz] Modulation format Symbol rate [ksps] Frequency deviation [kHz] Programmed RX_BW [kHz] FB2PLL [yes/no] Sensitivity [dBm] Comment eJournal number
CC1120 470 GFSK 0.6 1.8 7.8 no -125 Possible improvement: LNA = 3, Extended Data filter on 26996
CC1200 868 GFSK 0.6 1.2 16.7 yes -121 Extended Data filter on 26995
CC1200 868 GFSK 0.6 1.2 16.7 yes -116.5 Extended Data filter off 26995