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Cc112x cc120x fb2pll
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]|
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]|
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|