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Texas Instruments provides a number of recommended settings in SmartRF Studio for CC13xx and CC26xx. If a different data rate, deviation or frequency band than covered by those listed in SmartRF Studio is needed, it is required to adjust a few settings to get the best possible performance. This may also be the case if changing the RX bandwidth for an existing setting. This application report provides a guideline on how settings for a wanted data rate can be found. The steps covered in this application report may not cover all possible combinations.
The guidelines listed in this application report are only valid when the following cpe patch is used:
CC13x0: rf_patch_cpe_genfsk
CC13x2: rf_patch_cpe_prop
To fully verify a setting, extensive testing is required.
A mimimum to run all the measurements suggested in this document is:
All testing should be done conducted. Strong unwanted transmitters could impact the results and use of a shielded test setup should be considered if a base station or similar impacts the results.
A VNA or similar should be used to measure the cable loss if exact results are needed. For sensitivity measurements, it is recommended to adjust for the frequency offset for the DUT measured. First, set the DUT to send a CW. Measure the frequency offset using the spectrum analyzator and adjust the frequency set on the signal generator to account for the offset.
Measurements should be done on a sufficient number of devices to give statistical significance. Ideally measurements should also be done for min and max temperature the device will be used in.
The parameters the modem uses are set in a series of registers. Some registers change meaning dependent on which patch is used and the registers are placed in different domains. Overrides are therefore used to set or change the registers the modem is using. In this document, the following commands are typically used to set the wanted override:
Overrides are typically placed in the override list. This is a uint32_t array named pOverrides_X in the examples from Texas Instruments where X is a descriptive name of the override list. An override list is always terminated by (uint32_t)0xFFFFFFFF. Some of the overrides listed through this document already exist in the override list. In these cases, the override should just be changed to the new value. If the override needed does not exist in the current override list it has to be added to the list. The recommended is to add the new override at the end of the list but before the termination word.
In SmartRF Studio, overrides can be edited using the Override Editor.
The transmitted signal will have a certain signal bandwidth (SBW), which depends on the symbol rate and modulation format. This bandwidth can be approximated by Carson's rule given that the modulation format is FSK:
Where
Since GFSK uses a shaping to reduce the side lobes the GFSK signal bandwidth will be less than estimated by Carson’s rule. The exact signal bandwidth can be measured with a spectrum analyzer.
On the receiver side there is an RX channel filter, which is centered on the down-converted received RF frequency, such that, the intermediate frequency (IF). The RX filter has a programmable bandwidth (RX BW). The transmitted signal bandwidth has to be less than RX BW, but the frequency error of the transmitter and receiver must also be taken into account.
If there is an error in the transmitter carrier frequency and the receiver LO frequency, there will also be an error in the IF frequency. For simplicity assume the frequency error in the transmitter and receiver is equal (same type of crystal). If the receiver has an error of –X ppm and the transmitter has an error of +X ppm the IF frequency will have an error of +2*X ppm. Conversely, if the receiver has an error of +X ppm and the transmitter an error of -X ppm the IF frequency will have an error of -2*X ppm.
RX BW has to be larger than the maximum SBW plus the maximum frequency error due to crystal inaccuracies. Worst case scenario will be for the crystal errors on TX and RX side to be of opposite signs.
where