SWRS045F January 2006 – November 2018 CC1021
PRODUCTION DATA.
In order to meet different channel spacing requirements, the receiver channel filter bandwidth is programmable. It can be programmed from 38.4 to 307.2 kHz.
The minimum receiver channel filter bandwidth depends on data rate, frequency separation and crystal tolerance.
The signal bandwidth must be smaller than the available receiver channel filter bandwidth. The signal bandwidth (SBW) can be approximated by (Carson’s rule) as shown in Equation 13.
Where: fm is the modulating signal.
In Manchester mode, the maximum modulating signal occurs when transmitting a continuous sequence of 0s (or 1s). In NRZ mode the maximum modulating signal occurs when transmitting a 0-1-0 sequence. In both Manchester and NRZ mode 2 × fm is then equal to the programmed baud rate. The equation for SBW can then be rewritten as shown in Equation 14.
Furthermore, the frequency offset of the transmitter and receiver must also be considered. Assuming equal frequency error in the transmitter and receiver (same type of crystal) the total frequency error is shown in Equation 15.
Where:
XTAL_ppm is the total accuracy of the crystal including initial tolerance, temperature drift, loading and ageing.
f_RF is the RF operating frequency.
The minimum receiver channel filter bandwidth (ChBW) can then be estimated as shown in Equation 16.
The DEC_DIV[2:0] bits in the FILTER register control the receiver channel filter bandwidth. The 6 dB bandwidth is given by Equation 17.
Where: IF frequency is set to 307.2 kHz.
Table 5-5 shows the available channel filter bandwidths.
There is a tradeoff between selectivity as well as sensitivity and accepted frequency tolerance. In applications where larger frequency drift is expected, the filter bandwidth can be increased, but with reduced adjacent channel rejection (ACR) and sensitivity.
FILTER BANDWIDTH
[kHz] |
FILTER.DEC_DIV[2:0]
[decimal(binary)] |
---|---|
38.4 | 7 (111b) |
43.9 | 6 (110b) |
51.2 | 5 (101b) |
61.4 | 4 (100b) |
76.8 | 3 (011b) |
102.4 | 2 (010b) |
153.6 | 1 (001b) |
307.2 | 0 (000b) |