5.9.2 Receiver Channel Filter Bandwidth

In order to meet different channel spacing requirements, the receiver channel filter bandwidth is programmable. It can be programmed from 9.6 to 307.2 kHz.

The minimum receiver channel filter bandwidth depends on baud 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) shown in Equation 13.

Where:

f_m 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 × f_m 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[4:0] bits in the FILTER register control the receiver channel filter bandwidth. The 6 dB bandwidth is given by Equation 17.

Equation 17.

Where:

the IF frequency is set to 307.2 kHz.

In SmartRF Studio, the user specifies the channel spacing and the channel filter bandwidth is set according to Table 5-5.

For narrowband systems with channel spacings of 12.5 and 25 kHz, the channel filter bandwidth is 12.288 kHz and 19.2 kHz respectively to comply with ARIB STD-T67 and EN 300 220.

For wideband systems (channel spacing of 50 kHz and above), it is possible to use different channel filter bandwidths than given in Table 5-5.

There is a trade-off 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.