2.8.7.2 Channel Filter
The channel filter determines the receiver bandwidth. Its output is used for both receive paths; therefore, it has to be configured to be suitable for path A and path B simultaneously. The channel filter bandwidth is set in the eepServices.CHCR.BWM[3:0] and eepServices.CHDN.BBDN[4:0] variables in the range of 25 kHz to 366 kHz (-3 dB bandwidth).
|
Address Service0 |
Name |
Bit 7 |
Bit 6 |
Bit 5 |
Bit 4 |
Bit 3 |
Bit 2 |
Bit 1 |
Bit 0 |
|---|---|---|---|---|---|---|---|---|---|
|
0x00BB |
CHCR | — | — | — | — |
BWM[3:0] | |||
|
0x00BC |
CHDN | — | — |
ADCDN |
BBDN[4:0] | ||||
To determine the needed channel filter bandwidth, the frequency error between the receiver and transmitter and the occupied bandwidth of the used modulation signal has to be taken into account. A detailed consideration of the trade-offs can be found in Frequency Accuracy in System Design.
The settling time of the channel filter is highly dependent upon its bandwidth. As a result, the setting has considerable influence on the channel start-up time. A wide channel filter has a short settling time (approximately 45 µs at 360 kHz), a narrow filter has a long settling time (approximately 380 µs at 25 kHz). The corresponding settings are done for each service separately in the eepServices.CHSTARTFILTER EEPROM variable.
| Address Service0 | Name | Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
|---|---|---|---|---|---|---|---|---|---|
| 0x00BD | CHSTARTFILTER | — | — | PLDT | HADT | DFDT | FID[2:0] | ||
The channel filter bandwidth must always be configured by the configuration tool because it determines the baseband clock frequency, which affects several other settings, such as the correct data rate configuration. Direct manipulations in the SRAM or hardware registers are not recommended.