3.4.3.1 Channel Filter

The channel filter performs a decimation filtering (IF filter) of the ADC samples, mixing of the intermediate frequency signal (IQ Mixer) to baseband and finally, low-pass filtering (baseband filter) of the result, providing a high resolution output (IQ_samples) and a processing clock (CLK_BB) for the demodulator. A corresponding block diagram is shown in the following figure.
Figure 3-6. Channel Filter Block Diagram

The correct decimation for each RF frequency band is selected by the CHDN.ADCDN register. The receiving bandwidth is selected by configuring the CHDN.BBDN and CHCR.BWM registers. Table 3-10 describes the correct settings for these registers for each bandwidth.

A typical frequency response of the channel filter, including the phase noise effect, is shown in the following figure.

Figure 3-7. Typical Channel Filter Frequency Response for the 165 kHz Bandwidth

Channel Filter Bandwidths

The register bits CHDN.BBDN and CHCR.BWM are used to select the desired bandwidth. The following table shows the typical bandwidths for an RF signal (fRF) of 433.92 MHz with the corresponding register settings.

Table 3-10. Filter Settings for Each Supported Bandwidth

No.

BW-3 dB, 433.92

-3 dB Bandwidth

(fRF =433.92 MHz)

-30 dB Bandwidth

(fRF = 433.92 MHz)

BBDN

BWM

Resulting CLK_BB433.92 Frequency

125 kHz41 kHz20075.3 kHz
227 kHz43 kHz19079.3 kHz
329 kHz49 kHz17088.6 kHz
433 kHz55 kHz150100.4 kHz
535 kHz59 kHz140107.6 kHz
641 kHz69 kHz120125.5 kHz
745 kHz75 kHz110136.9 kHz
850 kHz83 kHz100150.6 kHz
955 kHz92 kHz90167.3 kHz
1061 kHz103 kHz80188.2 kHz
1171 kHz117 kHz70215.1 kHz
1280 kHz139 kHz74215.1 kHz
1393 kHz151 kHz78215.1 kHz
1499 kHz167 kHz51301.2 kHz
15110 kHz193 kHz55301.2 kHz
16123 kHz205 kHz41376.5 kHz
17134 kHz234 kHz45376.5 kHz
18146 kHz177 kHz512301.2 kHz
19165 kHz270 kHz32502.0 kHz
20185 kHz328 kHz36502.0 kHz
21219 kHz351 kHz39502.0 kHz
22237 kHz368 kHz23753.0 kHz
23243 kHz295 kHz313502.0 kHz
24276 kHz474 kHz27753.0 kHz
25325 kHz494 kHz210753.0 kHz
26366 kHz446 kHz214753.0 kHz

Bandwidth Scaling

The channel filter is realized as a digital filter and the sampling rate depends on the RF signal. It is, therefore, subject to scaling with the RF signal and the corresponding sampling clock (CLK_BB). This scaling effect can modify the actual -3 dB and -30 dB bandwidth in the range of -5.6% to +3.6% versus the typical bandwidths given for fRF = 433.92 MHz in Table 3-10.

If the target application frequency, fRF, differs from 433.92 MHz in Table 3-10, the following procedure can be used to calculate the resulting application bandwidth BW-3 dB, APPL:
  1. The target division factor (DIV_IF) is selected from the following table.
    Table 3-11. Frequency Dependent Division Factor (DIV_IF)
    No.fRFDIV_IF
    1310 MHz to 318 MHz204
    2418 MHz to 440 MHz288
    3441 MHz to 477 MHz306
    4836 MHz to 870 MHz576
    5871 MHz to 956 MHz612
  2. The target (-3 dB) bandwidth from the Table 3-10 selected and the corresponding BBDN and CLK_BB433.92 are remembered.
  3. The application sampling clock, CLK_BBAPPL, for the target application frequency, fRF,APPL, is calculated using
    CLK_BBAPPL⁢ = fRFDIV_IF⁢  ×  BBDN......(13)
  4. The resulting frequency-dependent scaling factor is calculated using
    SFRQAPPL⁢ = CLK_BBAPPLCLK_BB433.92......(14)
  5. Apply the scaling to the typical bandwidth
    BW−3 dB,APPL⁢ = BW−3 dB,433.92⁢ × SFRQAPPL......(15)
  6. If the resulting BWAPPL is too narrow for the target data rate, deviation and crystal tolerances, the procedure must be restarted at the next higher bandwidth.

Usage Example

The application has a target application frequency of fRF = 316 MHz and requires a bandwidth of 366 kHz.

  1. DIV_IF = 205 is taken from line 1 of Table 3-11
  2. BBDN = 2 and CLK_BB433.92 = 753.0 kHz are taken from line 26 of Table 3-10
  3. The application sampling clock is calculated using
    CLK_BBAPPL⁢ = 316 MHz204⁢  × 2⁢ =  774.5 kHz
  4. The resulting scaling is calculated using
    SFRQAPPL⁢ = 774.5 kHz753.0 kHz⁢ = 376.6 kHz
  5. Apply the scaling to the typical bandwidth
    BW−3 dB,APPL⁢ = 366 kHz⁢ × 1.029⁢= 376.6 kHz