5.9.1 IF Frequency

The IF frequency is derived from the crystal frequency as shown in Equation 12.

Equation 12.

Where:

ADC_DIV[2:0] is set in the MODEM register.

The analog filter succeeding the mixer is used for wideband and anti-alias filtering, which is important for the blocking performance at 1 MHz and larger offsets. This filter is fixed and centered on the nominal IF frequency of 307.2 kHz. The bandwidth of the analog filter is about 160 kHz.

Using crystal frequencies which gives an IF frequency within 300 to 320 kHz means that the analog filter can be used (assuming low frequency deviations and low data rates).

Large offsets, however, from the nominal IF frequency will give an un-symmetric filtering (variation in group delay and different attenuation) of the signal, resulting in decreased sensitivity and selectivity. See AN022 CC1020 Crystal Frequency Selection (SWRA070) for more details.

For IF frequencies other than 300 to 320 kHz and for high frequency deviation and high data rates (typically ≥ 76.8 kBaud), the analog filter must be bypassed by setting FILTER_BYPASS = 1 in the FILTER register. In this case the blocking performance at 1 MHz and larger offsets will be degraded.

The IF frequency is always the ADC clock frequency divided by 4. The ADC clock frequency should therefore be as close to 1.2288 MHz as possible.