The AAF used here is the same filter as the optional bandpass filter for its high performance, small footprint, and low cost. This filter is critical in filtering out-of-band noise that would otherwise alias back into the first Nyquist zone at the ADC output. An advantage of the RF sampling ADC is the very wide Nyquist bandwidth it can provide, typically greater than 1 GHz. This gives more relaxed requirements for the anti-alias filtering. In practice, the AAF passband can be as large as the ADC Nyquist bandwidth and as small as the desired band as long as the aliased noise does not fold on top of the wanted band. Additional band specific selection and anti-alias filtering is then carried out by the built-in DDC in the ADC.
The other function of the AAF in a co-location deployment is to provide further filtering of the blocker to prevent desensitization of the receiver. The amount of filtering depends on the ADC alias location of the blocker with the chosen ADC sampling rate. There are two scenarios:
- Scenario A: The blocker alias falls directly onto the wanted C-band, implying the blocker must be ≤ –109 dBm at the input of the ADC (referenced to the antenna input).
- Scenario B: The blocker alias falls outside the wanted C-band, and no filtering of the blocker is needed if it does not saturate the ADC input. This scenario is where direct RF sampling offers more flexibility through appropriate frequency planning.