INTRODUCTION
_________________________________________________________________________________________________________
2-8
COM-00-21-05 OCTOBER 2021
Version No.: A
The radio incorporates filtering at the following stages:
●
RFM
○
RF (Image) Filter which is centered at the RF frequency of operation
○
Intermediate Frequency (IF) Filter which is centered at the IF frequency of
operation
●
Configurable analog filter which is embedded in the AD9364 analog front-end chip
●
Digital filter
The configurable analog filter and the digital filter are software configurable filters embedded on
the CBB. These filters are very specific, narrow filters. The filter in the RFM receive path is a
broad filter meant to eliminate distant, noisy RF neighbors and is used to attenuate interference
to avoid saturating the receiver before the analog to digital conversion. When the adjacent
channel interferer’s power is greater than the Airlink signal, the gain in the receive chain needs
to be reduced to avoid saturation. The stronger the interferer, the lower the receive gain. As the
receive gain is reduced, the Airlink signal occupies a smaller portion of the analog to digital
converter (ADC) amplitude range which reduces the resolution and increases the quantization
noise until a level at which the signal can no longer be decoded.
The RFM filter consists of components within the RFM and the IRF, as shown in the figure
below. The receive signal comes into the TX/RX switch on the RF frequency, goes through an
LNA, and into the RF SAW filter. From the RF SAW filter, it goes into a mixer where it is
transformed to the IF frequency, goes into an amplifier, and the CBB. The low bandpass filter
and the RF SAW filter are at the RF frequency whereas the IF SAW (surface acoustic wave)
filter is at the IF frequency.
The RF SAW filter and the IF SAW filter are located on a separate daughter board and are not
physically part of the RFM.
Figure 2-4 RF and IF SAW Filters
