CMX983 Datasheet, PDF (37/93 Pages) CML Microcircuits – Programmable Channel Filter

Analogue Front End (AFE) for Digital Radio
10 Transmit Channel
CMX983
Figure 14 Tx Channel A and B
The CMX983 has two Tx channels (Figure 14). Within each Tx channel, data can be upsampled twice. Data
from the serial port, formatted as 16-bit 2’s complement values, optionally passes through a first upsampler, a
programmable FIR interpolation filter and a bit selector. The data then passes through a second upsampler
and into a sigma-delta modulator. The resulting signal then passes through a reconstruction filter and variable
gain block before being driven onto the Tx output pins.
First upsampler
The first upsampler takes 16-bit 2’s complement values from the serial port at a rate equal to CT1 and outputs
them to the interpolation filter at a rate equal to CT2. The upsampler uses zero padding, so for an upsample
factor of L1 (= f_CT2/f_CT1) there are L1-1 zero-valued samples inserted between each of the original time
samples. This upsampling process results in unwanted spectral images at multiples of the CT1 frequency, but
 2015 CML Microsystems Plc
37
D/983/6