16
Constellation: Symbol Mapping
The packing of serial data stream into symbols is
done with the Most Significant bit first.
Symbol Timing
The demodulator is designed to acquire and track
timing differences up to +/- 100ppm between the
nominal (expected) symbol rate and the actual
received symbol rate.
AGC1
The purpose of this AGC is to prevent saturation at
the input signal A/D converter(s) while making full
use of the A/D converters dynamic range.
Therefore, AGC1 reacts to the composite input
signal which may comprise not only the useful
signal but also adjacent channel interferers and
noise. The principle of operations is outlined below:
(a)
Digital input samples are first subsampled
according to the user-defined AGC1
response time.
(b)
Near-saturation events are detected from
the subsampled digital input samples and
the AGC gain is adjusted accordingly.
(c)
A 12-bit D/A converter generates the
analog gain control signal RX_AGC1 for
use by the external variable gain amplifiers.
(pin J6/B13 left connector)
(d)
Alternatively, the gain is controlled through
the COM-3504 auxiliary 12-bit DAC1.
(e)
The AGC1 loop can be closed or open, with
the gain frozen at a user-specified level, by
software command.
Adjust
gain
AGC
A/D
12-bit
DAC
pin
J4/B13
Subsample
(loop
response
time)
Detect
near-
saturation
COM-1503
AGC1 principle
The user is responsible for selecting a preamble
extension length (see control register REG18) long
enough to give the receiver AGC enough time to
converge at the beginning of a packet. Selecting the
AGC response time time (see control register
REG39) is a tradeoff between fast convergence and
loop stability.
The figure below illustrates the AGC converging
during the 010101 preamble extension and being
stable during the 32-bit sync word (delineated by
the two cursors).
Orange trace: received signal
Purple trace: receiver gain under AGC control
Blue trace: detected start of frame
Input Modulated Signal Pre-
Processing
Prior to being routed to the demodulator, the input
signal is subject to
AGC1
, variable decimation, and
frequency translation to near-zero frequency.
The variable decimation consists of two half-band
FIR filters and a Cascaded Integrated Comb (CIC)
filter.
Output Modulated Signal Post-
Processing
Several filters are used to clean the out-of-band
output spectrum:
-
two 10-taps half-band FIR filters in series
-
a CIC interpolation filter.The interpolation
factor R is set automatically.